Covariance and contravariance concepts in .NET generics. Covariance allows implicit conversion from derived to base types, like converting a List<string> to IEnumerable<object>. Contravariance allows converting parameters from base to derived types, like allowing a method that takes an object to be assigned to a delegate that expects a string. In .NET 4, generics support covariance for out type parameters and contravariance for in parameters through interface and delegate type variance annotations.

1. Covariance, Contravariance 觀念分享
Howard

2. Covariance
string[] strings = new string[3];
object[] objects = strings;
// RunTimeError !! 發生 ArrayTypeMismatchException!
objects[0] = DateTime.Now;
string s = (string)objects[0];

3. Covariance – generic types
List<string> stringList = new List<string>();
List<object> objectList = stringList;
Cannot implicitly convert type 'System.Collections.Generic.List<string>' to
'System.Collections.Generic.List<object>’
泛型本身具有不變性（ invariance ）

4. Covariance in .Net 4.0
List<string> stringList = new List<string>();
// C# 2/3 不允許， C# 4.0 OK!
IEnumerable<object> objects = stringList;
WHY ?

5. Covariance in .Net 4.0
public interface IEnumerable<out T> : Ienumerable
{
IEnumerator<T> GetEnumerator();
}
型別 T 在這個泛型介面中只能用於 method 的傳回值，而不能用來當
作 method 的參數
public interface IFoo<out T>
{
// 編譯失敗：型別 T 在這裡只能用於方法的傳回值，不可當
作參數。
string Convert(T obj);
T GetInstance(); // OK!
}

6. Covariance in .Net 4.0
 用 IEnumerable<T> 把一串物件包起來，難道不會跟前面
的陣列 covariance 範例一樣產生型別安全的問題嗎？
答案是不會。因為 IEnumerable<T> 的操作都是唯讀的，你無法透
過它去替換或增刪串列中的元素。

7. Contravariance
 跟 Covariance 相反 - in
static object GetObject() { return null; }
static void SetObject(object obj) { }
static string GetString() { return ""; }
static void SetString(string str) { }
static void Test()
{
// Covariance. A delegate specifies a return type as object,
// but you can assign a method that returns a string.
Func<object> del = GetString;
// Contravariance. A delegate specifies a parameter type as string,
// but you can assign a method that takes an object.
Action<string> del2 = SetObject;
}

8. Contravariance
public interface MyIComparer<in Tin>
{
int Compare(Tin left, Tin right);
}

9. 結論
 In .NET Framework 4, both C# and Visual Basic support
covariance and contravariance in generic interfaces and
delegates and allow for implicit conversion of generic type
parameters.

10. Reference
 C# 4.0 ： Covariance 與 Contravariance 觀念入門
 http://huan-lin.blogspot.com/2009/10/c-40covariance-and-
contravariance.html
 Covariance and Contravariance (C# and Visual Basic)
 http://msdn.microsoft.com/en-us/library/ee207183.aspx

11. Q&A