Why is conversion between with different type parameters is NOT allowed?

Question

I just don't get it as it would be so useful to convert one generic container into the other?

Stack <IType> stack = new Stack<SomeType>();

Answer 1

Are you talking about conversions like so?

IFoo<Child> child = ...;
IFoo<Parent> parent = child;

If so, this is what is known as covariance. This is usually paired with it's counterpart contravariant. This feature is indeed not available in the current version of C#. But it will be available in next version of both C# and VB.Net.

Some articles about the upcoming release

• http://blogs.msdn.com/ericlippert/archive/2007/10/16/covariance-and-contravariance-in-c-part-one.aspx
• http://www.leading-edge-dev.de/?p=246

Answer 2

While what @JaredPar says is true, there are some work-arounds for collections that are available today. For instance, you can use the Cast IEnumerable extension if there is a legal cast from one type to another.

List<Foo> list = barList.Cast<Foo>().ToList();

Or you could explicitly convert from one type to another using Select.

List<double> dList = intList.Select( i => Convert.ToDouble( i ) ).ToList();

Note that both of these methods will produce a new collection of the appropriate type, not simply assign the collection to a variable of a different type as will be available under certain conditions in the next versions of C#/VB.

Answer 3

With regard to the example you gave:

Stack <IType> stack = new Stack<SomeType>();

What would happen if we did this:

stack.Add(new SomeOtherTypeNotDerivedFromSomeType());

This is the basic reason why covariance is not allowed in .Net, I guess because the underlying collection for the generic container does not necessarily match the declared type. The articles by Eric Lippert go into great detail (more than I really can).