ISO/IEC 23271:2012

Information technology - Common Language Infrastructure (CLI)

ISO/IEC 23271:2012 defines the Common Language Infrastructure (CLI) in which applications written in multiple high-level languages can be executed in different system environments without the need to rewrite those applications to take into consideration the unique characteristics of those environments. It consists of six partitions. Partition I describes the overall architecture of the CLI, and provides the normative description of the Common Type System (CTS), the Virtual Execution System (VES), and the Common Language Specification (CLS). It also provides an informative description of the metadata. Partition II provides the normative description of the metadata: its physical layout (as a file format), its logical contents (as a set of tables and their relationships), and its semantics (as seen from a hypothetical assembler, ILAsm). Partition III describes the Common Intermediate Language (CIL) instruction set. Partition IV provides an overview of the CLI Libraries, and a specification of their factoring into Profiles and Libraries. A companion file, CLILibrary.xml, considered to be part of this partition, but distributed in XML format, provides details of each class, value type, and interface in the CLI Libraries. Partition V describes a standard way to interchange debugging information between CLI producers and consumers. Partition VI contains some sample programs written in CIL Assembly Language (ILAsm), information about a particular implementation of an assembler, a machine-readable description of the CIL instruction set which can be used to derive parts of the grammar used by this assembler as well as other tools that manipulate CIL, a set of guidelines used in the design of the libraries of Partition IV, and portability considerations.

Technologies de l'information — Infrastructure commune de langage (ICL)

General Information

• Status: Published
• Publication Date: 12-Feb-2012
• ICS: 35.060 - Languages used in information technology
• Technical Committee: ISO/IEC JTC 1/SC 22 - Programming languages, their environments and system software interfaces
• Drafting Committee: ISO/IEC JTC 1/SC 22 - Programming languages, their environments and system software interfaces
• Current Stage: 9093 - International Standard confirmed
• Start Date: 29-Apr-2021
• Completion Date: 30-Oct-2025

Relations

• Revises: ISO/IEC 23271:2006 - Information technology - Common Language Infrastructure (CLI) Partitions I to VI
• Effective Date: 15-Jan-2011

Overview

ISO/IEC 23271:2012 - Information technology - Common Language Infrastructure (CLI) - defines a platform-neutral framework that enables applications written in multiple high-level languages to be executed across different system environments without rewriting them for each environment. The standard is organized into six partitions that together give normative specifications for the Common Type System (CTS), the Virtual Execution System (VES), the Common Language Specification (CLS), metadata formats, the Common Intermediate Language (CIL), library factoring, debugging interchange, and implementation guidance (including sample ILAsm programs and machine-readable CIL descriptions).

Key Topics and Technical Requirements

• Common Type System (CTS): normative rules for values, types, classes, interfaces, value vs. reference types, inheritance, member semantics, and type safety.
• Virtual Execution System (VES): execution semantics, machine state, method calling conventions, exception handling, memory model, threading and synchronization, and verification constraints.
• Common Language Specification (CLS): language interoperability rules and conformance views for CLS consumers, extenders, and frameworks.
• Metadata: normative description of metadata physical layout (file format), logical tables and relationships, metadata tokens, and semantics as seen by ILAsm.
• Common Intermediate Language (CIL): full instruction set, opcode encodings, stack transitions, operand types, verifiability rules, and restrictions on CIL sequences.
• CLI Libraries and CLILibrary.xml: overview of library factoring into Profiles and Libraries and a companion XML file describing classes, value types, and interfaces.
• Debugging Interchange: standardized format to exchange debugging information between CLI producers and consumers.
• Implementation guidance: sample ILAsm programs, assembler grammar aids, and portability considerations; extensions to PE file format and method header layouts are specified.

Applications and Users

ISO/IEC 23271:2012 is essential for:

• Compiler and language implementers designing front ends that target CIL and conform to CLS/CTS rules for cross-language interoperability.
• Runtime and virtual machine developers implementing VES semantics, verification, memory model, and thread/exception behavior.
• Tool vendors building assemblers/disassemblers (ILAsm), linkers, debuggers, profilers, and static analyzers that read or emit CLI metadata and CIL.
• Library designers and architects who must factor APIs into profiles and ensure compatibility across environments.
• Portability engineers and security auditors validating type safety, metadata integrity, and runtime conformity.

Using ISO/IEC 23271:2012 ensures consistent behavior for CIL, metadata handling, and cross-language interoperability across diverse system environments.

Related Standards

Refer to companion and industry specifications that address runtime environments, executable file formats (PE extensions mentioned in the standard), and language interoperability for broader ecosystem alignment.

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 23271
Third edition
2012-02-15
Information technology — Common
Language Infrastructure (CLI)
Technologies de l'information — Infrastructure commune de langage
(ICL)
Reference number
©
ISO/IEC 2012
This CD-ROM contains:
1) the publication ISO/IEC 23271:2012 in portable document format (PDF), which can be viewed using
Adobe® Acrobat® Reader;
2) a companion XML file that provides details of each class, value type, and interface in the CLI
Libraries.
Adobe and Acrobat are trademarks of Adobe Systems Incorporated.
©  ISO/IEC 2012
All rights reserved. Unless required for installation or otherwise specified, no part of this CD-ROM may be reproduced, stored in a retrieval
system or transmitted in any form or by any means without prior permission from ISO. Requests for permission to reproduce this product
should be addressed to
ISO copyright office  Case postale 56  CH-1211 Geneva 20  Switzerland
Internet [e-mail]
Reproduction may be subject to royalty payments or a licensing agreement.
Violators may be prosecuted.
Published in Switzerland
ii © ISO/IEC 2012 – All rights reserved

Installation
If this publication has been packaged as a zipped file, do NOT open the file from the CD-ROM, but copy it to
the desired location in your local environment. Onc
...

