When to pass ref keyword in

Question

I've read the difference between passng and not passing ref in parameters, however, when would I want to use them?

For example, I had some logic in a method which could be refactored into its own method. Resharper 4.5 made one of the parameters a ref type but I didn't think I would have of done this if I did the refactoring manually.

Obviously I am missing some understanding. Perhaps an example of what happens when certain types or certain scenarios in coding miss the ref keyword will help?

Thanks

Answer 1

Let me break that down into two questions:

1) When should one use ref/out formal parameter declarations when writing a method?

Use ref/out when you desire your method to be able to read and write a variable passed in from the caller, rather than merely reading a value.

2) Why does an "extract method" refactoring produce a ref parameter?

I don't know the details of Resharper, but I can make a guess. Consider the following evil mutable value type:

struct S 
{ 
  private int x;
  public int X() { return this.x; } 
  public void M() { this.x += 1; } 
}

You have a method:

void Foo() 
{
    S s = new S();
    Fred(s);
    Blah(s);
    Bar(s);
    s.M();
    Console.WriteLine(s.X()); // prints 1
}

and you do "extract method" on the middle bit:

void NewMethod(ref S s)
{
    Blah(s);
    Bar(s);
    s.M();
}

void Foo() 
{
    S s = new S();
    Fred(s);
    NewMethod(ref s);
    Console.WriteLine(s.X()); // still prints 1
}

If instead you made a method without "ref" then calling NewMethod(s) would pass a copy of s to NewMethod. Remember, value types are copied by value; that's why we called them "value types". It would be the copy that gets mutated, and then s.X() returns zero. It is a bad idea for a refactoring to introduce a semantic change in a program, and it is difficult for a refactoring engine to know whether a given method relies on the mutability of a value type or not.

This is just another reason why you should avoid mutable value types.