To reproduce:

1.Setup microsoft speech recognition engine
2.Download and open attached project
3.Attach UsePlugin script to MainCamera object
4.Press play

Reproduced with : 5.3.1f1, 5.2.5f1, 5.4.3p3, 5.3.7p2, 5.5.0f3, 5.6.0b3

Expected result: editor not crashing.
Actual result: editor crashes.

Notes: Crash occurs with speech dictation settings both enabled and disabled.

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We generally don't support COM Interop in Unity. The best way to use this functionality is access it in native and expose C/PInvoke wrappers.