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Fix: Address missing sizeHint in BuffersExtensions write logic #126776

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Fix: Address missing sizeHint in BuffersExtensions write logic #126776

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4 changes: 2 additions & 2 deletions src/libraries/System.Memory/src/System/Buffers/BuffersExtensions.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -115,7 +115,7 @@ public static T[] ToArray<T>(in this ReadOnlySequence<T> sequence)
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static void Write<T>(this IBufferWriter<T> writer, ReadOnlySpan<T> value)
{
Span<T> destination = writer.GetSpan();
Span<T> destination = writer.GetSpan(value.Length);

// Fast path, try copying to the available memory directly
Comment on lines 116 to 120
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Copilot AI Apr 10, 2026

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This change alters the observable behavior of Write for writers that allocate based on sizeHint (e.g., the existing TestBufferWriterMultiSegment in System.Memory tests). With GetSpan(value.Length) / GetSpan(input.Length), those tests will no longer write one byte per segment (they'll typically write the whole span in a single segment), so current assertions like Comitted.Count == 12 will fail. Please update/add tests to validate the new contract (that the sizeHint passed equals the remaining input length) rather than relying on the old sizeHint == 0 growth behavior.

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if (value.Length <= destination.Length)
Expand All @@ -140,7 +140,7 @@ private static void WriteMultiSegment<T>(IBufferWriter<T> writer, in ReadOnlySpa
input = input.Slice(writeSize);
if (input.Length > 0)
{
destination = writer.GetSpan();
destination = writer.GetSpan(input.Length);

if (destination.IsEmpty)
{
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