The result is: xhello
The thing here is that because the stringbuilder is a reference type, when you change the content of the stringbuilder inside the method AppendHello() this change is reflected in the caller method –> Problem() but when you set the stringbuilder to null this change is not visible inside the caller method.
This is because in .Net all parameters are passed by value, even reference types. For reference types it’s not the value of the object itself that is passed by value but a pointer to the underlying value (the reference). The pointer is copied on the stack and is passed by value. When you change the value inside the called method this change is reflected in the underlying value and is visible for the calling method. When you set the pointer to null inside the called method you only destroy the copied pointer not the underlying value.