INTERNATIONAL ISO/IEC
STANDARD 23271
Third edition
2012-02-15
Information technology — Common
Language Infrastructure (CLI)
Technologies de l'information — Infrastructure commune de langage
(ICL)
Reference number
©
ISO/IEC 2012
©  ISO/IEC 2012
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56  CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland
ii © ISO/IEC 2012 – All rights reserved

Table of Contents
Foreword xxi
I.1 Scope 1
I.2 Conformance 2
I.3 Normative references 3
I.4 Conventions 5
I.4.1 Organization 5
I.4.2 Informative text 5
I.5 Terms and definitions 6
I.6 Overview of the Co mmon Language Infrastructure 9
I.6.1 Relationship to type safety 9
I.6.2 Relationship to managed metadata -driven execution 10
I.6.2.1 Managed code 10
I.6.2.2 Managed data 11
I.6.2.3 Summary 11
I.7 Common Language Specification 12
I.7.1 Introduction 12
I.7.2 Views of CLS co mpliance 12
I.7.2.1 CLS framework 12
I.7.2.2 CLS consumer 13
I.7.2.3 CLS extender 13
I.7.3 CLS co mpliance 14
I.7.3.1 Marking items as CLS -compliant 14
I.8 Common Type System 16
I.8.1 Relationship to object -oriented programming 19
I.8.2 Values and types 19
I.8.2.1 Value types and reference types 19
I.8.2.2 Built-in value and reference types 20
I.8.2.3 Classes, interfaces, and objects 21
I.8.2.4 Boxing and unboxing of values 21
I.8.2.5 Identity and equality of values 22
I.8.3 Locations 23
I.8.3.1 Assignment-compatible locations 23
I.8.3.2 Coercion 23
I.8.3.3 Casting 24
© ISO/IEC 2012 – All rights reserved.                                                iii

I.8.4 Type members 24
I.8.4.1 Fields, array elements, and values 24
I.8.4.2 Methods 24
I.8.4.3 Static fields and static methods 25
I.8.4.4 Virtual methods 25
I.8.5 Naming 25
I.8.5.1 Valid names 25
I.8.5.2 Assemblies and scoping 26
I.8.5.3 Visibility, accessibility, and security 27
I.8.6 Contracts 30
I.8.6.1 Signatures 30
I.8.7 Assignment compatibility 34
I.8.7.1 Assignment compatibility for signature types 37
I.8.7.2 Assignment compatibility for location types 38
I.8.7.3 General assignment compatibility 39
I.8.8 Type safety and verification 39
I.8.9 Type definers 39
I.8.9.1 Array types 40
I.8.9.2 Unmanaged pointer types 41
I.8.9.3 Delegates 41
I.8.9.4 Interface type definition 42
I.8.9.5 Class type definition 43
I.8.9.6 Object type definitions 44
I.8.9.7 Value type definition 47
I.8.9.8 Type inheritance 47
I.8.9.9 Object type inheritance 47
I.8.9.10 Value type inheritance 48
I.8.9.11 Interface type derivation 48
I.8.10 Member inheritance 48
I.8.10.1 Field inheritance 48
I.8.10.2 Method inheritance 48
I.8.10.3 Property and event inheritance 49
I.8.10.4 Hiding, overriding, and layout 49
I.8.11 Member definitions 50
I.8.11.1 Method definitions 50
I.8.11.2 Field definitions 51
I.8.11.3 Property definitions 51
I.8.11.4 Event definitions 52
I.8.11.5 Nested type definitions 52
iv                                                © ISO/IEC 2012 – All rights reserved.

I.9 Metadata 53
I.9.1 Components and assemblies 53
I.9.2 Accessing metadata 53
I.9.2.1 Metadata tokens 54
I.9.2.2 Member signatures in metadata 54
I.9.3 Unmanaged code 54
I.9.4 Method implementation metadata 54
I.9.5 Class layout 55
I.9.6 Assemblies: name scopes for types 55
I.9.7 Metadata extensibility 56
I.9.8 Globals, imports, and exports 57
I.9.9 Scoped statics 58
I.10 Name and type rules for the Co mmon Language
Specification 59
I.10.1 Identifiers 59
I.10.2 Overloading 59
I.10.3 Operator overloading 60
I.10.3.1 Unary operators 60
I.10.3.2 Binary operators 61
I.10.3.3 Conversion operators 62
I.10.4 Naming patterns 62
I.10.5 Exceptions 63
I.10.6 Custom attributes 63
I.10.7 Generic types and methods 64
I.10.7.1 Nested type parameter re -declaration 64
I.10.7.2 Type names and arity encoding 65
I.10.7.3 Type constraint re -declaration 66
I.10.7.4 Constraint type restrictions 67
I.10.7.5 Frameworks and accessibility of nested types 67
I.10.7.6 Frameworks and abstract or virtual methods 68
I.11 Collected Co mmon Language Specification rules 69
I.12 Virtual Execution System 72
I.12.1 Supported data types 72
I.12.1.1 Native size: native int, native unsigned int, O and & 73
I.12.1.2 Handling of short integer data types 74
I.12.1.3 Handling of floating -point data types 75
I.12.1.4 CIL instructions and numeric types 76
I.12.1.5 CIL instructions and pointer types 77
I.12.1.6 Aggregate data 78
© ISO/IEC 2012 – All rights reserved.                                                v

I.12.2 Module information 81
I.12.3 Machine state 81
I.12.3.1 The global state 81
I.12.3.2 Method state 82
I.12.4 Control flow 85
I.12.4.1 Method calls 86
I.12.4.2 Exception handling 89
I.12.5 Proxies and remoting 99
I.12.6 Memory model and optimizations 100
I.12.6.1 The memory store 100
I.12.6.2 Alignment 100
I.12.6.3 Byte ordering 100
I.12.6.4 Optimization 100
I.12.6.5 Locks and threads 101
I.12.6.6 Atomic reads and writes 102
I.12.6.7 Volatile reads and writes 102
I.12.6.8 Other memory model issues 103
II.1 Introduction 105
II.2 Overview 106
II.3 Validation and verification 107
II.4 Introductory exa mples 108
II.4.1 “Hello world!” 108
II.4.2 Other examples 108
II.5 General syntax 109
II.5.1 General syntax notation 109
II.5.2 Basic syntax categories 109
II.5.3 Identifiers 110
II.5.4 Labels and lists of labels 111
II.5.5 Lists of hex bytes 111
II.5.6 Floating-point numbers 111
II.5.7 Source line information 112
II.5.8 File names 112
II.5.9 Attributes and metadata 112
II.5.10 ilasm source files 112
II.6 Assemblies, manifests and modules 114
II.6.1 Overview of modules, assemblies, and files 114
II.6.2 Defining an assembly 115
II.6.2.1 Information about the assembly ( AsmDecl) 115
vi                                                © ISO/IEC 2012 – All rights reserved.

