Variance in Generic Interfaces (C#)

.NET Framework 4 introduced variance support for several existing generic interfaces. Variance support enables implicit conversion of classes that implement these interfaces. The following interfaces are now variant:

Covariance permits a method to have a more derived return type than that defined by the generic type parameter of the interface. To illustrate the covariance feature, consider these generic interfaces: IEnumerable<Object> and IEnumerable<String>. The IEnumerable<String> interface does not inherit the IEnumerable<Object> interface. However, the String type does inherit the Object type, and in some cases you may want to assign objects of these interfaces to each other. This is shown in the following code example.

IEnumerable<String> strings = new List<String>();  
IEnumerable<Object> objects = strings;  

In earlier versions of the .NET Framework, this code causes a compilation error in C# with Option Strict On. But now you can use strings instead of objects, as shown in the previous example, because the IEnumerable<T> interface is covariant.

Contravariance permits a method to have argument types that are less derived than that specified by the generic parameter of the interface. To illustrate contravariance, assume that you have created a BaseComparer class to compare instances of the BaseClass class. The BaseComparer class implements the IEqualityComparer<BaseClass> interface. Because the IEqualityComparer<T> interface is now contravariant, you can use BaseComparer to compare instances of classes that inherit the BaseClass class. This is shown in the following code example.

// Simple hierarchy of classes.  
class BaseClass { }  
class DerivedClass : BaseClass { }  

// Comparer class.  
class BaseComparer : IEqualityComparer<BaseClass>   
{  
    public int GetHashCode(BaseClass baseInstance)  
    {  
        return baseInstance.GetHashCode();  
    }  
    public bool Equals(BaseClass x, BaseClass y)  
    {  
        return x == y;  
    }  
}  
class Program  
{  
    static void Test()  
    {  
        IEqualityComparer<BaseClass> baseComparer = new BaseComparer();  

        // Implicit conversion of IEqualityComparer<BaseClass> to   
        // IEqualityComparer<DerivedClass>.  
        IEqualityComparer<DerivedClass> childComparer = baseComparer;  
    }  
}  

For more examples, see Using Variance in Interfaces for Generic Collections (C#).

Variance in generic interfaces is supported for reference types only. Value types do not support variance. For example, IEnumerable<int> cannot be implicitly converted to IEnumerable<object>, because integers are represented by a value type.

IEnumerable<int> integers = new List<int>();  
// The following statement generates a compiler errror,  
// because int is a value type.  
// IEnumerable<Object> objects = integers;  

It is also important to remember that classes that implement variant interfaces are still invariant. For example, although List<T> implements the covariant interface IEnumerable<T>, you cannot implicitly convert List<Object> to List<String>. This is illustrated in the following code example.

// The following line generates a compiler error  
// because classes are invariant.  
// List<Object> list = new List<String>();  

// You can use the interface object instead.  
IEnumerable<Object> listObjects = new List<String>();  

See Also

Using Variance in Interfaces for Generic Collections (C#)
Creating Variant Generic Interfaces (C#)
Generic Interfaces
Variance in Delegates (C#)