C# Why does passing of an class instance to a method which accepts object as parameter and boxing it back work

For example lets consider the following example.

 class A
 {
  int i;
  string j;
  double t;
 }

 A a =new A();
 MethodCalled(a);

 void Method(object a)
 {
  A newA = a as A; // This will work even though Class A is down casted to System.Object
 }

Can somebody please help me understand this. A link reference to explanation ?开发者_运维百科

Thanks

I don't see any boxing going on. Boxing is when a value type (e.g. int) is converted to a reference type. In your example, the value passed to the method is a reference type (class A). Link for boxing:

http://msdn.microsoft.com/en-us/library/yz2be5wk.aspx

So all that's happening when you call Method (or MethodCalled, I assume that's a typo) is that the method is accepting the argument of type class A because it is an object. All reference types derive from object.

I think your question really boils down to "what does the 'as' operator do?" This line of code:

A newA = a as A;

logically translates to this pseudo-code:

A newA = null;
if (a can be cast to type A)
    newA = a;

So you can see that the 'as' operator will set newA correctly because the parameter's type is actually class A. If you passed another object of type class B, the as operator would return null (assuming class B didn't derive from class A). Here's a link with a sample on the as operator, which might help explain a bit better:

http://msdn.microsoft.com/en-us/library/cscsdfbt.aspx