[XmlSerializer] Avoid IList/Array.GetValue path for array item serialization in reflection writer

[Copilot]

XmlSerializer’s reflection path was serializing arrays through IList, which can devolve to Array.GetValue per element and add avoidable overhead in tight loops. This change introduces an explicit array path in ReflectionXmlSerializationWriter.WriteArrayItems to keep array iteration on the direct array/enumerator path.

• Problem

  • WriteArrayItems treated arrays as IList, so element access used arr[i] and could hit slower Array.GetValue behavior for primary-type arrays.

• Change

  • Added an early if (o is Array array) branch in ReflectionXmlSerializationWriter.WriteArrayItems.
  • Iterates array items directly and forwards each item to WriteElements.
  • Hoisted choiceSources cast to a single Array? choiceArray and reused it across array/list/enumerable paths.

• Behavioral scope

  • Serialization output semantics are unchanged.
  • Change is localized to iteration strategy in the reflection serializer array-item loop.

Array? choiceArray = choiceSources as Array;

if (o is Array array)
{
    IEnumerator e = array.GetEnumerator();
    int c = 0;
    while (e.MoveNext())
    {
        object ai = e.Current;
        object? choiceSource = choiceArray?.GetValue(c++);
        WriteElements(ai, choiceSource, elements, text, choice, true, true);
    }

    return;
}

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• bla
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing (dns block)
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing urity.Cryptograp-DURTBLDENV_FRIENDLY=Debug c.c.o.d -o CMake/usr/bin/sh _ALIGNED SSUME_ALIGNED TARGET_UNIX -DUR-D_TIME_BITS=64 (dns block)
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing k/_temp/ghcca-node/node/bin/bash -DFALLBACK_OS_Igit d/lib -DDEBUG Native_EXPORTS -DTARGET_64BIT -D/home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-U0 (dns block)
• foo
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing (dns block)
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing urity.Cryptograp-DURTBLDENV_FRIENDLY=Debug c.c.o.d -o CMake/usr/bin/sh _ALIGNED SSUME_ALIGNED TARGET_UNIX -DUR-D_TIME_BITS=64 (dns block)
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing k/_temp/ghcca-node/node/bin/bash -DFALLBACK_OS_Igit d/lib -DDEBUG Native_EXPORTS -DTARGET_64BIT -D/home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-U0 (dns block)
• notfound.invalid.corp.microsoft.com
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing (dns block)
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing urity.Cryptograp-DURTBLDENV_FRIENDLY=Debug c.c.o.d -o CMake/usr/bin/sh _ALIGNED SSUME_ALIGNED TARGET_UNIX -DUR-D_TIME_BITS=64 (dns block)
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing k/_temp/ghcca-node/node/bin/bash -DFALLBACK_OS_Igit d/lib -DDEBUG Native_EXPORTS -DTARGET_64BIT -D/home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-U0 (dns block)
• test.test
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing (dns block)
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing urity.Cryptograp-DURTBLDENV_FRIENDLY=Debug c.c.o.d -o CMake/usr/bin/sh _ALIGNED SSUME_ALIGNED TARGET_UNIX -DUR-D_TIME_BITS=64 (dns block)
  • Triggering command: /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet /home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-linux-Debug-x64/dotnet exec --runtimeconfig System.Private.Xml.Tests.runtimeconfig.json --depsfile System.Private.Xml.Tests.deps.json /home/REDACTED/.nuget/packages/microsoft.dotnet.xunitconsoleREDACTED/2.9.3-beta.26257.113/build/../tools/net/xunit.console.dll System.Private.Xml.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing k/_temp/ghcca-node/node/bin/bash -DFALLBACK_OS_Igit d/lib -DDEBUG Native_EXPORTS -DTARGET_64BIT -D/home/REDACTED/work/runtime/runtime/artifacts/bin/testhost/net11.0-U0 (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)