c# what is ??

The null-coalescing operator ?? returns the value of its left-hand operand if it isn't null; otherwise, it evaluates the right-hand operand and returns its result. The ?? operator doesn't evaluate its right-hand operand if the left-hand operand evaluates to non-null.

refernce: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-coalescing-operator