ISO/IEC 23270:2018

INTERNATIONAL ISO/IEC
STANDARD 23270
Third edition
2018-12
Information technology —
Programming languages — C#
Technologies de l'information — Langages de programmation — C#
Reference number
©
ISO/IEC 2018
© ISO/IEC 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
ii © ISO/IEC 2018 – All rights reserved

Table of Contents
Foreword . xix
Introduction . xxi
1. Scope . 1
2. Normative references . 3
3. Terms and definitions. 5
4. Acronyms and abbreviations . 7
5. General description . 9
6. Conformance . 11
7. Lexical structure . 13
7.1 Programs . 13
7.2 Grammars . 13
7.2.1 General . 13
7.2.2 Grammar notation . 13
7.2.3 Lexical grammar . 14
7.2.4 Syntactic grammar . 15
7.2.5 Grammar ambiguities . 15
7.3 Lexical analysis . 16
7.3.1 General . 16
7.3.2 Line terminators . 16
7.3.3 Comments . 17
7.3.4 White space . 18
7.4 Tokens . 19
7.4.1 General . 19
7.4.2 Unicode character escape sequences . 19
7.4.3 Identifiers . 20
7.4.4 Keywords . 21
7.4.5 Literals . 22
7.4.5.1 General . 22
7.4.5.2 Boolean literals . 23
7.4.5.3 Integer literals . 23
7.4.5.4 Real literals . 24
7.4.5.5 Character literals . 25
7.4.5.6 String literals . 26
7.4.5.7 The null literal . 28
7.4.6 Operators and punctuators . 28
7.5 Pre-processing directives . 28
7.5.1 General . 28
7.5.2 Conditional compilation symbols . 30
7.5.3 Pre-processing expressions . 30
7.5.4 Definition directives . 31
7.5.5 Conditional compilation directives . 32
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7.5.6 Diagnostic directives . 34
7.5.7 Region directives . 35
7.5.8 Line directives . 35
7.5.9 Pragma directives . 36
8. Basic concepts. 37
8.1 Application startup . 37
8.2 Application termination . 38
8.3 Declarations. 38
8.4 Members . 41
8.4.1 General . 41
8.4.2 Namespace members . 41
8.4.3 Struct members . 41
8.4.4 Enumeration members . 42
8.4.5 Class members . 42
8.4.6 Interface members . 42
8.4.7 Array members. 42
8.4.8 Delegate members . 42
8.5 Member access . 42
8.5.1 General . 42
8.5.2 Declared accessibility . 42
8.5.3 Accessibility domains . 43
8.5.4 Protected access . 46
8.5.5 Accessibility constraints . 47
8.6 Signatures and overloading . 48
8.7 Scopes. 49
8.7.1 General . 49
8.7.2 Name hiding . 52
8.7.2.1 General . 52
8.7.2.2 Hiding through nesting . 52
8.7.2.3 Hiding through inheritance . 53
8.8 Namespace and type names . 54
8.8.1 General . 54
8.8.2 Unqualified names . 56
8.8.3 Fully qualified names . 56
8.9 Automatic memory management . 57
8.10 Execution order . 59
9. Types . 61
9.1 General . 61
9.2 Reference types . 61
9.2.1 General . 61
9.2.2 Class types . 62
9.2.3 The object type . 62
9.2.4 The dynamic type . 63
9.2.5 The string type . 63
9.2.6 Interface types . 63
9.2.7 Array types . 63
9.2.8 Delegate types . 63
9.3 Value types . 63
9.3.1 General . 63

