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• Issue: ParseUsingDeclaration builds two content strings by chaining .Skip(), .Where(), .Select() LINQ extensions over a SyntaxToken[] and passing the results to string.Concat(IEnumerable<string>). Each string.Concat call internally allocates a StringBuilder, iterates through multiple LINQ enumerator objects (one per .Skip(), .Where(), .Select()), and calls StringBuilder.ToString() to allocate the final string. This creates 4+ enumerator objects and 2 intermediate StringBuilder instances per @using directive parse — a hot path triggered on every keystroke.

  This matches the allocation stack flow showing ParseMarkupNodes → ParseCodeTransition → OtherParserBlock → ParseBlock → TryParseKeyword → ParseUsingKeyword → ParseUsingDeclaration → String.Concat → StringBuilder.ToString → FramedAllocateString → TypeAllocated!System.String

microsoft.codeanalysis.razor.compiler.dll!Microsoft.AspNetCore.Razor.Language.Legacy.CSharpCodeParser.ParseUsingDeclaration
mscorlib.dll!System.String.Concat
mscorlib.dll!System.Text.StringBuilder.ToString
clr.dll!FramedAllocateString
clr.dll!SVR::GCHeap::Alloc
clr.dll!SVR::gc_heap::try_allocate_more_space
TypeAllocated!System.String
...
 TypeAllocated!SZGenericArrayEnumerator`1[Microsoft.AspNetCore.Razor.Language.Syntax.InternalSyntax.SyntaxToken]
...
TypeAllocated!WhereEnumerableIterator`1[Microsoft.AspNetCore.Razor.Language.Syntax.InternalSyntax.SyntaxToken]

• Issue type: Eliminate unnecessary LINQ enumerator and string.Concat allocations in hot path

• Proposed fix: Replace the two LINQ chains (.Skip().Where().Select()) + string.Concat(IEnumerable<string>) calls with a single for loop over the already-snapshotted SyntaxToken[] array, appending to two plain StringBuilder instances. The TokenBuilder.ToList().Nodes snapshot is kept at its original position (before TryAccept mutates TokenBuilder) to preserve exact functional parity. This eliminates 4+ LINQ enumerator allocations and avoids string.Concat's internal StringBuilder allocation, while producing identical output strings.

Best practices wiki
See related failure in PRISM
ADO work item