Question

I have the following code in C#

// test.Program
private static void Main()
{
    int x = 5;
    int y = 100;
    Console.WriteLine(y + ", " + x);
}

And I’m reading the IL code, I’ve never programmed assembly before so I’m asking if what I each line does is correct.

.method private hidebysig static 
    void Main () cil managed 
{
    // Method begins at RVA 0x2058
    // Code size 33 (0x21)
    .maxstack 3 // maximum stack in this method is 3
    .entrypoint // method is initial entry point
    .locals init ( // reserves memory for x and y variables
        [0] int32 x, // x variable is reserved on position 0 of the stack
        [1] int32 y  // y variable is reserved on position 1 of the stack
    )
    IL_0000: ldc.i4.5     // integer of 4 bytes in size and the value of 5 is loaded onto the evaluation stack position 0
    IL_0001: stloc.0      // put evaluation stack position 0 into the stack position 0, the evaluation stack is emptied
    IL_0002: ldc.i4.s 100 // integer of 4 bytes in size and the value of 100 is loaded onto the evaluation stack position 0
    IL_0004: stloc.1      // put evaluation stack position 0 onto the stack position 1, the evaluation stack is emptied
    IL_0005: ldloc.1      // load stack position 1 into the evaluation stack position 0
    IL_0006: box [mscorlib]System.Int32 // box last valuetype placed on evaluation stack, replace valuetype with reference on evaluation stack position 0, do not empty stack
    IL_000b: ldstr ", "   // put reference to string on evaluation stack position 1
    IL_0010: ldloc.0      // load stack position 0 into the evaluation stack position 2
    IL_0011: box [mscorlib]System.Int32 // box last valuetype placed on evaluation stack, replace valuetype with reference on evaluation stack position 0, do not empty stack
    IL_0016: call string [mscorlib]System.String::Concat(object, object, object) // call Concat, pass values on evaluation stack, empty evaluation stack, put result of concat on evaluationstack
    IL_001b: call void [mscorlib]System.Console::WriteLine(string) // pass first value in evaluation stack
    IL_0020: ret         // return
} // end of method Program::Main

Do I understand this program correctly?

Answer 1

Pretty much; only thing I’d clarify is that the box (IL_0006 and IL_0011) is type specific, so it is explicitly constructing a box of type int (it isn’t just the “last valuetype”).

Also, “empty evaluation stack” is misleading; that isn’t quite right – for example, call consumes a given number of positions – it doesn’t “empty” it. There is never an “empty evaluation stack” semantic – it is always “consume a number of values, put back a number of values” (either of which may be zero).