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9.3.2 The System.ValueType type . 64
9.3.3 Default constructors . 64
9.3.4 Struct types . 65
9.3.5 Simple types . 65
9.3.6 Integral types . 66
9.3.7 Floating-point types . 67
9.3.8 The decimal type . 68
9.3.9 The bool type . 69
9.3.10 Enumeration types . 69
9.3.11 Nullable value types . 69
9.3.12 Boxing and unboxing . 70
9.4 Constructed types . 70
9.4.1 General . 70
9.4.2 Type arguments . 71
9.4.3 Open and closed types . 71
9.4.4 Bound and unbound types . 72
9.4.5 Satisfying constraints . 72
9.5 Type parameters . 73
9.6 Expression tree types . 73
9.7 The dynamic type . 74
10. Variables . 77
10.1 General . 77
10.2 Variable categories . 77
10.2.1 General . 77
10.2.2 Static variables . 77
10.2.3 Instance variables. 77
10.2.3.1 General . 77
10.2.3.2 Instance variables in classes . 78
10.2.3.3 Instance variables in structs . 78
10.2.4 Array elements . 78
10.2.5 Value parameters . 78
10.2.6 Reference parameters . 78
10.2.7 Output parameters . 79
10.2.8 Local variables . 79
10.3 Default values . 80
10.4 Definite assignment . 80
10.4.1 General . 80
10.4.2 Initially assigned variables . 81
10.4.3 Initially unassigned variables . 81
10.4.4 Precise rules for determining definite assignment . 81
10.4.4.1 General . 81
10.4.4.2 General rules for statements . 82
10.4.4.3 Block statements, checked, and unchecked statements . 82
10.4.4.4 Expression statements . 82
10.4.4.5 Declaration statements . 83
10.4.4.6 If statements . 83
10.4.4.7 Switch statements . 83
10.4.4.8 While statements . 83
10.4.4.9 Do statements . 83

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10.4.4.10 For statements . 84
10.4.4.11 Break, continue, and goto statements . 84
10.4.4.12 Throw statements . 84
10.4.4.13 Return statements . 84
10.4.4.14 Try-catch statements . 85
10.4.4.15 Try-finally statements . 85
10.4.4.16 Try-catch-finally statements . 85
10.4.4.17 Foreach statements . 86
10.4.4.18 Using statements . 86
10.4.4.19 Lock statements . 87
10.4.4.20 Yield statements . 87
10.4.4.21 General rules for constant expressions . 87
10.4.4.22 General rules for simple expressions . 88
10.4.4.23 General rules for expressions with embedded expressions . 88
10.4.4.24 Invocation expressions and object creation expressions . 88
10.4.4.25 Simple assignment expressions . 89
10.4.4.26 && expressions . 89
10.4.4.27 || expressions . 90
10.4.4.28 ! expressions . 91
10.4.4.29 ?? expressions . 91
10.4.4.30 ?: expressions . 91
10.4.4.31 Anonymous functions . 92
10.5 Variable references . 92
10.6 Atomicity of variable references . 92
11. Conversions . 93
11.1 General . 93
11.2 Implicit conversions. 93
11.2.1 General . 93
11.2.2 Identity conversion . 94
11.2.3 Implicit numeric conversions . 94
11.2.4 Implicit enumeration conversions . 94
11.2.5 Implicit nullable conversions. 94
11.2.6 Null literal conversions . 94
11.2.7 Implicit reference conversions . 95
11.2.8 Boxing conversions. 95
11.2.9 Implicit dynamic conversions . 97
11.2.10 Implicit constant expression conversions . 97
11.2.11 Implicit conversions involving type parameters . 97
11.2.12 User-defined implicit conversions . 98
11.2.13 Anonymous function conversions and method group conversions . 98
11.3 Explicit conversions . 98
11.3.1 General . 98
11.3.2 Explicit numeric conversions . 99
11.3.3 Explicit enumeration conversions . 101
11.3.4 Explicit nullable conversions . 101
11.3.5 Explicit reference conversions . 101
11.3.6 Unboxing conversions . 102
11.3.7 Explicit dynamic conversions . 103
11.3.8 Explicit conversions involving type parameters . 103

