Why can '=' not be overloaded in C#?

If you overloaded '=' you would never be able to change an object reference after it's been created. ... think about it - any call to theObjectWithOverloadedOperator=something inside the overloaded operator would result in another call to the overloaded operator... so what would the overloaded operator really be doing ? Maybe setting some other properties - or setting the value to a new object (immutability) ? Generally not what '=' implies..

You can, however, override the implicit & explicit cast operators: http://www.blackwasp.co.uk/CSharpConversionOverload.aspx

Memory managed languages usually work with references rather than objects. When you define a class and its members you are defining the object behavior, but when you create a variable you are working with references to those objects.

Now, the operator = is applied to references, not objects. When you assign a reference to another you are actually making the receiving reference point to the same object that the other reference is.

Type var1 = new Type();
Type var2 = new Type();

var2 = var1;

In the code above, two objects are created on the heap, one referred by var1 and the other by var2. Now the last statement makes the var2 reference point to the same object that var1 is referring. After that line, the garbage collector can free the second object and there is only one object in memory. In the whole process, no operation is applied to the objects themselves.

Going back to why = cannot be overloaded, the system implementation is the only sensible thing you can do with references. You can overload operations that are applied to the objects, but not to references.