Use of new() while applying a constraint on generic class

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Hi,
I have been using generics for quite a while and have already used them extensively.Recently when i was working on a project involving generics some thing struck me which i want somebody to please clarify. Suppose i declare a class as following:

C#

public class Stack<t> where T : IComparable,new()
{
....
}

Here, the class which takes the position of T should implement IComparable interface and should also implement a default constructor. However we also know the fact that if we don`t provide a default constructor, the compiler automatically inserts a default constructor. So whats the use of writing a "new()" constraint on the generic class ?

Posted 27-May-12 4:14am

www.Developerof.NET

Updated 27-May-12 4:15am

Wendelius

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Wendelius · Accepted Answer · 2012-05-27T04:21:00

That means, phe parameterless constructor must exist, one way or another.

Consider the following:

C#

public class Test1 {
   public Test1(string somestring) { }
}

public class Test2<T1> where T1 : new() {
}

Now if I try to define an instance

C#

Test2<Test1> testInstance = new Test2<Test1>();

That would fail with

'App.Test1' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T1' in the generic type or method 'App.Test2<T1>'