II.6.2.2 Manifest resources 118
II.6.2.3 Associating files with an assembly 118
II.6.3 Referencing assemblies 118
II.6.4 Declaring modules 119
II.6.5 Referencing modules 120
II.6.6 Declarations inside a module or assembly 120
II.6.7 Exported type definitions 120
II.6.8 Type forwarders 121
II.7 Types and signatures 122
II.7.1 Types 122
II.7.1.1 modreq and modopt 123
II.7.1.2 pinned 123
II.7.2 Built-in types 124
II.7.3 References to user -defined types ( TypeReference ) 124
II.7.4 Native data types 125
II.8 Visibility, accessibility and hiding 127
II.8.1 Visibility of top -level types and accessibility of nested types 127
II.8.2 Accessibility 127
II.8.3 Hiding 127
II.9 Generics 128
II.9.1 Generic type definitions 128
II.9.2 Generics and recursive inheritance graphs 129
II.9.3 Generic method definitions 130
II.9.4 Instantiating generic types 131
II.9.5 Generics variance 132
II.9.6 Assignment compatibility of instantiated types 132
II.9.7 Validity of member signatures 133
II.9.8 Signatures and binding 134
II.9.9 Inheritance and overriding 135
II.9.10 Explicit method overrides 136
II.9.11 Constraints on generic parameters 137
II.9.12 References to members of generic types 138
II.10 Defining types 139
II.10.1 Type header ( ClassHeader) 139
II.10.1.1 Visibility and accessibility attributes 140
II.10.1.2 Type layout attributes 141
II.10.1.3 Type semantics attributes 141
II.10.1.4 Inheritance attributes 142
© ISO/IEC 2012 – All rights reserved.                                                vii

II.10.1.5 Interoperation attributes 142
II.10.1.6 Special handling attributes 142
II.10.1.7 Generic parameters ( GenPars) 143
II.10.2 Body of a type definition 146
II.10.3 Introducing and overriding virtual methods 147
II.10.3.1 Introducing a virtual method 147
II.10.3.2 The .override directive 147
II.10.3.3 Accessibility and overriding 148
II.10.3.4 Impact of overrides on derived classes 149
II.10.4 Method implementation requirements 150
II.10.5 Special members 150
II.10.5.1 Instance constructor 150
II.10.5.2 Instance finalizer 151
II.10.5.3 Type initializer 151
II.10.6 Nested types 153
II.10.7 Controlling instance layout 153
II.10.8 Global fields and methods 154
II.11 Semantics of classes 156
II.12 Semantics of interfaces 157
II.12.1 Implementing interfaces 157
II.12.2 Implementing virtual methods on interfaces 157
II.12.2.1 Interface Implementation Examples 159
II.13 Semantics of value types 162
II.13.1 Referencing value types 163
II.13.2 Initializing value types 163
II.13.3 Methods of value types 164
II.14 Semantics of special types 166
II.14.1 Vectors 166
II.14.2 Arrays 166
II.14.3 Enums 168
II.14.4 Pointer types 169
II.14.4.1 Unmanaged pointers 170
II.14.4.2 Managed pointers 171
II.14.5 Method pointers 171
II.14.6 Delegates 172
II.14.6.1 Delegate signature compatibility 173
II.14.6.2 Synchronous calls to delegates 174
II.14.6.3 Asynchronous calls to delegates 175
viii                                                © ISO/IEC 2012 – All rights reserved.

II.15 Defining, referencing, and calling methods 177
II.15.1 Method descriptors 177
II.15.1.1 Method declarations 177
II.15.1.2 Method definitions 177
II.15.1.3 Method references 177
II.15.1.4 Method implementations 177
II.15.2 Static, instance, and virtual methods 177
II.15.3 Calling convention 178
II.15.4 Defining methods 179
II.15.4.1 Method body 180
II.15.4.2 Predefined attributes on methods 182
II.15.4.3 Implementation attributes of methods 184
II.15.4.4 Scope blocks 186
II.15.4.5 vararg methods 186
II.15.5 Unmanaged methods 187
II.15.5.1 Method transition thunks 187
II.15.5.2 Platform invoke 188
II.15.5.3 Method calls via function pointers 189
II.15.5.4 Data type marshaling 189
II.16 Defining and referencing fields 190
II.16.1 Attributes of fields 190
II.16.1.1 Accessibility information 191
II.16.1.2 Field contract attributes 191
II.16.1.3 Interoperation attributes 191
II.16.1.4 Other attributes 192
II.16.2 Field init metadata 192
II.16.3 Embedding data in a PE file 193
II.16.3.1 Data declaration 193
II.16.3.2 Accessing data from the PE file 194
II.16.4 Initialization of non -literal static data 194
II.16.4.1 Data known at link time 194
II.16.5 Data known at load time 195
II.16.5.1 Data known at run time 195
II.17 Defining properties 196
II.18 Defining events 198
II.19 Exception handling 201
II.19.1 Protected blocks 201
II.19.2 Handler blocks 201
© ISO/IEC 2012 – All rights reserved.                                                ix

