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Casting Restrictions ???

, 30 Jul 2010 CPOL
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About casting restrictions

We all know that the runtime can detect the actual type of a System.Object instance. The primitive data types provided by the runtime are compatible with one another for casting (assuming that we do not truncate the values). So if I have an int, it can be cast to long or ulong. All that is fine. Watch this:

interface IAppDataTypeBase
{
   // Other methods
   GetValue();
}

Since IAppDataTypeBase represents the mother of all types of data in my application, I have made GetValue to return the value as object (I could have used generics, that is for another day!).

IAppDataTypeBase longType = GetLongInstanceFromSomeWhere();
int i = (int)longType.GetValue();

So are we discussing any problems here? Yes, we are. The problem is that the value returned by GetValue - System.Object - despite being inherently long cannot be cast to an int. It would result in an 'Specified cast is invalid' exception. If an object is one of the primitive types, it can only be cast to its actual type. In the above case, the object returned by GetValue can only be cast to long, and nothing else. The user defined data types do not have this restriction if the base type and target type are related.

class X { };
class DX : X { };
class Y { };

If GetValue returns an instance of DX, it can be cast to X or any of its base interfaces (if any). The same goes good for structs too.

So why do we have this casting restriction for the primitive types? Was this unintentional or is there an advanced CLR internals web page somewhere talking about this? Probably fixed in C# 4.0? Until I learn why, the question is open.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Vivek Ragunathan
Software Developer (Senior)
United States United States
No Biography provided

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