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11.3.9 User-defined explicit conversions . 105
11.4 Standard conversions . 105
11.4.1 General . 105
11.4.2 Standard implicit conversions . 105
11.4.3 Standard explicit conversions . 105
11.5 User-defined conversions. 105
11.5.1 General . 105
11.5.2 Permitted user-defined conversions . 105
11.5.3 Evaluation of user-defined conversions . 106
11.5.4 User-defined implicit conversions . 107
11.5.5 User-defined explicit conversions . 107
11.6 Conversions involving nullable types . 109
11.6.1 Nullable Conversions. 109
11.6.2 Lifted conversions . 109
11.7 Anonymous function conversions . 109
11.7.1 General . 109
11.7.2 Evaluation of anonymous function conversions to delegate types . 111
11.7.3 Evaluation of anonymous function conversions to expression tree types . 112
11.8 Method group conversions . 112
12. Expressions . 115
12.1 General . 115
12.2 Expression classifications . 115
12.2.1 General . 115
12.2.2 Values of expressions . 116
12.3 Static and Dynamic Binding . 116
12.3.1 General . 116
12.3.2 Binding-time . 117
12.3.3 Dynamic binding . 117
12.3.4 Types of subexpressions . 118
12.4 Operators . 118
12.4.1 General . 118
12.4.2 Operator precedence and associativity . 118
12.4.3 Operator overloading . 119
12.4.4 Unary operator overload resolution . 121
12.4.5 Binary operator overload resolution. 121
12.4.6 Candidate user-defined operators . 121
12.4.7 Numeric promotions . 122
12.4.7.1 General . 122
12.4.7.2 Unary numeric promotions . 122
12.4.7.3 Binary numeric promotions . 122
12.4.8 Lifted operators . 123
12.5 Member lookup . 124
12.5.1 General . 124
12.5.2 Base types . 125
12.6 Function members . 126
12.6.1 General . 126
12.6.2 Argument lists . 128
12.6.2.1 General . 128
12.6.2.2 Corresponding parameters . 129

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12.6.2.3 Run-time evaluation of argument lists. 130
12.6.3 Type inference. 131
12.6.3.1 General . 131
12.6.3.2 The first phase . 132
12.6.3.3 The second phase . 133
12.6.3.4 Input types . 133
12.6.3.5 Output types . 133
12.6.3.6 Dependence . 133
12.6.3.7 Output type inferences . 133
12.6.3.8 Explicit parameter type inferences . 134
12.6.3.9 Exact inferences . 134
12.6.3.10 Lower-bound inferences . 134
12.6.3.11 Upper-bound inferences . 134
12.6.3.12 Fixing . 135
12.6.3.13 Inferred return type . 135
12.6.3.14 Type inference for conversion of method groups . 137
12.6.3.15 Finding the best common type of a set of expressions . 137
12.6.4 Overload resolution . 137
12.6.4.1 General . 137
12.6.4.2 Applicable function member . 138
12.6.4.3 Better function member . 139
12.6.4.4 Better conversion from expression . 140
12.6.4.5 Better conversion from type . 140
12.6.4.6 Better conversion target . 140
12.6.4.7 Overloading in generic classes . 140
12.6.5 Compile-time checking of dynamic member invocation . 141
12.6.6 Function member invocation . 142
12.6.6.1 General . 142
12.6.6.2 Invocations on boxed instances . 143
12.7 Primary expressions . 143
12.7.1 General . 143
12.7.2 Literals . 144
12.7.3 Simple names . 144
12.7.3.1 General . 144
12.7.3.2 Invariant meaning in blocks . 146
12.7.4 Parenthesized expressions . 146
12.7.5 Member access . 147
12.7.5.1 General . 147
12.7.5.2 Identical simple names and type names . 149
12.7.6 Invocation expressions . 149
12.7.6.1 General . 149
12.7.6.2 Method invocations . 150
12.7.6.3 Extension method invocations . 151
12.7.6.4 Delegate invocations . 153
12.7.7 Element access . 154
12.7.7.1 General . 154
12.7.7.2 Array access . 154
12.7.7.3 Indexer access . 155
12.7.8 This access . 156
12.7.9 Base access . 156

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12.7.10 Postfix increment and decrement operators . 157
12.7.11 The new operator . 158
12.7.11.1 General . 158
12.7.11.2 Object creation expressions . 158
12.7.11.3 Object initializers .
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