From cdad000ccfa0eba542d94b798eb0695e80ee8015 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 0760dd7eeb0255e570c7a8d5d9992ab1b7647ba3 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 304f4e8eb..58d96cfe7 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 5959bfe4fb4ec2a74c44f7e71c2570f417ab8dd1 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 4fa2d44b1..15feb8ed3 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 { @@ -1390,6 +1397,7 @@ 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 6ae600160d4e752df2a6b29c3d5a87527efb020f 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 58d96cfe7..26f5b1d21 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 e7d8643b4868591d889472072d75c99f0acddd50 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 26f5b1d21..1a7efc908 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 169d6f8b37fdf6f7819df2a24b4207e4a56a50fe 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 1a7efc908..4caec84e8 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 3ad941027c1a90a2e4206d255513a17daf37034b 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 4caec84e8..61feabf0f 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 d2bdab1289332745217a949fa07e5bb06fc5a7fa 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 61feabf0f..de5c00967 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 15feb8ed3..901e3a204 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)] @@ -1396,8 +1396,8 @@ 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`