ref对象参数的方法

Question

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";

MethodName(ref (object) c);

It doesn't work.

How can i do?

thanks

Answer 1

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.

Answer 2

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.

Answer 3

Does

MethodName(ref c);

not work?