[release/3.1] Support dictionaries that don't implement non-generic IDictionary

[steveharter]

note: this PR was created and made stand-alone since the previous PR that included this fix takes a dependency on collection PR that will likely not be accepted.

Ports #41903

Issue:

Support dictionaries that implement IDictionary<TKey, TValue> but do not implement the non-generic IDictionary

Description

An assumption was made during serialization that a call to a dictionary's GetEnumerator() (either through the IEnumerable or IDictionary interfaces) returns an IDictionaryEnumerable (which is required for types that implement IDictionary). However, there are certain "newer" generic collection types outside of the BCL that don't implement IDictionary or return IDictionaryEnumerable from their IEnumerable.GetEnumerator() method (note that IDictionary<TKey, TValue> does not implement IDictionary). This caused a InvalidCastException at runtime. The fix is to assume IEnumerable<KeyValuePair<TKey, TValue>> instead of IDictionaryEnumerable for generic dictionaries.

Customer Impact

Without this PR, there is no support for generic dictionaries that implement IDictionary<TKey, TValue> but don't support the older non-generic IDictionary. This is a community-reported issue and is blocking adoption.

Regression?

No. The InvalidCastException is no longer encountered.

Risk

Low. Dictionaries have good coverage already and additional tests were added. A community member verified a fix based on the sames changes to master.

[danmoseley]

Thanks for breaking this out. Approved: customer reported, impeding adoption, relatively low risk and localized, good tests, verified by customer.

Please merge when green/signed off and hopefully by 4pm

[safern]

/azp run corefx-ci

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ahsonkhan]

@danmosemsft - I am assuming this is supposed to have "servicing-approved" label rather than "api-approved".

[danmoseley]

Thank you.