Covariance & Contravariance with a simple example

Sumit Arora

I need to understand the difference between Covariance & Contravariance with a simple example as there are many articles which only talke about covariance but not about contravariance.

5 Replies

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Rajesh Singh10y ago
Hi
Plz refer this.
http://www.c-sharpcorner.com/UploadFile/hasansheik/covariance-and-contravariance-in-C-Sharp/
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Francis Susaimichael11y ago
It would be good if you could send me the example of covariance as well

The above example that i posted contains both.

why delegates comes to picture with both of these concepts

In this book, Herbert schildt explains "Covariance" and "Contravariance" with the concept of delegates. Also, it is possible to use this concept in Arrays, functions etc.

I would suggest the below MSDN blog post for more info:

http://blogs.msdn.com/b/csharpfaq/archive/2010/02/16/covariance-and-contravariance-faq.aspx

Hope this helps!
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Sumit Arora11y ago
Thanks for update Francis, It would be good if you could send me the example of covariance as well and also why delegates comes to picture with both of these concepts
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Francis Susaimichael11y ago
Since you need a simple example, i am here to highlight the example given in the book "C# 4.0 : The complete Reference" by Herbert Schildt.

Normally, the method that you pass to a delegate must have the same return type and signature as the delegate. However, covariance and contravariance relax this rule slightly, as it pertains to derived types. Covariance enables a method to be assigned to a delegate when the method’s return type is a class derived from the class specified by the return type of the delegate. Contravariance enables a method to be assigned to a delegate when a method’s parameter type is a base class of the class specified by the delegate’s declaration.
Here is an example that illustrates both covariance and contravariance:
// Demonstrate covariance and contravariance.

using System;

class X {
public int Val;
}

// Y is derived from X.
class Y : X { }

// This delegate returns X and takes a Y argument.
delegate X ChangeIt(Y obj);

class CoContraVariance {

// This method returns X and has an X parameter.
static X IncrA(X obj) {
X temp = new X();
temp.Val = obj.Val + 1;
return temp;
}

// This method returns Y and has a Y parameter.
static Y IncrB(Y obj) {
Y temp = new Y();
temp.Val = obj.Val + 1;
return temp;
}

static void Main() {
Y Yob = new Y();

// In this case, the parameter to IncrA
// is X and the parameter to ChangeIt is Y.
// Because of contravariance, the following
// line is OK.
ChangeIt change = IncrA;

X Xob = change(Yob);

Console.WriteLine("Xob: " + Xob.Val);

// In the next case, the return type of
// IncrB is Y and the return type of
// ChangeIt is X. Because of covariance,
// the following line is OK.
change = IncrB;

Yob = (Y) change(Yob);

Console.WriteLine("Yob: " + Yob.Val);
}
}

Hope this helps to understand!
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Ravi Patel11y ago
Hi Sumit,

go through following references

http://www.codeproject.com/Tips/635451/Contravariance-in-Csharp

i hope you will get some idea about Contravariance.
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