Fix: using default value for nullable enum parameter throws ArgumentException

[buyaa-n]

Fix: Type.Missing doesn't convert to nullable enum in Method.Invoke

The root cause is: ParameterInfo.DefaultValue returns the raw value if the enum is nullable. So the root cause is a known issue and we choose to not fix for compatibility reason:

runtime/src/coreclr/System.Private.CoreLib/src/System/Reflection/MdConstant.cs

Lines 16 to 20 in fd31f1f

	// NOTE: Unlike in `TypeBuilder.SetConstantValue`, if `fieldType` describes
	// a nullable enum type `Nullable<TEnum>`, we do not unpack it to `TEnum` to
	// successfully enter this `if` clause. Default values of `TEnum?`-typed
	// parameters have been reported as values of the underlying type, changing
	// this now might be a breaking change.

Though for MethodInfo.Invoke we could parse the raw value into the corresponding Enum and avoid throwing ArgumentException

Fixes #70925

[ghost]

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Issue Details

---

Fix: Type.Missing doesn't convert to nullable enum in Method.Invoke

The root cause is: ParameterInfo.DefaultValue returns the raw value if the enum is nullable. So the root cause is a known issue and we choose to not fix for compatibility reason:

runtime/src/coreclr/System.Private.CoreLib/src/System/Reflection/MdConstant.cs

Lines 16 to 20 in fd31f1f

	// NOTE: Unlike in `TypeBuilder.SetConstantValue`, if `fieldType` describes
	// a nullable enum type `Nullable<TEnum>`, we do not unpack it to `TEnum` to
	// successfully enter this `if` clause. Default values of `TEnum?`-typed
	// parameters have been reported as values of the underlying type, changing
	// this now might be a breaking change.

Though for MethodInfo.Invoke we could parse the raw value into the corresponding Enum and avoid throwing ArgumentException

Fixes #70925

Author:	buyaa-n
Assignees:	-
Labels:	area-System.Reflection
Milestone:	-

[steveharter]

Looks like some of the new tests are failing in NativeAOT. You likely will want an ActiveIssue there.

[steveharter]

LGTM once CI green for NativeAOT

[steveharter]

Apparently, the new tests work on Mono?

[jkotas]

Looks like some of the new tests are failing in NativeAOT. You likely will want an ActiveIssue there.

Could you please look into fixing the failing tests as part of this PR?