Fix TagList.CopyTo to not throw when TagList is empty

[Copilot]

Description

TagList.CopyTo(array, arrayIndex) throws ArgumentOutOfRangeException when the TagList is empty and an empty array is passed. This is inconsistent with ActivityTagsCollection.CopyTo and standard collection semantics.

let emptyArray: KeyValuePair<string,obj> array = [||]
TagList().CopyTo(emptyArray, 0)  // Throws: arrayIndex ('0') must be less than '0'
TagList() |> Seq.toArray          // Also throws (uses CopyTo internally)

Fix: Allow arrayIndex == array.Length when the TagList is empty (since nothing will be copied), while still validating that arrayIndex is within valid bounds (non-negative and not greater than array length) regardless of whether the TagList is empty.

Also fixed a typo in the XML documentation ("greater that" → "greater than") in both TagList.netcore.cs and TagList.netfx.cs.

Customer Impact

Users cannot safely call CopyTo or use LINQ's ToArray() on empty TagList instances without catching exceptions.

Regression

No.

Testing

• Added TestEmptyTagListCopyTo test covering empty TagList with CopyTo(array, index), CopyTo(span), and ToArray() for both empty and non-empty destination arrays
• Added tests verifying that invalid arrayIndex values (negative or exceeding array length) still throw ArgumentOutOfRangeException even when the TagList is empty
• Extended TestCopyTo to exercise ICollection<T>.CopyTo for all sizes including 0
• All tests in System.Diagnostics.DiagnosticSource.Tests pass

Risk

Low. The change only affects the empty case and aligns behavior with existing collection types. The fix is applied consistently to both TagList.netcore.cs and TagList.netfx.cs.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Diagnostics.TagList does not support IEnumerable or CopyTo when Empty</issue_title>
<issue_description>### Description

TagList throws exception when empty and consuming as in IEnumerable or when coping to an array. This is not the case with ActivityTagsCollection.

(Example in dotnet fsi using F#):

open System.Collections.Generic;;
open System.Diagnostics;;

let myArray: KeyValuePair<string,obj> array = [||]

// ActivityTagsCollection.CopyTo works as expected when empty
ActivityTagsCollection().CopyTo(myArray, 0);;

// TagList.CopyTo throws exception when empty
System.Diagnostics.TagList().CopyTo(myArray, 0);;
// System.ArgumentOutOfRangeException: arrayIndex ('0') must be less than '0'. (Parameter 'arrayIndex')
// Actual value was 0.
//    at System.ArgumentOutOfRangeException.ThrowGreaterEqual[T](T value, T other, String paramName)
//    at System.ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual[T](T value, T other, String paramName)
//    at System.Diagnostics.TagList.CopyTo(KeyValuePair`2[] array, Int32 arrayIndex)
// Stopped due to error


// Converting an empty ActivityTagsCollection IEnumerable to an array works
System.Diagnostics.ActivityTagsCollection() |> Seq.toArray;;
// KeyValuePair<string,obj> array = [||]

// Converting an empty TagList (as an IEnumerable) to an array throws exception because
// it uses CopyTo internally
TagList() |> Seq.toArray;;
// System.ArgumentOutOfRangeException: arrayIndex ('0') must be less than '0'. (Parameter 'arrayIndex')
// Actual value was 0.
//    at System.ArgumentOutOfRangeException.ThrowGreaterEqual[T](T value, T other, String paramName)
//    at System.ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual[T](T value, T other, String paramName)
//    at System.Diagnostics.TagList.CopyTo(KeyValuePair`2[] array, Int32 arrayIndex)
// Stopped due to error

Reproduction Steps

let arr: KeyValuePair<string,obj> array = [||]
TagList().CopyTo(arr, 0)

Expected behavior

Should do the same as the other CopyTo method. This does not throw an exception:

let arr: KeyValuePair<string,obj> array = [||]
TagList().CopyTo(arr)

Actual behavior

System.ArgumentOutOfRangeException: arrayIndex ('0') must be less than '0'. (Parameter 'arrayIndex')
Actual value was 0.
   at System.ArgumentOutOfRangeException.ThrowGreaterEqual[T](T value, T other, String paramName)
   at System.ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual[T](T value, T other, String paramName)
   at System.Diagnostics.TagList.CopyTo(KeyValuePair`2[] array, Int32 arrayIndex)

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

Line where exception is thrown:

runtime/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/TagList.netcore.cs

Line 161 in 9d5a6a9

ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual((uint)arrayIndex, (uint)array.Length, nameof(arrayIndex));

</issue_description>

<agent_instructions>If the source is empty, the destination index doesn't matter because nothing will be copied.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Diagnostics.TagList does not support IEnumerable or CopyTo when Empty #122848

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[Copilot]

Pull request overview

This PR fixes a bug where TagList.CopyTo(array, arrayIndex) incorrectly throws an ArgumentOutOfRangeException when called on an empty TagList with an empty array, making it inconsistent with standard collection semantics and with ActivityTagsCollection.CopyTo.

Key Changes:

• Modified CopyTo(KeyValuePair<string, object?>[] array, int arrayIndex) to skip bounds checking when _tagsCount == 0
• Added comprehensive test coverage for empty TagList operations including CopyTo variants and ToArray()
• Extended existing TestCopyTo to exercise ICollection<T>.CopyTo interface method for all sizes

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/TagList.netcore.cs	Wraps the arrayIndex bounds check in a conditional that only executes when the TagList is non-empty
src/libraries/System.Diagnostics.DiagnosticSource/tests/TagListTests.cs	Adds TestEmptyTagListCopyTo test for empty TagList scenarios and enhances TestCopyTo to test ICollection<T>.CopyTo

[stephentoub]

@copilot, please address the review feedback

[Copilot]

@copilot, please address the review feedback

Addressed in ece9db1:

• Added ThrowIfGreaterThan check before the empty check to validate arrayIndex is within bounds even when TagList is empty
• Added tests for empty TagList with invalid arrayIndex (negative and exceeding array length)
• Fixed the typo in the XML documentation

[tarekgh]

@copilot need to apply the change to the file TagList.netfx.cs too.

[Copilot]

@copilot need to apply the change to the file TagList.netfx.cs too.

Applied the same fix to TagList.netfx.cs in 97b36bd.

[stephentoub]

/ba-g dead letter