II.19.3 Catch blocks 202
II.19.4 Filter blocks 202
II.19.5 Finally blocks 203
II.19.6 Fault handlers 203
II.20 Declarative security 204
II.21 Custo m attributes 205
II.21.1 CLS conventions: custom attribute usage 205
II.21.2 Attributes used by the CLI 205
II.21.2.1 Pseudo custom attributes 206
II.21.2.2 Custom attributes defined by the CLS 207
II.21.2.3 Custom attributes for security 207
II.21.2.4 Custom attributes for TLS 207
II.21.2.5 Custom attributes, various 208
II.22 Metadata logical forma t: tables 209
II.22.1 Metadata validation rules 210
II.22.2 Assembly : 0x20 211
II.22.3 AssemblyOS : 0x22 212
II.22.4 AssemblyProcessor : 0x21 212
II.22.5 AssemblyRef : 0x23 212
II.22.6 AssemblyRefOS : 0x25 213
II.22.7 AssemblyRefProcessor : 0x24 213
II.22.8 ClassLayout : 0x0F 214
II.22.9 Constant : 0x0B 216
II.22.10 CustomAttribute : 0x0C 216
II.22.11 DeclSecurity : 0x0E 218
II.22.12 EventMap : 0x12 220
II.22.13 Event : 0x14 220
II.22.14 ExportedType : 0x27 222
II.22.15 Field : 0x04 223
II.22.16 FieldLayout : 0x10 225
II.22.17 FieldMarshal : 0x0D 226
II.22.18 FieldRVA : 0x1D 227
II.22.19 File : 0x26 227
II.22.20 GenericParam : 0x2A 228
II.22.21 GenericParamConstraint : 0x2C 229
II.22.22 ImplMap : 0x1C 230
II.22.23 InterfaceImpl : 0x09 231
II.22.24 ManifestResource : 0x28 231
II.22.25 MemberRef : 0x0A 232
x                                                © ISO/IEC 2012 – All rights reserved.

II.22.26 MethodDef : 0x06 233
II.22.27 MethodImpl : 0x19 236
II.22.28 MethodSemantics : 0x18 237
II.22.29 MethodSpec : 0x2B 238
II.22.30 Module : 0x00 239
II.22.31 ModuleRef : 0x1A 239
II.22.32 NestedClass : 0x29 240
II.22.33 Param : 0x08 240
II.22.34 Property : 0x17 241
II.22.35 PropertyMap : 0x15 242
II.22.36 StandAloneSig : 0x11 243
II.22.37 TypeDef : 0x02 243
II.22.38 TypeRef : 0x01 247
II.22.39 TypeSpec : 0x1B 248
II.23 Metadata logical forma t: other structures 249
II.23.1 Bitmasks and flags 249
II.23.1.1 Values for AssemblyHashAlgorithm 249
II.23.1.2 Values for AssemblyFlags 249
II.23.1.3 Values for Culture 249
II.23.1.4 Flags for events [EventAttributes] 250
II.23.1.5 Flags for fields [FieldAttributes] 250
II.23.1.6 Flags for files [FileAttributes] 251
II.23.1.7 Flags for Generic Parameters [GenericParamAttributes] 251
II.23.1.8 Flags for ImplMap [PInvokeAttributes] 251
II.23.1.9 Flags for ManifestResource [ManifestResourceAttributes] 252
II.23.1.10 Flags for methods [MethodAttributes] 252
II.23.1.11 Flags for methods [MethodImplAttributes] 253
II.23.1.12 Flags for MethodSemantics [MethodSemanticsAttributes] 253
II.23.1.13 Flags for params [ParamAttributes] 253
II.23.1.14 Flags for properties [PropertyAttributes] 254
II.23.1.15 Flags for types [TypeAttributes] 254
II.23.1.16 Element types used in signatures 255
II.23.2 Blobs and signatures 257
II.23.2.1 MethodDefSig 259
II.23.2.2 MethodRefSig 260
II.23.2.3 StandAloneMethodSig 261
II.23.2.4 FieldSig 262
II.23.2.5 PropertySig 262
II.23.2.6 LocalVarSig 263
© ISO/IEC 2012 – All rights reserved.                                                xi

II.23.2.7 CustomMod 263
II.23.2.8 TypeDefOrRefOrSpecEncoded 264
II.23.2.9 Constraint 264
II.23.2.10 Param 264
II.23.2.11 RetType 265
II.23.2.12 Type 265
II.23.2.13 ArrayShape 265
II.23.2.14 TypeSpec 266
II.23.2.15 MethodSpec 266
II.23.2.16 Short form signatures 267
II.23.3 Custom attributes 267
II.23.4 Marshalling descriptors 269
II.24 Metadata physical layout 271
II.24.1 Fixed fields 271
II.24.2 File headers 271
II.24.2.1 Metadata root 271
II.24.2.2 Stream header 272
II.24.2.3 #Strings heap 272
II.24.2.4 #US and #Blob heaps 272
II.24.2.5 #GUID heap 272
II.24.2.6 #~ stream 273
II.25 File format extensions to PE 277
II.25.1 Structure of the runtime file format 277
II.25.2 PE headers 277
II.25.2.1 MS-DOS header 278
II.25.2.2 PE file header 278
II.25.2.3 PE optional header 279
II.25.3 Section headers 281
II.25.3.1 Import Table and Import Address Table (IAT) 282
II.25.3.2 Relocations 282
II.25.3.3 CLI header 283
II.25.4 Common Intermediate Language physical layout 284
II.25.4.1 Method header type values 285
II.25.4.2 Tiny format 285
II.25.4.3 Fat format 285
II.25.4.4 Flags for method headers 285
II.25.4.5 Method data section 286
II.25.4.6 Exception handling clauses 286
III.1 Introduction 289
xii                                                © ISO/IEC 2012 – All rights reserved.

