C# is an Object-oriented programming language. C# comes with simplicity, expressiveness and great performance to meet the programmer productivity.
Introduction
So far, we have studied about
variance in
C#. Now, let's study co-variance and contra-variance with different types in this chapter.
Objective
The main objective of this article is to learn co-variance and contra-variance in
C# programming.
Co-variance and Contra-variance
Co-variance and Contra-variance are the C# terms which enables the implicit reference conversion for
delegates,
arrays and
generic types.
Meanwhile Co-variance preserves the assignment compatibility and Contra-variance reverses the Co-variance functionality.
Assignment Compatibility:
Assignment compatibility is the concept which provides to store a value of one particular type into the storage location of another type (different than first one) without losing the data.
Example:
string Name = "Krishna";
object obj = Name; // Assigning
In the above example, it is clearly explained that an object of more derived type is assigned to a less derived type's object.
Now observe the below lines of code which demonstrates co-variance,
IEnumerable<string> Names = new List<string>();
IEnumerable<object> objs = Names;
In the above lines, the object instantiated with more derived type is assigned to the object instantiated with less derived type. This is called
Co-variance.
And,
class Program
{
static void MyObj(object ob)
{
// static method
}
static void Main()
{
Action<object> ConObj = MyObj;
Action<string> ConName = ConObj;
}
}In the above code, we have a static method in our class for that we have instantiated a less derived object. Here, we are assigning the object of less derived type to the object of more derived type. This is called as
Contra-variance. This is exactly opposite (reverse) to
Co-variance.
Note: The above seen codes are the examples of Co-variance and Contra-variance for generic types.
Co-variance for Arrays
In order to do implicit conversion from an array of more derived type to an array of less derived type, Array Co-variance is needed.
Example,
object[] MyArray = new String[5]; // run-time error
The above line will not give any compile time error. But if you run, it will through a run-time error. It is stated that array co-variance is not safe at all. For instance,
MyArray[0] = 3;
This statement causes an
ArrayTypeMismatchException. This is because our
MyArray variable is holding the reference of a string array.
Similar to the co-variance, contra-variance is the reverse functionality of co-variance.
Co-variance for Delegates
Similar to arrays, delegate co-variance allows a method that has parameter types more derived than the specified type in the delegate.
Example,
static string MyString()
{
return "Hello";
}
static void Main()
{
Func<object> delObj = MyString;
}In the above code, we are implementing the conversion of more derived type to less derived type.
Contra-variance means the reverse functionality of above code.
Conclusion
In this article, we have studied about co-variance and contra-variance in C#. Hope you understand.
Thanks for reading,
Regards,
Krishna.