From 711428bd66ed136c256f06e8de3e438d487a405a 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 a286e6936b0273867c748db21ca4e0c54de70fc5 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 8ab493677..25157fd9e 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -494,7 +494,7 @@ 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. @@ -845,6 +845,76 @@ For invocations that occur within declarations of instance constructors, static For an invocation that occurs within a local function, the name of the method that calls that local function is used. Consider the following: if method `M` calls local function `F1`, which in turn calls local function `F2`, and `F2` has a parameter marked with this attribute, the method name passed to `F2` is `M`, because a local function is *not* a function member! +#### §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 3ac1f9de0fa4676aa7131b8a6ad281b2762e4d39 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 1756dc02523963b901d534ac24d0bfe496a9e24a 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 25157fd9e..067f1779a 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -900,7 +900,7 @@ 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 @@ -1262,5 +1262,3 @@ For interoperation with other languages, an indexer may be implemented using ind > Now, the indexer’s name is `TheItem`. > > *end example* - - From 740a3b3aa88ccdecd294c6946ab590d3fe477e4c 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 067f1779a..e02e9a7e0 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -849,7 +849,7 @@ For an invocation that occurs within a local function, the name of the method th 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: @@ -882,7 +882,8 @@ 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. @@ -896,6 +897,7 @@ 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); @@ -909,6 +911,7 @@ 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 943c55e77a57ce40eb5f90c1bd8cb01902aa6fcf 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 e02e9a7e0..13a520caa 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -911,7 +911,7 @@ 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 712c6661b08e3ea6d166723dfaa6a178d00fbc03 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 13a520caa..307981ee7 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -897,7 +897,7 @@ 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); @@ -911,7 +911,7 @@ 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 f3c17e46f5eda5726f22e1e10a53a7105d45fd3e 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 307981ee7..9c5769df9 100644 --- a/standard/attributes.md +++ b/standard/attributes.md @@ -797,13 +797,13 @@ 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 @@ -847,9 +847,9 @@ For an invocation that occurs within a local function, the name of the method th #### §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`