Method invocation on a struct?

Question

When we invoke a method on a object, then the reference of the object is passed implicitly to the method.

So my question is what happens when a method is invoked on a struct ? Is it similar to classes in this aspect?

Answer 1

According to the CIL spec the this pointer is passed by reference / as managed pointer. As Binary Worrier assumed this is a CIL feature.

Instance and virtual methods of classes shall be coded to expect a reference to an instance of the class as the this pointer. By contrast, instance and virtual methods of value types shall be coded to expect a managed pointer (see Partition I) to an unboxed instance of the value type. The CLI shall convert a boxed value type into a managed pointer to the unboxed value type when a boxed value type is passed as the this pointer to a virtual method whose implementation is provided by the unboxed value type.

So from a high-level perspective a call to an instance method of a reference type (class) looks like this:

MyClass myClass = MyClass_Constructor();  MyClass_MyInstanceMethod(myClass, myFirstParameter); //                       ^ //                       The "this" argument 

And a call to an instance method of a value type (struct) like this:

MyStruct myStruct = MyStruct_Constructor();  MyStruct_MyInstanceMethod(ref myStruct, myFirstParameter); //                        ^ //                        The "this" argument 

Answer 2

Florian is right; a few more details while we're on the subject:

When we invoke a method on a object, then the reference of the object is passed implicitly to the method.

Correct. One way to think about this is that a method call like:

class C {     int y;     public void M(int x)      {          Console.WriteLine(x + y);      } } ... C c = new C(); c.M(10); 

is actually the same as

class C {     int y;     public static void M(C _this, int x)      {          Console.WriteLine(x + _this.y);      } } ... C c = new C(); C.M(c, 10); 

That is, every instance method has a hidden "this" parameter, and the method is "really" static.

So my question is what happens when a method is invoked on a struct ? Is it similar to classes in this aspect?

Yes. Instead of a reference to an instance, what is passed is an alias to the variable that contains the struct. That's how struct methods can mutate the struct. (Of course, mutating a struct is a bad practice, but sometimes necessary.)

struct S {     int y;     public void M(int x)      {          Console.WriteLine(x + y);      } } ... S s = new S(); s.M(10); 

is logically the same as

struct S {     int y;     public static void M(ref S _this, int x)      {          Console.WriteLine(x + _this.y);      } } ... S s = new S(); S.M(ref s, 10); 

An interesting question then arises: what if the "receiver" is not a variable? (*) You can only take a ref to a variable. Suppose you have:

GetAnS().M(10);  

??? What happens then?

We make a variable for you. That becomes

S temporary = GetAnS(); temporary.M(10); 

and now the receiver is a variable, so we can make the "hidden this parameter" an alias to it.

(*) There are other interesting cases here as well, like what if the struct is mutable but the variable is readonly, and so on.