The thing that is nearly always forgotten is that a class isn’t passed by reference, the reference to the class is passed by value.
This is important. Instead of copying the entire class (pass by value in the stereotypical sense), the reference to that class (I’m trying to avoid saying “pointer”) is copied. This is 4 or 8 bytes; much more palatable than copying the whole class and in effect means the class is passed “by reference”.
At this point, the method has it’s own copy of the reference to the class. Assignment to that reference is scoped within the method (the method re-assigned only its own copy of the reference).
Dereferencing that reference (as in, talking to class members) would work as you’d expect: you’d see the underlying class unless you change it to look at a new instance (which is what you do in your failing test).
Using the ref keyword is effectively passing the reference itself by reference (pointer to a pointer sort of thing).
As always, Jon Skeet has provided a very well written overview:
http://www.yoda.arachsys.com/csharp/parameters.html
Pay attention to the “Reference parameters” part:
Reference parameters don’t pass the values of the variables used in
the function member invocation – they use the variables themselves.
If the method assigns something to a ref reference, then the caller’s copy is also affected (as you have observed) because they are looking at the same reference to an instance in memory (as opposed to each having their own copy).
The default convention for parameters in C# is pass by value. This is true whether the parameter is a class or struct. In the class case just the reference is passed by value while in the struct case a shallow copy of the entire object is passed.
When you enter the TestAssignmentMethod there are 2 references to a single object: testObj which lives in AssignmentTest and testClass which lives in TestAssignmentMethod. If you were to mutate the actual object via testClass or testObj it would be visible to both references since they both point to the same object. In the first line though you execute
testClass = new TestRefClass() { TestInt = 1 }
This creates a new object and points testClass to it. This doesn’t alter where the testObj reference points in any way because testClass is an independent copy. There are now 2 objects and 2 references which each reference pointing to a different object instance.
If you want pass by reference semantics you need to use a ref parameter.