How do I cast a List<T> effectively?

Both .OfType<T> and .Cast<T> will return a list of T, but the meaning of the two methods is different.

list.OfType() filters the original list and returns all items which are of type T, and skips the ones that are not of that type.

list.Cast() casts all items in the original list to type T, and throws an exception for items which cannot be cast to that type.

In your case both would give the same result, but using .Cast() would communicate your intent a lot more clearly, so I would recommend using that.

List<InputField> list = (from i .... select i).Cast<IDataField>().ToList();

List<InputField> raw = (from i .... select i).ToList();
List<IDataField> result = raw.OfType<IDataField>().ToList();

You could also use List.ConvertAll.

Documentation: http://msdn.microsoft.com/en-us/library/73fe8cwf.aspx

Example:

List<IDataField> newList = oldList.ConvertAll(i => i as IDataField);