Can cast as T fail if parameter is T?

Up to now I always thought that if I check if a parameter "is" T, then a cast "as" T should be safe. So this would always succeed and never be null:

if (myParam is MyClass)
{
    MyClass instance = myParam as MyClass;
}

However, the compiler warns me that the right hand side might be null:

"CS8600: Converting null literal or possible null value to non-nullable type."

Can this actually happen or does the compiler simply not get the preceding check? And if it still could be null, how would this be possible?

According to the Microsoft documentation, it seems impossible:
https://learn.microsoft.com/en-au/dotnet/csharp/language-reference/operators/type-testing-and-cast

>The expression of the form
>
> E as T
>
>where E is an expression that returns a value and T is the name of a type or a type parameter, produces the same result as
>
>E is T ? (T)(E) : (T)null

Note: (I get myParam as parameter in a generic class in a quite complicated way; in principle, with a bit of effort I can create a minimum example if necessary, but I was quite sure up to now that there is no situation where this cast could ever fail.)

This is safe. The compiler warning is due to the nullable reference types feature.

When analysing whether an expression can be null, C# only does control flow analysis for the null-checks that you did, and not the type checks that you did. For example, C# can understand this:

if (myParam != null) {
    // C# understands that myParam is not null here
}

But not:

if (myParam is MyClass) {
    // C# does not understand that myParam is of type MyClass here
}

Because of this, C# thinks that the expression myParam as MyClass is "maybe null". And it sees that you are assigning it to a non-nullable variable instance, hence the warning. See here for a reference.

In older versions of C#, nullable reference types weren't a thing, so there were no warnings for this. Every reference type were nullable.

You should check the type with a declaration pattern instead.

if (myParam is MyClass instance) {
    // use instance here...
}