How does the ref keyword work (in terms of memory)

Question

C# has a ref keyword. Using ref you can pass an int to a method by reference. What goes on the stack frame when you call a method that accepts an int by reference?

Answer 1

Here is a simple example in C# code:

void Main()
{
    int i = 1;
    inc(ref i);
    Console.WriteLine(i);
}

public void inc(ref int i) { 
  i++;
}

Here is the generated IL code

IL_0000:  nop         
IL_0001:  ldc.i4.1    
IL_0002:  stloc.0     // i
IL_0003:  ldarg.0     
IL_0004:  ldloca.s    00 // i
IL_0006:  call        inc
IL_000B:  nop         
IL_000C:  ldloc.0     // i
IL_000D:  call        System.Console.WriteLine
IL_0012:  nop         
IL_0013:  ret         

inc:
IL_0000:  nop         
IL_0001:  ldarg.1     
IL_0002:  dup         
IL_0003:  ldind.i4    
IL_0004:  ldc.i4.1    
IL_0005:  add         
IL_0006:  stind.i4    
IL_0007:  ret     

Note with this simple case there is really only one difference ldloca.s 00 or ldloc.0. Load local or load address (of offset 00)

That is the difference at the simplest level (which is what you asked for in your comment) -- if you load the value of the variable or you load the address of the variable. Thing can get complicated quickly -- if the function you are calling is not local, if the variable you are passing is not local etc etc etc. But at a basic level this is the difference.

I used linqpad to do my quick diss-assembly -- I recommend it. http://www.linqpad.net/