From 613c188fc9a0217c99b1e9f07bc2501a6f14c40b Mon Sep 17 00:00:00 2001 From: Rex Jaeschke Date: Mon, 19 Jan 2026 13:09:46 -0500 Subject: [PATCH 1/8] Add files via upload --- .../additional-files/CallerArgumentAttrM.cs | 10 ++++++++++ 1 file changed, 10 insertions(+) create mode 100644 tools/example-templates/additional-files/CallerArgumentAttrM.cs diff --git a/tools/example-templates/additional-files/CallerArgumentAttrM.cs b/tools/example-templates/additional-files/CallerArgumentAttrM.cs new file mode 100644 index 000000000..4184b801d --- /dev/null +++ b/tools/example-templates/additional-files/CallerArgumentAttrM.cs @@ -0,0 +1,10 @@ +using System; +using System.Runtime.CompilerServices; +#nullable enable +class Test +{ + public static void M(int val = 0, [CallerArgumentExpression("val")] string? text = null) + { + Console.WriteLine($"val = {val}, text = <{text}>"); + } +} From 12333dc06701c54b08ce1ece24d8a8d8992fe0da Mon Sep 17 00:00:00 2001 From: Rex Jaeschke Date: Mon, 19 Jan 2026 13:22:12 -0500 Subject: [PATCH 2/8] add the new attribute --- standard/attributes.md | 72 +++++++++++++++++++++++++++++++++++++++++- 1 file changed, 71 insertions(+), 1 deletion(-) diff --git a/standard/attributes.md b/standard/attributes.md index fdae044fd..8ce17429e 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -497,7 +497,7 @@ A number of attributes affect the language in some way. These attributes include - `System.Diagnostics.ConditionalAttribute` ([§23.5.3](attributes.md#2353-the-conditional-attribute)), is a multi-use attribute class which is used to define conditional methods and conditional attribute classes. This attribute indicates a condition by testing a conditional compilation symbol. - `System.ObsoleteAttribute` ([§23.5.4](attributes.md#2354-the-obsolete-attribute)), which is used to mark a member as obsolete. - `System.Runtime.CompilerServices.AsyncMethodBuilderAttribute` ([§23.5.5](attributes.md#2355-the-asyncmethodbuilder-attribute)), which is used to establish a task builder for an async method. -- `System.Runtime.CompilerServices.CallerLineNumberAttribute` ([§23.5.6.2](attributes.md#23562-the-callerlinenumber-attribute)), `System.Runtime.CompilerServices.CallerFilePathAttribute` ([§23.5.6.3](attributes.md#23563-the-callerfilepath-attribute)), and `System.Runtime.CompilerServices.CallerMemberNameAttribute` ([§23.5.6.4](attributes.md#23564-the-callermembername-attribute)), which are used to supply information about the calling context to optional parameters. +- `System.Runtime.CompilerServices.CallerLineNumberAttribute` ([§23.5.6.2](attributes.md#23562-the-callerlinenumber-attribute)), `System.Runtime.CompilerServices.CallerFilePathAttribute` ([§23.5.6.3](attributes.md#23563-the-callerfilepath-attribute)), `System.Runtime.CompilerServices.CallerMemberNameAttribute` ([§23.5.6.4](attributes.md#23564-the-callermembername-attribute)), and `System.Runtime.CompilerServices.CallerArgumentExpressionAttribute` (§callargexpattr), which are used to supply information about the calling context to optional parameters. - `System.Runtime.CompilerServices.EnumeratorCancellationAttribute` ([§23.5.8](attributes.md#2358-the-enumeratorcancellation-attribute)), which is used to specify parameter for the cancellation token in an asynchronous iterator. - `System.Runtime.CompilerServices.ModuleInitializer` ([§23.5.9](attributes.md#2359-the-moduleinitializer-attribute)), which is used to mark a method as a module initializer. @@ -881,6 +881,76 @@ For an invocation that occurs within a local function or an anonymous function, > > This attribute supplies the name of the calling function member, which for local function `F1` is the method `Main`. And even though `F2` is called by `F1`, a local function is *not* a function member, so the reported caller of `F2` is also `Main`. *end example* +#### §callargexpattr The CallerArgumentExpression attribute + +The attribute `System.Runtime.CompilerServices.CallerArgumentExpression` is applied to a *target parameter*, and can result in the capture of the source-code text of a sibling parameter’s argument as a string, referred to here as the *captured string*. + +The target parameter shall have a *default_argument*. + +Consider the following method declaration: + + +```csharp +using System; +using System.Runtime.CompilerServices; +#nullable enable +class Test +{ + public