[release/5.0] Add support for variance to IDIC

[github-actions]

Backport of #58646 to release/5.0

/cc @AaronRobinsonMSFT

Customer Impact

This enables support for variance in an IDynamicInterfaceCastable scenario. The primary user of this, at present, is C#/WinRT. Without this there can be significant performance impact implementing variance in WinRT collections.

The precise impact of this change is difficult to define since it dependent on managed type and the generated source for a particular application. However, a thought experiment to predict potential impact can be done using IEnumerable<T>. Imagine the case where T is defined to be string. The generated C#/WinRT tools generated a projection for IEnumerable<string> of a particular type. Consider the case where a user requests IEnumerable<object>. Based on the current implementation this would fail because IEnumerable<object> is not IEnumerable<string>. What this means is C#/WinRT would now need to dynamically create an additional wrapper, at runtime, to wrap the IEnumerable<string> projection into a IEnumerable<object>. This fix avoids that because it enables the runtime to apply its own variance checks via IsAssignableTo – which is true based on covariance.

Testing

Tests have been added.

Risk

Minimal

[AaronRobinsonMSFT]

/cc @jkoritzinsky @elinor-fung @davidwrighton

[jeffschwMSFT]

Approved. Please request a code review and we can take for consideration in 5.0.x.

[Anipik]

@jeffschwMSFT do we need to add back the servicing-consider label here for next release ? cc @aik-jahoda

[jeffschwMSFT]

Thanks for checking. We are not considering this for servicing for now. We are waiting for the e2e to come together.

[jkotas]

5.0 is going out of support in a few months.