Generics and Constraints over Generics

In this post,  I am going to discuss about Generic Classes and bit more on this.  Just for a smooth start,

“Generics were introduced in .NET 2.0, which provides us a way to create Classes/Methods/Types and many more without specifing the specific type of parameters. Parameters must be provided at the time of creating the instances. So we can say Generics are only a blueprint/templated version and actual type is defined at Runtime.”

So lets first create a simple Class say Point

public class Point<T>
    {
    }

Here as you can see, I have created a generic class Point, its coordinates X and Y can be of any type.

So in this post, I am going to mainly discuss, Constraint with generics.

Its very imperative to have a Generic Class, which can accept any type of parameter. But sometimes one might require that your generic Class can be instantiated with only specific types. Like say you want that a Generic can be instantiated with only say a Class Point Type or of its derived types.  Is it possible ?

Yes this is possible.

And also, C# provides a very robust way to implement this feature. So using it, It can be very helpful at certain times to define a constraint over Generic.

So what are the ways to do this. We’ll first take few examples then. Later we will conclude .

So First, How can we define constraint with Generics. It is done using where clause.

We have two types of variable in C#. reference and Value..

No.. …

Now we have three types:

• Reference
• Value
• Nullable ( It is having feature of both types)

public class Point
          where T1 : struct
    {
       ...
    }

Public Class Queue: where T:Class
{
}

public class Point
          where T1 : class
    {
       ...
    }

Public class Point where T: struct, class
    {
       ...
    }

Public class Point where T: new()
    {
       ...
    }

public class Point
          where T1 : class, new()
          where T2 : IList
    {
       ...
    }

public class Point
          where T1 : int
    {
       ...
    }

But this is not possible. The constraint must be an interface, a non-sealed class or a type.

When you will compile will get an ERROR “A type used as a constraint must be an interface, a non-sealed class or a type”.

I was thinking to have this one also, but not there. But you might except it in the coming version of C#.

Hope you all must have liked this. Do share your feedback

Cheers,

Brij

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