How can I return NULL from a generic method in C#?

return default(T);

Three options:

Return default (or default(T) for older versions of C#) which means you'll return null if T is a reference type (or a nullable value type), 0 for int, '\0' for char, etc. (Default values table (C# Reference))
If you're happy to restrict T to be a reference type with the where T : class constraint and then return null as normal
If you're happy to restrict T to be a non-nullable value type with the where T : struct constraint, then again you can return null as normal from a method with a return value of T? - note that this is not returning a null reference, but the null value of the nullable value type.

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