static void M(int val = 0, [CallerArgumentExpression("val")] string? text = null) + { + Console.WriteLine($"val = {val}, text = <{text}>"); + } +} +``` + +in which the target parameter is `text` and the sibling parameter is `val`, whose corresponding argument’s source-code text can be captured in `text` when `M` is called. + +The attribute constructor takes an argument of type `string`. That string + +- Shall contain the name of a sibling parameter; otherwise, the attribute is ignored. +- Shall omit the leading `@` from a parameter name having that prefix. + +A *parameter_list* may contain multiple target parameters. + +The type of the target parameter shall have a standard conversion from `string`. + +> *Note:* This means no user-defined conversions from `string` are allowed, and in practice means the type of such a parameter must be `string`, `object`, or an interface implemented by `string`. *end note* + +If an explicit argument is passed for the target parameter, no string is captured, and that parameter takes on that argument’s value. Otherwise, the text for the argument corresponding to the sibling parameter is converted to a captured string, according to the following rules: + +- Leading and trailing white space is removed. +- All other *input_element*s are retained verbatim (including white space, comments, *Unicode_Escape_Sequence*s, and `@` prefixes on identifiers). + +The captured string is then passed as the argument corresponding to the target parameter. However, if the argument for the sibling parameter is omitted, the target parameter takes on its *default_argument* value. + +> *Example*: Given the declaration of `M` above, consider the following calls to `M`: +> +> +> ```csharp +> Test.M(); +> Test.M(123); +> Test.M(123, null); +> Test.M(123, "xyz"); +> Test.M( 1 + 2 ); +> int local = 10; +> Test.M(l\u006fcal /*...*/ + // xxx +> 5); +> ``` +> +> the output produced is +> +> ```console +> val = 0, text = <> +> val = 123, text = <123> +> val = 123, text = <> +> val = 123, text = +> val = 3, text = <1 + 2> +> val = 15, text = 5> +> ``` +> +> *end example* + ### 23.5.7 Code analysis attributes #### 23.5.7.1 General From f3a0960c02cc06a40b3821d4aa9c196bf328c2db Mon Sep 17 00:00:00 2001 From: Rex Jaeschke Date: Mon, 19 Jan 2026 13:29:05 -0500 Subject: [PATCH 3/8] Add new attribute type --- standard/standard-library.md | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/standard/standard-library.md b/standard/standard-library.md index d2ee3622f..525cce666 100644 --- a/standard/standard-library.md +++ b/standard/standard-library.md @@ -789,6 +789,13 @@ namespace System.Runtime.CompilerServices public Type BuilderType { get; } } + [System.AttributeUsage(System.AttributeTargets.Parameter, AllowMultiple=false, + Inherited=false)] + public sealed class CallerArgumentExpressionAttribute : Attribute + { + public CallerArgumentExpressionAttribute (string parameterName); + } + [AttributeUsage(AttributeTargets.Parameter, Inherited = false)] public sealed class CallerFilePathAttribute : Attribute { @@ -1426,6 +1433,7 @@ The following library types are referenced in this specification. The full names - `global::System.Reflection.MemberInfo` - `global::System.Runtime.CompilerServices.AsyncMethodBuilderAttribute` - `global::System.Runtime.CompilerServices.CallerFilePathAttribute` +- `global::System.Runtime.CompilerServices.CallerArgumentExpressionAttribute` - `global::System.Runtime.CompilerServices.CallerLineNumberAttribute` - `global::System.Runtime.CompilerServices.CallerMemberNameAttribute` - `global::System.Runtime.CompilerServices.FormattableStringFactory` From 0887c6fcc44ac7a2b076bea62420fe6e0ff15505 Mon Sep 17 00:00:00 2001 From: Rex Jaeschke Date: Mon, 19 Jan 2026 13:45:10 -0500 Subject: [PATCH 4/8] fix md spacing --- standard/attributes.md | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/standard/attributes.md b/standard/attributes.md index 8ce17429e..4cfacd2c2 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -936,7 +936,7 @@ The captured string is then passed as the argument corresponding to the target p > Test.M(l\u006fcal /*...