No, you only have user-definable type constructors that happen to be monads if you jump to a metalanguage. (Whatever formal or informal system you use to reason about C# code.) Inside C#, you don't even have the means to assert that they are monads, let alone take advantage of the fact that they are monads.
That just shows that the result of specializing the monad laws to one specific type constructor, holds. But the monad laws themselves are inexpressible in a general form.
If you can't even postulate a theory (the monad signature and its equational laws), it doesn't make sense to talk about what models of the theory exist (concrete instances).
C# does have user-definable monads. There are many in this project (Option, Either, Try, TryOption, Task, Reader, Writer, State, Parser, ...)