III.1.1 Data types 289
III.1.1.1 Numeric data types 290
III.1.1.2 Boolean data type 292
III.1.1.3 Character data type 292
III.1.1.4 Object references 292
III.1.1.5 Runtime pointer types 292
III.1.2 Instruction variant table 294
III.1.2.1 Opcode encodings 294
III.1.3 Stack transition diagram 300
III.1.4 English description 301
III.1.5 Operand type table 301
III.1.6 Implicit argument coercion 304
III.1.7 Restrictions on CIL code sequences 305
III.1.7.1 The instruction stream 306
III.1.7.2 Valid branch targets 306
III.1.7.3 Exception ranges 306
III.1.7.4 Must provide maxstack 307
III.1.7.5 Backward branch constraints 307
III.1.7.6 Branch verification constraints 307
III.1.8 Verifiability and correctness 307
III.1.8.1 Flo w control restrictions for verifiable CIL 308
III.1.9 Metadata tokens 312
III.1.10 Exceptions thro wn 313
III.2 Prefixes to instructions 314
III.2.1 constrained. – (prefix) invoke a member on a value of a
variable type 315
III.2.2 no. – (prefix) possibly skip a fault check 317
III.2.3 readonly. (prefix) – following instruction returns a controlled -
mutability managed pointer 318
III.2.4 tail. (prefix) – call terminates current method 319
III.2.5 unaligned. (prefix) – pointer instruction might be unaligned 320
III.2.6 volatile. (prefix) – pointer reference is volatile 321
III.3 Base instructions 322
III.3.1 add – add numeric values 323
III.3.2 add.ovf. – add integer values with overflo w check 324
III.3.3 and – bitwise AND 325
III.3.4 arglist – get argument list 326
III.3.5 beq. – branch on equal 327
III.3.6 bge. – branch on greater than or equal to 328
© ISO/IEC 2012 – All rights reserved.                                                xiii

III.3.7 bge.un. – branch on greater than or equal to,
unsigned or unordered 329
III.3.8 bgt. – branch on greater than 330
III.3.9 bgt.un. – branch on greater than, unsigned or
unordered 331
III.3.10 ble. – branch on less than or equal to 332
III.3.11 ble.un. – branch on less than or equal to, unsigned or
unordered 333
III.3.12 blt. – branch on less than 334
III.3.13 blt.un. – branch on less than, unsigned or unordered 335
III.3.14 bne.un – branch on not equal or unordered 336
III.3.15 br. – unconditional branch 337
III.3.16 break – breakpoint instruction 338
III.3.17 brfalse. – branch on false, null, or zero 339
III.3.18 brtrue. – branch on non-false or non-null 340
III.3.19 call – call a method 341
III.3.20 calli – indirect method call 343
III.3.21 ceq – compare equal 345
III.3.22 cgt – compare greater than 346
III.3.23 cgt.un – compare greater than, unsigned or unordered 347
III.3.24 ckfinite – check for a finite real number 348
III.3.25 clt – compare less than 349
III.3.26 clt.un – compare less than, unsigned or unordered 350
III.3.27 conv. – data conversion 351
III.3.28 conv.ovf. – data conversion with overflow detection 352
III.3.29 conv.ovf..un – unsigned data conversion with
overflo w detection 353
III.3.30 cpblk – copy data from memory to memory 354
III.3.31 div – divide values 355
III.3.32 div.un – divide integer values, unsigned 356
III.3.33 dup – duplicate the top value of the stack 357
III.3.34 endfilter – end exception handling filter clause 358
III.3.35 endfinally – end the finally or fault clause of an exception
block 359
III.3.36 initblk – initialize a block of memory to a value 360
III.3.37 jmp – jump to method 361
III.3.38 ldarg. – load argument onto the stack 362
III.3.39 ldarga. – load an argument address 363
III.3.40 ldc. – load numeric constant 364
III.3.41 ldftn – load method pointer 365
III.3.42 ldind. – load value indirect onto the stack 366
xiv                                                © ISO/IEC 2012 – All rights reserved.

III.3.43 ldloc – load local variable onto the stack 368
III.3.44 ldloca. – load local variable address 369
III.3.45 ldnull – load a null pointer 370
III.3.46 leave. – exit a protected region of code 371
III.3.47 localloc – allocate space in the local dynamic memory pool 372
III.3.48 mul – multiply values 373
III.3.49 mul.ovf. – multiply integer values with overf lo w check 374
III.3.50 neg – negate 375
III.3.51 nop – no operation 376
III.3.52 not – bitwise complement 377
III.3.53 or – bitwise OR 378
III.3.54 pop – remove the top element of the stack 379
III.3.55 rem – compute remainder 380
III.3.56 rem.un – compute integer remainder, unsigned 381
III.3.57 ret – return from method 382
III.3.58 shl – shift integer left 383
III.3.59 shr – shift integer right 384
III.3.60 shr.un – shift integer right, unsigned 385
III.3.61 starg. – store a value in an argument slot 386
III.3.62 stind. – store value indirect from stack 387
III.3.63 stloc – pop value from stack to local variable 388
III.3.64 sub – subtract numeric values 389
III.3.65 sub.ovf. – subtract integer values, checking for
overflo w 390
III.3.66 switch – table switch based on value 391
III.3.67 xor – bitwise XOR 392
III.4 Object model instructions 393
III.4.1 box – convert a boxable value to its boxed form 393
III.4.2 callvirt – call a method associated, at runtime, with an object 395
III.4.3 castclass – cast an object to a class 397
III.4.4 cpobj – copy a value from one address to another 398
III.4.5 initobj – initialize the value at an address 399
III.4.6 isinst – test if an object is an instance of a class or interface 400
III.4.7 ldelem – load element from array 401
III.4.8 ldelem. – load an element of an array 402
III.4.9 ldelema – load address of an element of an array 404
III.4.10 ldfld – load field of an object 405
III.4.11 ldflda – load field address 406
III.4.12 ldlen – load the length of an array 407
III.4.13 ldobj – copy a value from an address to the stack 408
© ISO/IEC 2012 – All rights reserved.                                                xv