*/ + // xxx > 5); > ``` -> +> > the output produced is > > ```console @@ -1298,5 +1298,3 @@ For interoperation with other languages, an indexer may be implemented using ind > Now, the indexer’s name is `TheItem`. > > *end example* - - From acf68e4b3b1d224dceaada1963426a20dc075d51 Mon Sep 17 00:00:00 2001 From: Rex Jaeschke Date: Mon, 19 Jan 2026 15:52:29 -0500 Subject: [PATCH 5/8] mention extension methods, and grouping parens --- standard/attributes.md | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/standard/attributes.md b/standard/attributes.md index 4cfacd2c2..7f19d59ba 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -885,7 +885,7 @@ For an invocation that occurs within a local function or an anonymous function, The attribute `System.Runtime.CompilerServices.CallerArgumentExpression` is applied to a *target parameter*, and can result in the capture of the source-code text of a sibling parameter’s argument as a string, referred to here as the *captured string*. -The target parameter shall have a *default_argument*. +Except when it is the first parameter in an extension method, the target parameter shall have a *default_argument*. Consider the following method declaration: @@ -918,7 +918,8 @@ The type of the target parameter shall have a standard conversion from `string`. If an explicit argument is passed for the target parameter, no string is captured, and that parameter takes on that argument’s value. Otherwise, the text for the argument corresponding to the sibling parameter is converted to a captured string, according to the following rules: -- Leading and trailing white space is removed. +- Leading and trailing white space is removed both before and after any outermost grouping parentheses are removed. +- All outermost grouping parentheses are removed both before and after any leading and trailing white space is removed. - All other *input_element*s are retained verbatim (including white space, comments, *Unicode_Escape_Sequence*s, and `@` prefixes on identifiers). The captured string is then passed as the argument corresponding to the target parameter. However, if the argument for the sibling parameter is omitted, the target parameter takes on its *default_argument* value. @@ -932,6 +933,7 @@ The captured string is then passed as the argument corresponding to the target p > Test.M(123, null); > Test.M(123, "xyz"); > Test.M( 1 + 2 ); +> Test.M( ( ((123) + 0) ) ); > int local = 10; > Test.M(l\u006fcal /*...*/ + // xxx > 5); @@ -945,6 +947,7 @@ The captured string is then passed as the argument corresponding to the target p > val = 123, text = <> > val = 123, text = > val = 3, text = <1 + 2> +> val = 123, text = <(123 + 0)> > val = 15, text = 5> > ``` From 3384bbff7b7cfe13ebd26146bfc11a69c6273f9a Mon Sep 17 00:00:00 2001 From: Rex Jaeschke Date: Mon, 19 Jan 2026 15:59:53 -0500 Subject: [PATCH 6/8] correct expected console output --- standard/attributes.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/standard/attributes.md b/standard/attributes.md index 7f19d59ba..61d0621d9 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -947,7 +947,7 @@ The captured string is then passed as the argument corresponding to the target p > val = 123, text = <> > val = 123, text = > val = 3, text = <1 + 2> -> val = 123, text = <(123 + 0)> +> val = 123, text = <((123) + 0)> > val = 15, text = 5> > ``` From 26cbd8645540dc119d1cd3e03cba3f4ab429556d Mon Sep 17 00:00:00 2001 From: Rex Jaeschke Date: Mon, 19 Jan 2026 16:07:01 -0500 Subject: [PATCH 7/8] fix more console output --- standard/attributes.md | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/standard/attributes.md b/standard/attributes.md index 61d0621d9..f4867f4be 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -933,7 +933,7 @@ The captured string is then passed as the argument corresponding to the target p > Test.M(123, null); > Test.M(123, "xyz"); > Test.M( 1 + 2 ); -> Test.M( ( ((123) + 0) ) ); +> Test.M(( ( (123) + 0) ) ); > int local = 10; > Test.M(l\u006fcal /*...