Change ParameterDefaultValue to use GetUninitializedObject instead of Activator.CreateInstance

[eerhardt]

Activator.CreateInstance has the ability to call a parameterless constructor (if one is defined) on a value type. This is incorrect for ParameterDefaultValue. It is more correct to call GetUninitializedObject, which is the same as using default(T).

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Issue Details

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Activator.CreateInstance has the ability to call a parameterless constructor (if one is defined) on a value type. This is incorrect for ParameterDefaultValue. It is more correct to call GetUninitializedObject, which is the same as using default(T).

Author:	eerhardt
Assignees:	-
Labels:	area-Extensions-DependencyInjection
Milestone:	-

[pakrym]

I think we should add a test.

[pakrym]

LGTM

[eerhardt]

I think we should add a test.

I'm not sure how to add a test without dotnet/csharplang#99. I would have to write a value type in IL that defines a parameterless constructor. Maybe we could do it with Microsoft.NET.Sdk.IL, but I'm not sure how much work that would be. My plan was to wait for dotnet/csharplang#99 to add a test. Thoughts?

[stephentoub]

We have a few IL test projects, e.g.
https://github.com/dotnet/runtime/tree/master/src/libraries/System.Runtime/tests/TestModule
https://github.com/dotnet/runtime/tree/master/src/libraries/System.Runtime/tests/TestStructs
https://github.com/dotnet/runtime/tree/master/src/libraries/System.Reflection.Metadata/tests/Resources/Interop
This one even has what you want :)

runtime/src/libraries/System.Runtime/tests/TestStructs/System.TestStructs.il

Lines 25 to 37 in 2ffb39e

	.class public sequential sealed StructWithPublicDefaultConstructor
	extends [System.Runtime]System.ValueType
	{
	.field public initonly bool ConstructorInvoked
	.method public hidebysig specialname rtspecialname instance void .ctor () cil managed
	{
	ldarg.0
	ldc.i4.1
	stfld bool System.Tests.StructWithPublicDefaultConstructor::ConstructorInvoked
	ret
	}
	}

as part of testing CreateInstance:

runtime/src/libraries/System.Runtime/tests/System/ActivatorTests.Generic.cs

Lines 47 to 49 in 1d9e50c

	[Fact]
	public void CreateInstanceT_StructWithPublicDefaultConstructor_InvokesConstructor() =>
	Assert.True(Activator.CreateInstance<StructWithPublicDefaultConstructor>().ConstructorInvoked);

[eerhardt]

Thanks @stephentoub! I didn't know about those tests. I added a test using that struct.

[eerhardt]

Test failures are #47728

[GrabYourPitchforks]

The logic in ParameterDefaultValue.cs LGTM.