III.4.14 ldsfld – load static field of a class 409
III.4.15 ldsflda – load static field address 410
III.4.16 ldstr – load a literal string 411
III.4.17 ldtoken – load the runtime representation of a metadata token 412
III.4.18 ldvirtftn – load a virtual method pointer 413
III.4.19 mkrefany – push a typed reference on the stack 414
III.4.20 newarr – create a zero -based, one-dimensional array 415
III.4.21 newobj – create a new object 416
III.4.22 refanytype – load the type out of a typed reference 419
III.4.23 refanyval – load the address out of a typed reference 420
III.4.24 rethrow – rethrow the current exception 421
III.4.25 sizeof – load the size, in bytes,of a type 422
III.4.26 stelem – store element to array 423
III.4.27 stelem. – store an element of an array 424
III.4.28 stfld – store into a field of an object 426
III.4.29 stobj – store a value at an address 427
III.4.30 stsfld – store a static field of a class 428
III.4.31 throw – thro w an exception 429
III.4.32 unbox – convert boxed value type to its raw form 430
III.4.33 unbox.any – convert boxed type to value 431
IV.1 Overview 433
IV.2 Libraries and Profiles 434
IV.2.1 Libraries 434
IV.2.2 Profiles 434
IV.2.3 The relationship between Libraries and Profiles 435
IV.3 The Standard Profiles 436
IV.3.1 The Kernel Profile 436
IV.3.2 The Compact Profile 436
IV.4 Kernel Profile feature requirements 437
IV.4.1 Features excluded from the Kernel Profile 437
IV.4.1.1 Floating point 437
IV.4.1.2 Non-vector arrays 437
IV.4.1.3 Reflection 437
IV.4.1.4 Application domains 438
IV.4.1.5 Remoting 438
IV.4.1.6 Vararg 438
IV.4.1.7 Frame growth 438
IV.4.1.8 Filtered exceptions 438
xvi                                                © ISO/IEC 2012 – All rights reserved.

IV.5 The standard libraries 439
IV.5.1 General comments 439
IV.5.2 Runtime infrastructure library 439
IV.5.3 Base Class Library (BCL) 439
IV.5.4 Network library 439
IV.5.5 Reflection library 439
IV.5.6 XML library 439
IV.5.7 Extended numerics library 440
IV.5.8 Extended array library 440
IV.5.9 Vararg library 440
IV.5.10 Parallel library 440
IV.6 Implementation-specific modifications to the system
libraries 442
IV.7 The XML specification 443
IV.7.1 Semantics 443
IV.7.1.1 Value types as objects 451
IV.7.1.2 Exceptions 451
IV.7.2 XML signature notation issues 451
IV.7.2.1 Serialization 451
IV.7.2.2 Delegates 451
IV.7.2.3 Properties 452
IV.7.2.4 Nested types 452
V.1 Portable CILDB files 454
V.1.1 Encoding of integers 454
V.1.2 CILDB header 454
V.1.2.1 Version GUID 454
V.1.3 Tables and heaps 454
V.1.3.1 SymConstant table 455
V.1.3.2 SymDocument table 455
V.1.3.3 SymMethod table 455
V.1.3.4 SymSequencePoint table 456
V.1.3.5 SymScope table 456
V.1.3.6 SymVariable table 456
V.1.3.7 SymUsing table 457
V.1.3.8 SymMisc heap 457
V.1.3.9 SymString heap 457
V.1.4 Signatures 457
VI.Annex A Introduction 459
© ISO/IEC 2012 – All rights reserved.                                                xvii

VI.Annex B Sample progra ms 460
VI.B.1 Mutually recursive program (with tail calls) 460
VI.B.2 Using value types 461
VI.B.3 Custom attributes 463
VI.B.4 Generics code and metadata 466
VI.B.4.1 ILAsm version 466
VI.B.4.2 C# version 467
VI.B.4.3 Metadata 467
VI.Annex C CIL assembler implementation 469
VI.C.1 ILAsm keywords 469
VI.C.2 CIL opcode descriptions 481
VI.C.3 Complete grammar 492
VI.C.4 Instruction syntax 507
VI.C.4.1 Top-level instruction syntax 508
VI.C.4.2 Instructions with no operand 508
VI.C.4.3 Instructions that refer to parameters or local variables 509
VI.C.4.4 Instructions that take a single 32 -bit integer argument 510
VI.C.4.5 Instructions that take a single 64 -bit integer argument 510
VI.C.4.6 Instructions that take a single floating -point argument 510
VI.C.4.7 Branch instructions 511
VI.C.4.8 Instructions that take a method as an argument 511
VI.C.4.9 Instructions that take a field of a class as an argument 511
VI.C.4.10 Instructions that take a type as an argument 511
VI.C.4.11 Instructions that take a string as an argument 512
VI.C.4.12 Instructions that take a signature as an argument 512
VI.C.4.13 Instructions that take a metadata token as an argument 512
VI.C.4.14 Switch instruction 513
VI.Annex D Class library design guidelines 514
VI.Annex E Portability considerations 515
VI.E.1 Uncontrollable behavior 515
VI.E.2 Language- and compiler -controllable behavior 515
VI.E.3 Programmer-controllable behavior 515
VI.Annex F Imprecise faults 517
VI.F.1 Instruction reordering 517
VI.F.2 Inlining 517
VI.F.3 Finally handlers still guaranteed once a try block is entered 517
VI.F.4 Interleaved calls 518
VI.F.4.1 Rejected notions for fencing 518
xviii                                                © ISO/IEC 2012 – All rights reserved.

VI.F.5 Examples 518
VI.F.5.1 Hoisting checks out of a loop 519
VI.F.5.2 Vectorizing a loop 519
VI.F.5.3 Autothreading a loop 519
VI.Annex G Parallel library 521
VI.G.1 Considerations 521
VI.G.2 ParallelFor 521
VI.G.3 ParallelForEach 521
VI.G.4 ParallelWhile 522
VI.G.5 Debugging 522
Index 523
© ISO/IEC 2012 – All rights reserved.                                                xix

Common Language Infrastructure (CLI)
Partition I:
Concepts and Architecture
Foreword
ISO (the International Organization for Standardization) and IEC (the International
Electrotechnical Commission) form the specialized system for worldwide standardization.
National bodies that are members of ISO or IEC participate in the development of International
Standards through technical committees established by the respective organization to deal with
particular fields of technical activity. ISO and IEC technical committees collaborate in fields of
mutual interest. Other international organizations, governmental and non-governmental, in
liaison with ISO and IEC, also take part in the work. In the field of information technology, ISO
and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives,
Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft
International Standards adopted by the joint technical committee are circulated to national bodies
for voting. Publication as an International Standard requires approval by at least 75 % of the
national bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights. ISO and IEC shall not be held responsible for identifying any or all such
patent rights.
ISO/IEC 23271 was prepared by Ecma International (as ECMA-335) and was adopted, under a
special “fast-track procedure”, by Joint Technical Committee ISO/IEC JTC 1, Information
technology, in parallel with its approval by national bodies of ISO and IEC.
This third edition cancels and replaces the second edition (ISO/IEC 23271:2006), which has been
technically revised.
The following features have been added, extended or clarified in the Standard:

