Question

According To reflector , ExpandoObject Does implemenet IDictionary<string, object>

How ever I have this code which i dont understand why Does Interface co-variance doesn’t work here

From what Ive read – it does suppose to work :

Answer 1

There are three things wrong here:

• IDictionary<TKey, TValue> is invariant in both its type parameters
• Generic variance doesn’t work across value types (int here)
• If it were covariant, you’d be trying to use it the wrong way round: you’re actually trying to use contravariance

You should ignore the dynamic part here – it’s a red herring. Try working out some code you’d expect to work that doesn’t use dynamic. Here’s an example of why IDictionary<,> is invariant:

// Suppose it were covariant in value...
Dictionary<string, string> stringToString = new Dictionary<string, string>();
IDictionary<string, object> stringToObject = stringToString;
stringToObject["foo"] = new MemoryStream();
string value = stringToString["foo"]; // Um?

// Suppose it were contravariant in value...
Dictionary<string, object> stringToObject = new Dictionary<string, object>();
IDictionary<string, string> stringToString = stringToString;
stringToObject["foo"] = new MemoryStream();
string value = stringToString["foo"]; // Um?