Compiler Error: Cannot convert from 'List<string>' to 'IList<object>'

Question

How would I change the following code to make it compile? (Other than casting/changing strings to List<object>).

Action<IList<object>> DoSomething = (list) => { /*list is never modified*/ };
var strings = new List<string>() { "one", "two" };
DoSomething(strings);

Answer 1

As the compiler error indicates, you can't cast a IList<string> to an IList<object>. This is because the IList<T> interface is invariant with respect to T. Imagine if your did something like this:

Action<IList<object>> DoSomething = (list) => list.Add(1);

This would be valid with a IList<object> but not with an IList<string>.

As long as you're not trying to modify the collection, a simple solution is to change the IList<T> to IEnumerable<T>, which is covariant with respect to T:

Action<IEnumerable<object>> DoSomething = (list) => { };
var strings = new List<string>() { "one", "two" };
DoSomething(strings);

Further Reading

• Covariance and Contravariance in Generics
• Covariance and contravariance (computer science)