 The presentation of the rules for assignment compatibility (§I.8.7, §III.1.8.1.2.3) has
been extensively revised to a more precise and clearer relation-based format.
 The presentation of the verification rules for many IL instructions has been revised to
be more precise and clearer by building upon the revisions to the presentation of
assignment compatibility.
 The presentation of delegate signature compatibility has been revised along the same
lines as assignment compatibility.
 The verification rules for the IL newobj instruction have been extended to cover
general delegate creation.
 The dispatch rules for variance (§II.12.2) have been extended to define resolutions for
the ambiguities that can arise.
 Type forwarders have been added to support the relocation of types between libraries
(§II.6.8)
The following changes of behavior have been made to the Standard:

 The semantics of variance has been redefined making it a core feature of the CLI. In
the previous edition of the Standard variance could be ignored by languages not
wishing to support it (§I.1.8); as exact type matches always took precedence over
matches-by-variance. In this edition the dispatch rules for interfaces (§II.12.2) allow a
match-by-variance to take precedence over an exact match, so all language
© ISO/IEC 2012 – All rights reserved.                                                xxi

implementation targeting the CLI must be aware of the behavior even if it is not
supported in the language (§I.1.8).
 Additional requirements on ilasm to metadata conversion. The left-to-right order of
interfaces listed in a type header (§II.10.2) must now be preserved as a top-to-bottom
order in the InterfaceImpl table (§II.22.23); and the top-to-bottom of method
definitions (§II.10.2, §II.25) must now be preserved as a top-to-bottom order in the
MethodDef table (§II.22.26). Both these additional requirements are required to
support the revised variance semantics.
 System.Math and System.Double have been modified to better conform to IEEE (see
Partition IV and IEC 60559:1989)

The following types have been added to the Standard or have been significantly updated (*
represents an update).
Type Library
System.Action BCL
System.Action`1<-T>* … System.Action`8<-T1.-T8> BCL
System.Comparison`1<-T>* BCL
System.Converter`2<-T,+U>* BCL
System.IComparable`1<-T>* BCL
System.Predicate`1<-T>* BCL
System.Collections.Generic.IComparer`1<-T>* BCL
System.Collections.Generic.IEnumerable`1<+T>* BCL
System.Collections.Generic.IEqualityComparer`1<-T>* BCL
System.Guid BCL
System.MulticastDelegate BCL
System.Reflection.CallingConventions Runtime Infrastructure
System.Runtime.InteropServices.GuidAttribute Runtime Infrastructure
System.Func`1<+TResult>…System.Func`9<-T1.-T8, +TResult> BCL
System.Collections.Generic.Comparer`1 BCL
System.Collections.Generic.EqualityComparer`1 BCL
System.Collections.Generic.ISet`1 BCL
System.Collections.Generic.LinkedList`1 BCL
System.Collections.Generic.LinkedList`1.Enumerator BCL
System.Collections.Generic.LinkedListNode`1 BCL
System.Collections.Generic.Queue`1 BCL
System.Collections.Generic.Stack`1 BCL
System.Collections.Generic.Stack`1.Enumerator BCL
System.Collections.Stack BCL
System.DBNull BCL
System.Runtime.InteropServices.Marshal Runtime Infrastructure
System.Runtime.InteropServices.SafeBuffer Runtime Infrastructure
xxii                                                © ISO/IEC 2012 – All rights reserved.

System.Runtime.InteropServices.SafeHandle Runtime Infrastructure
System.Threading.AutoResetEvent BCL
System.Threading.EventWaitHandle BCL
System.Threading.ManualResetEvent BCL
System.WeakReference BCL
System.Runtime.CompilerServices.TypeForwardedToAttribute BCL
System.Runtime.CompilerServices.TypeForwardedFromAttribute BCL
System.Threading.EventResetMode BCL
System.Runtime.InteropServices.DllAttribute* Runtime Infrastructure
System.Math* BCL
One type, INullableValue, has been removed from the Standard. INullableValue is incompatible
with the semantics of boxing as defined in the previous edition of the Standard. The references to
it were included in error from an earlier draft and no implementations are known to have ever
included it.
Technical Report 89 (TR89), which was submitted during the third edition of this Ecma standard,
will no longer be part of the submission. TR89 specified a collection of generic types, to help
enhance inter-language interoperability, under consideration for inclusion in a future version of
the standard. That consideration has now occurred and TR89 has fulfilled its role. A selection of
the types covered in TR89 has been introduced into this edition of the standard. An
archive version of TR89 will continue to be available from Ecma.

The following companies and organizations have participated in the development of this
standard, and their contributions are gratefully acknowledged: Eiffel Software, Kahu Research,
Microsoft Corporation, Novell Corporation, Twin Roots. For previous editions, the following
companies and organizations are also acknowledged: Borland, Fujitsu Software Corporation,
Hewlett-Packard, Intel Corporation, IBM Corporation, IT University of Copenhagen, Jagger
Software Ltd., Monash University, Netscape, Phone.Com, Plum Hall, and Sun Microsystems.
© ISO/IEC 2012 – All rights reserved.                                                xxiii

I.1 Scope
This International Standard defines the Common Language Infrastructure (CLI) in which
applications written in multiple high-level languages can be executed in different system
environments without the need to rewrite those applications to take into consideration the unique
characteristics of those environments. This International Standard consists of the following parts:
 Partition I: Concepts and Architecture – Describes the overall architecture of the CLI, and
provides the normative description of the Common Type System (CTS), the Virtual
Execution System (VES), and the Common Language Specification (CLS). It also
provides an informative description of the metadata.
 Partition II: Metadata Definition and Semantics – Provides the normative description of
the metadata: its physical layout (as a file format), its logical contents (as a set of tables
and their relationships), and its semantics (as seen from a hypothetical assembler, ilasm).
 Partition III: CIL Instruction Set – Describes the Common Intermediate Language (CIL)
instruction set.
 Partition IV: Profiles and Libraries – Provides an overview of the CLI Libraries, and a
specification of their factoring into Profiles and Libraries. A companion file,
CLILibrary.xml, considered to be part of this Partition, but distributed in XML format,
provides details of each class, value type, and interface in the CLI Libraries.
 Partition V: Debug Interchange Format – Describes a standard way to interchange
debugging information between CLI producers and consumers.
 Partition VI: Annexes – Contains some sample programs written in CIL Assembly
Language (ILAsm), information about a particular implementation of an assembler, a
machine-readable description of the CIL instruction set which can be used to derive parts
of the grammar used by this assembler as well as other tools that manipulate CIL, a set of
guidelines used in the design of the libraries of Partition IV, and portability
considerations.
© ISO/IEC 2012 – All rights reserved.                                                1