*/ + // xxx > 5); @@ -947,7 +947,7 @@ The captured string is then passed as the argument corresponding to the target p > val = 123, text = <> > val = 123, text = > val = 3, text = <1 + 2> -> val = 123, text = <((123) + 0)> +> val = 123, text = <(123) + 0> > val = 15, text = 5> > ``` From aecff935952335d025e6b4e060f00b0ec087d39c Mon Sep 17 00:00:00 2001 From: Bill Wagner Date: Wed, 18 Mar 2026 21:14:51 -0400 Subject: [PATCH 8/8] Initial review Review the changes for the feature and make a few small updates. --- standard/attributes.md | 8 ++++---- standard/standard-library.md | 8 ++++---- 2 files changed, 8 insertions(+), 8 deletions(-) diff --git a/standard/attributes.md b/standard/attributes.md index f4867f4be..79eb26cba 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -800,13 +800,13 @@ Caller information is only substituted when a function is explicitly invoked in One exception is query expressions. These are considered syntactic expansions, and if the calls they expand to omit optional parameters with caller-info attributes, caller information will be substituted. The location used is the location of the query clause which the call was generated from. -If more than one caller-info attribute is specified on a given parameter, they are recognized in the following order: `CallerLineNumber`, `CallerFilePath`, `CallerMemberName`. Consider the following parameter declaration: +If more than one caller-info attribute is specified on a given parameter, they are recognized in the following order: `CallerLineNumber`, `CallerFilePath`, `CallerMemberName`, `CallerArgumentExpression`. Consider the following parameter declaration: ```csharp [CallerMemberName, CallerFilePath, CallerLineNumber] object p = ... ``` -`CallerLineNumber` takes precedence, and the other two attributes are ignored. If `CallerLineNumber` were omitted, `CallerFilePath` would take precedence, and `CallerMemberName` would be ignored. The lexical ordering of these attributes is irrelevant. +`CallerLineNumber` takes precedence, and the other three attributes are ignored. If `CallerLineNumber` were omitted, `CallerFilePath` would take precedence, and `CallerMemberName` and `CallerArgumentExpression` would be ignored. The lexical ordering of these attributes is irrelevant. #### 23.5.6.2 The CallerLineNumber attribute @@ -883,9 +883,9 @@ For an invocation that occurs within a local function or an anonymous function, #### §callargexpattr The CallerArgumentExpression attribute -The attribute `System.Runtime.CompilerServices.CallerArgumentExpression` is applied to a *target parameter*, and can result in the capture of the source-code text of a sibling parameter’s argument as a string, referred to here as the *captured string*. +The attribute `System.Runtime.CompilerServices.CallerArgumentExpressionAttribute` is applied to a *target parameter*, and can result in the capture of the source-code text of a sibling parameter’s argument as a string, referred to here as the *captured string*. -Except when it is the first parameter in an extension method, the target parameter shall have a *default_argument*. +Except when it is the first parameter in an extension method, the target parameter shall have a *default_argument*. When applied to the first parameter of an extension method, the captured string is the source text of the receiver expression in an extension method invocation. If the method is invoked using static method syntax, the captured string is the argument corresponding to the first parameter. Consider the following method declaration: diff --git a/standard/standard-library.md b/standard/standard-library.md index 525cce666..28282119b 100644 --- a/standard/standard-library.md +++ b/standard/standard-library.md @@ -789,11 +789,11 @@ namespace System.Runtime.CompilerServices public Type BuilderType { get; } } - [System.AttributeUsage(System.AttributeTargets.Parameter, AllowMultiple=false, - Inherited=false)] + [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, + Inherited = false)] public sealed class CallerArgumentExpressionAttribute : Attribute { - public CallerArgumentExpressionAttribute (string parameterName); + public CallerArgumentExpressionAttribute(string parameterName); } [AttributeUsage(AttributeTargets.Parameter, Inherited = false)] @@ -1432,8 +1432,8 @@ The following library types are referenced in this specification. The full names - `global::System.Linq.Expressions.Expression` - `global::System.Reflection.MemberInfo` - `global::System.Runtime.CompilerServices.AsyncMethodBuilderAttribute` -- `global::System.Runtime.CompilerServices.CallerFilePathAttribute` - `global::System.Runtime.CompilerServices.CallerArgumentExpressionAttribute` +- `global::System.Runtime.CompilerServices.CallerFilePathAttribute` - `global::System.Runtime.CompilerServices.CallerLineNumberAttribute` - `global::System.Runtime.CompilerServices.CallerMemberNameAttribute` - `global::System.Runtime.CompilerServices.FormattableStringFactory`