Unable to pass IEnumerable<Triangle> into a function that is looking for IEnumerable<IShape>

I have a collection of objects:

 IE开发者_如何学Pythonnumerable<Triangle>

These objects support an interface IShape but i am getting an error trying to pass this into a function that is asking for:

 IEnumerable<IShape>

1. Why can't I pass this in?
2. Is there any workaround to convert one to the other to get this to work?

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In .NET 4, this should be supported, as IEnumerable<T> is declared to be covariant, i.e. the declaration is really IEnumerable<out T>. In .NET 3.5, that is not the case, and you would need to use a workaround such as

triangles.Cast<IShape>()

(It's worth noting this was a language feature evolution as well; that is, if you somehow were able to use C# 3 with .NET 4, it would still not work, because support for co- and contravariance was not added to C# until version 4 of the language.)

An excellent explanation of co- and contravariance can be found in Jon Skeet's C# in Depth, section 13.3.

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It's simple:

IEnumerable<Triangle> does not inherit from and is not assignable to IEnumerable<IShape>^*

There is no relationship of that kind between the two types. It sounds like you really want a function more like this:

void MyFunction<T>(IEnumerable<T> items) where T : IShape

That says, "give me a function that accepts any IEnumerable<T> where the T inherits from IShape". Alternatively, you could use the .Cast<T>() extension method (it's surprisingly performant).

_{* On your version of the .Net Framework}