Step 1 of fix for disabled System.Dynamic.Runtime test on ILC

[ghost]

(https://github.com/dotnet/corefx/issues/19895)

Make Microsoft.CSharp use the the real HasMetadataDefinitionAs()
api if it exists on the running framework. Since this library
is built on netstandard, it still needs to fall back on its
emulation on older frameworks.

Actual removal of the ActiveIssue will have to wait until
the new System.Runtime reference assembly propagates to
the Project N tree and the new api becomes reflectable.

[VSadov]

I think this should be added to the self-modifying s_MemberEquivalence above. It woudl be just another try/catch before we check for the MetadataToken.

Note that the pattern not only checks for the presence of the API, but also that it does not throw right away. We had major issues with API being discovered and then throwing something like "Not supported" or "Method not avaialble" - I do not remember.
The pattern is to
1 - probe for the API
2 - try using the API for the given args
3 - if API worked without exceptions, self-modify the func to the new implementation so we are no longer wasting time on various conditional checks.
4 - return the result.

[VSadov]

"Before the probing for MetadataToken" is becasue I think this would be more efficient if both are present.

---

In reply to: 121757009 [](ancestors = 121757009)

[VSadov]

I'd expect it to be something like:

try
{
         MethodInfo apiMethod = typeof(MemberInfo).GetMethod(
                    "HasSameMetadataDefinitionAs",
                    BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly | BindingFlags.ExactBinding,
                    binder: null,
                    types: new Type[] { typeof(MemberInfo) },
                    modifiers: null);

        if (apiMethod != null)
        {
               var sameDefAs  = (Func<MemberInfo, MemberInfo, bool>)Delegate.CreateDelegate(typeof(Func<MemberInfo, MemberInfo, bool>), apiMethod);

               var result = sameDefAs  (m1, m2);

               // it worked, so publish it
               s_MemberEquivalence = sameDefAs;
               return result;
        }
}
catch
{
          // Platform might not allow access to the API or it is not implemented.
}

---

In reply to: 121757257 [](ancestors = 121757257,121757009)

[VSadov]

The new API should just be probed for in the same place where we probe/bind other possible implementations of s_MemberEquivalence.

And we should probe for this one first, since it seems to be preferable when present.

[ghost]

Merge probe with the existing probes.

[ghost]

@dotnet-bot test Portable Linux x64 Debug Build

[VSadov]

LGTM
Thanks!!