为什么我的代码不支持协变接口？

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?

Answer 2

1. IDictionary<TKey,TValue> does not support any variance.
   Imagine what would happen if you could convert IDictionary<string,object> to IDictionary<string,int> :
   Your interface now guarantees that every value it returns is an int . But since you can add arbitrary objects to it, it can't actually ensure that guarantee.
2. Even interfaces which support variance, only support it between reference types. int is no reference type.
3. This issue is unrelated to dynamic. You can't convert a class that only implements IDictionary<string,object> to IDictionary<string,int> . No matter if you use static casting or dynamic. It simply doesn't implement that interface.