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Exploring .NET sources on GitHub I stumbled upon

((SomeTypeToCast)variable!).SomeMethodToCall()

What's this postfix !?

I surmise this is a kind of "this value is never null". However there is no such operator in C# (at least publicly available) and such an expression fails to compile when I try it in a test project.

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This is the null-forgiving operator (also known as the "damn-it" operator) in C# 8, which effectively tells the compiler to assume that the value will be non-null. It's a little like a cast, in terms of telling the compiler that you know better than it does - but it has no effect at execution time, so you're effectively bypassing the safety of the compiler checks.

It's introduced as part of the C# 8 nullable-reference type feature. It is available in public preview builds of .NET Core 3.0 SDK.

Typical uses in my experience:

  • Testing your argument validation code, to prove that if you do pass null into a method, you've got validation to throw ArgumentNullException
  • Places where you're certain the value won't be null due to other invariants that the compiler isn't aware of. (For example, in Noda Time I have a ParseResult<T> which has fields of a value and an exception provider. Either the exception provider is null or the value is null, but never both, and I always check the exception provider before using the value.)
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