method with ref object parameter

Hi i have to call a method that has this signature:

int MethodName(ref object vIndexKey)

If i try to call it with

String c = "690";

MethodNam开发者_如何学Ce(ref (object) c);

It doesn't work.

How can i do?

thanks

You need to do it like this:

String c = "690"; 
object o = (object) c;
MethodName(ref o);

The reason is that the parameter must be assignable by the function. The function could do something like this:

o = new List<int>();

Which is not possible if the underlying type is a string that has been casted to an object during the method call, because the target of the assignment would still be a string and not an object.

When a method has a ref parameter, the argument type has to match the parameter type exactly. Suppose MethodName were implemented like this:

public void MethodName(ref object x)
{
    x = new object();
}

What would you expect to happen if you were able to call it with just ref c? It would be trying to write a reference to a plain System.Object into a variable of type System.String, thus breaking type safety.

So, you need to have a variable of type object. You can do that as shown in klausbyskov's answer, but be aware that the value won't then be copied back to the original variable. You can do this with a cast, but be aware that it may fail:

string c = "690";
object o = c;
MethodName(ref o);
// This will fail if `MethodName` has set the parameter value to a non-null
// non-string reference
c = (string) o;

Here's the relevant bit of the C# 3.0 spec, section 10.6.1.2 (emphasis mine):

When a formal parameter is a reference parameter, the corresponding argument in a method invocation must consist of the keyword ref followed by a variable-reference (§5.3.3) of the same type as the formal parameter. A variable must be definitely assigned before it can be passed as a reference parameter.

Does

MethodName(ref c);

not work?