Pass 'value type' on stack by ref - memory footprint

Question

What happens in memory when we pass a value type - which has been stored on the stack - by reference?

A temp value/pointer must be created somewhere to change the origninal value when the method completes. Could someone please explain or point me to the answer - lots of stuff on memory but none seem to answer this. ty

Answer 1

If you have a method like this:

static void Increment(ref int value)
{
   value = value + 1;
}

and call it like this:

int value = 5;
Increment(ref value);

then what happens is that, instead of the value 5 being pushed on the stack, the location of the variable value is pushed on the stack. I.e. the contents of value are changed directly by Increment and not after the method completes.

Here's the IL of the method and the method call respectively:

.method private hidebysig static void Increment(int32& 'value') cil managed
{
    .maxstack 8
    L_0000: nop 
    L_0001: ldarg.0 
    L_0002: ldarg.0 
    L_0003: ldind.i4         // loads the value at the location of 'value'
    L_0004: ldc.i4.1 
    L_0005: add 
    L_0006: stind.i4         // stores the result at the location of 'value'
    L_0007: ret 
}

.method private hidebysig static void Main() cil managed
{
    .entrypoint
    .maxstack 9
    .locals init ([0] int32 value) //   <-- only one variable declared
    L_0000: nop 
    L_0001: ldc.i4.5
    L_0002: stloc.0 
    L_0003: ldloca.s 'value'   // call Increment with the location of 'value'
    L_0005: call void Program::Increment(int32&)
    L_000a: ret 
}