Question

If a method takes a parameter of type System.Collections.IList can I legitimately/safely pass a value of type System.Collections.Generic.IEnumerable<T>?

I would appreciate a thorough explanation of why this is possible and what actually happens to the object T when the IEnumerable<T> is used inside of the method.

Is it converted to the base type of Object?

Is it used as an System.Collections.IEnumerable?

Are there any scenarios where this will cause problems (i.e. performance issues)?

Thanks in advance,
John

Answer 1

No, You cannot pass an IEnumerable<T> to a method which takes IList. The interface IEnumerable<T> is not convertible to IList and attempting to pass a variable of type IEnumerable<T> to a method taking IList will result in a compilation error.

In order to make this work you will need to do one of the following

1. Instead of having an IEnumerable<T>, maintain a reference to a type which implements both IEnumerable<T> and IList. List<T> is a good candidate here
2. Convert the IEnumerable<T> instance to a new object which is convertible to IList. For example, in 3.5+ you can call the .ToList() extension method to create a new List<T> over the enumeration.