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Fix DataContractSerializer nullable struct initialization cache lookup #128506

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Fix DataContractSerializer nullable struct initialization cache lookup #128506

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63c6e51
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Copilot May 22, 2026
4d570f0
Fix nullable DataContractSerializer cache lookup regression and add test
Copilot May 22, 2026
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2 changes: 1 addition & 1 deletion ...System.Private.DataContractSerialization/src/System/Runtime/Serialization/DataContract.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -392,7 +392,7 @@ internal static int GetIdForInitialization(ClassDataContract classContract)
int currentDataContractId = DataContractCriticalHelper.s_dataContractID;
for (int i = 0; i < currentDataContractId; i++)
{
if (ContractMatches(classContract, s_dataContractCache.GetItem(id)))
if (ContractMatches(classContract, s_dataContractCache.GetItem(i)))
{
return i;
}
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20 changes: 20 additions & 0 deletions src/libraries/System.Runtime.Serialization.Xml/tests/DataContractSerializer.cs
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Expand Up @@ -1488,6 +1488,21 @@ public static void DCS_Nullables()
Assert.Equal(item.Struct1.Value.B, actual.Struct1.Value.B);
}

[Fact]
public static void DCS_NullableDataContractStructAsRoot()
{
NullableRootDataContractStruct? value = new NullableRootDataContractStruct();
var serializer = new DataContractSerializer(typeof(NullableRootDataContractStruct?));

using var stream = new MemoryStream();
serializer.WriteObject(stream, value);

stream.Position = 0;
var roundTripped = (NullableRootDataContractStruct?)serializer.ReadObject(stream);

Assert.True(roundTripped.HasValue);
}

[Fact]
public static void DCS_SimpleStructWithProperties()
{
Expand Down Expand Up @@ -1545,6 +1560,11 @@ public static void DCS_PrivateTypeSerialization()
}

#region private type has to be in with in the class
[DataContract]
private struct NullableRootDataContractStruct
{
}

[DataContract]
private class PrivateType
{
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