CIL OpCode (Ldarg_0) is used even though there are no arguments

Question

I have the following C# code.

public void HelloWorld()
{
    Add(2, 2);
}

public void Add(int a, int b)
{
    //Do something
}

It produces the following CIL

.method public hidebysig instance void  HelloWorld() cil managed
{
  // Code size       11 (0xb)
  .maxstack  8
  IL_0000:  nop
  IL_0001:  ldarg.0
  IL_0002:  ldc.i4.2
  IL_0003:  ldc.i4.2
  IL_0004:  call       instance void ConsoleApplication3.Program::Add(int32,
                                                                      int32)
  IL_0009:  nop
  IL_000a:  ret
} // end of method Program::HelloWorld

Now, what I don't understand is the line at offset 0001:

ldarg.0

I know what that opcode is for, but I don't really understand why it's being used in this method, as there are no arguments, right?

Does someone know why? :)

Does this answer your question? [Why do I have to do ldarg.0 before calling a field in MSIL?](https://stackoverflow.com/questions/1785372/why-do-i-have-to-do-ldarg-0-before-calling-a-field-in-msil) — Pharap, May 13 '23 at 02:47
@Pharap this question has already received plenty of good answers, but thanks :) — animaonline, May 22 '23 at 09:30

score 23 · Answer 1 · answered May 02 '13 at 19:53

23

In instance methods there is an implicit argument with index 0, representing the instance on which the method is invoked. It can be loaded on the IL evaluation stack using ldarg.0 opcode.

answered May 02 '13 at 19:53

Zakharia Stanley

1,196
1
9
10

score 17 · Accepted Answer · edited May 23 '17 at 12:17

17

I think that the ldarg.0 is loading this onto the stack. See this answer MSIL Question (Basic)

edited May 23 '17 at 12:17

Community

1
1

answered May 02 '13 at 19:31

Twiltie

572
1
6
13

score 2 · Answer 3 · answered May 22 '13 at 15:56

The line at offset 0001: Loads the argument at index 0 onto the evaluation stack.

See more in: http://msdn.microsoft.com/en-us/library/system.reflection.emit.opcodes.ldarg_0.aspx

The argument at index 0 is the instance of the class that contains the methods HelloWorld and Add, as this (or self in other languajes)

 IL_0001:  ldarg.0   //Loads the argument at index 0 onto the evaluation stack.

 IL_0002:  ldc.i4.2  //Pushes a value 2 of type int32 onto the evaluation stack.

 IL_0003:  ldc.i4.2  //Pushes a value 2 of type int32 onto the evaluation stack.

 IL_0004:  call instance void ConsoleApplication3.Program::Add(int32, int32)

...this last line is as call: this.Add(2,2); in C#.

CIL OpCode (Ldarg_0) is used even though there are no arguments

3 Answers3