I.2 Conformance
A system claiming conformance to this International Standard shall implement all the normative
requirements of this standard, and shall specify the profile (see Partition IV Library – Profiles)
that it implements. The minimal implementation is the Kernel Profile. A conforming
implementation can also include additional functionality provided that functionality does not
prevent running code written to rely solely on the profile as specified in this standard. For
example, a conforming implementation can provide additional classes, new methods on existing
classes, or a new interface on a standardized class, but it shall not add methods or properties to
interfaces specified in this standard.
A compiler that generates Common Intermediate Language (CIL, see Partition III) and claims
conformance to this International Standard shall produce output files in the format specified in
this standard, and the CIL it generates shall be correct CIL as specified in this standard. Such a
compiler can also claim that it generates verifiable code, in which case, the CIL it generates shall
be verifiable as specified in this standard.
2                                                © ISO/IEC 2012 – All rights reserved.

I.3 Normative references
[Note that many of these references are cited in the XML description of the class libraries.]

Extensible Markup Language (XML) 1.0 (Third Edition), 2004 February 4,
http://www.w3.org/TR/2004/REC-xml-20040204/
Federal Information Processing Standard (FIPS 180-1), Secure Hash Standard (SHA-1), 1995,
April.
IEC 60559:1989, Binary Floating-point Arithmetic for Microprocessor Systems (previously
designated IEC 559:1989).
ISO 639, Codes for the representation of names of languages.
ISO 3166-1:2006, Codes for the representation of names of countries and their subdivisions —
Part 1: Country codeV
ISO/IEC 646:1991, Information technology — ISO 7-bit coded character set for information
interchange
ISO/IEC 9899:1999, Programming languages — C.
ISO/IEC 10646, Information technology — Universal Coded Character Set (UCS).
ISO/IEC 11578:1996, Information technology — Open Systems Interconnection - Remote
Procedure Call (RPC).
ISO/IEC 14882:2011, �nformation te�hnology — Programming languages — C++.
ISO/IEC 23270:2006, Information technology ²Programming languages — C#.
RFC-768, User Datagram Protocol. J. Postel. 1980, August.
RFC-791, Darpa Internet Program Protocol Specification. 1981, September.
RFC-792, Internet Control Message Protocol. Network Working Group. J. Postel. 1981,
September.
RFC-793, Transmission Control Protocol. J. Postel. 1981, September.
RFC-919, Broadcasting Internet Datagrams. Network Working Group. J. Mogul. 1984, October.
RFC-922, Broadcasting Internet Datagrams in the presence of Subnets. Network Working
Group. J. Mogul. 1984, October.
RFC-1035, Domain Names - Implementation and Specification. Network Working Group. P.
Mockapetris. 1987, November.
RFC-1036, Standard for Interchange of USENET Messages, Network Working Group. M.
Horton and R. Adams. 1987, December.
RFC-1112. Host Extensions for IP Multicasting. Network Working Group. S. Deering 1989,
August.
RFC-1222. Advancing the NSFNET Routing Architecture. Network Working Group. H-W
Braun, Y. Rekhter. 1991 May. http://tools.ietf.org/html/rfc1222
RFC-1510, The Kerberos Network Authentication Service (V5). Network Working Group. J.
Kohl and C. Neuman. 1993, September.
RFC-1741, MIME Content Type for BinHex Encoded Files: Format. Network Working Group.
P. Faltstrom, D. Crocker, and E. Fair. 1994, December.
RFC-1764. The PPP XNS IDP Control Protocol (XNSCP). Network Working Group. S. Senum.
1995, March.
RFC-1766, Tags for the Identification of Languages. Network Working Group. H. Alvestrand.
1995, March.
RFC-1792. TCP/IPX Connection Mib Specification. Network Working Group. T. Sung. 1995,
April.
© ISO/IEC 2012 – All rights reserved.                                                3

RFC-2236. Internet Group Management Protocol, Version 2. Network Working Group. W.
Fenner. 1997, November.
RFC-2045, Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet
Message Bodies. Network Working Group. N. Freed. 1996, November.
RFC-2616, Hypertext Transfer Protocol -- HTTP/1.1. Network Working Group. R. Fielding, J.
Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, and T. Berners-Lee. 1999 June.
http://www.ietf.org/rfc/rfc2616.txt
RFC-2617, HTTP Authentication: Basic and Digest Access Authentication. Network Working
Group. J. Franks, P. Hallam-Baker, J. Hostetler, S. Lawrence, P. Leach, A. Luotonen, and L.
Stewart. 1999 June, http://www.ietf.org/rfc/rfc2617.txt
The Unicode Consortium. The Unicode Standard, Version 4.0, defined by: The Unicode
Standard, Version 4.0 (Boston, MA, Addison-Wesley, 2003. ISBN 0-321-18578-1).
4                                                © ISO/IEC 2012 – All rights reserved.

I.4 Conventions
I.4.1 Organization
The divisions of this International Standard are organized using a hierarchy. At the top level is
the Partition. The next level is the clause, followed by subclause. Divisions within a subclause
are also referred to as subclauses. Partitions are numbered using Roman numerals. All other
divisions are numbered using Arabic digits with their place in the hierarchy indicated by nested
numbers. For example, Partition II, 14.4.3.2 refers to subclause 2 in subclause 3 in subclause 4 in
clause 14 in Partition II.
I.4.2 Informative text
This Inte
...