The technical term for this is that generics are invariant in C# 3.0 and earlier. From C#4.0 onward, the cast works.

What invariant means is that there is no relationship between two generic types just because their generic type parameters are related (i.e. are sub- or supertypes of each other).

In your example, there is no typing relationship between an IEnumerable<object> and an IEnumerable<string>, just because string is a subtype of object. They're just considered two completely unrelated types, like a string and an int (they still both are subtypes of object, but everything is)

There are a few workarounds and exceptions for this issue you've run into.

First, you can cast each string individually to object, if you're using .NET 3.0 you can do that using the Cast<T>() extension method. Otherwise, you can use a foreach and put the result into a new variable of the static type you want.

Second, arrays are an exception for reference type, i.e. passing in a string[] type to a method acccepting object[] types should work.