Generic class and Constraint

Question

I am working with Generic class and Constraint. Following is my class.

public class GenericList<T> where T : new()
    {

        private Node head;

        // constructor 
        public GenericList()
        {
            head = null;
        }

    }

when i create object with integer it works fine

GenericList<int> list = new GenericList<int>();

But when i try with string it gives me following compile time error.

GenericList<string> list1 = new GenericList<string>();

'string' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'GenericTest.GenericList' also when i passed reference parameter like any custom class it works fine.

What is the problem with string?

Konstantin has explained why it's failing - but why do you have the constraint on T to start with? When do you want to call new T()? That's all the constraint enables. — Jon Skeet
None of that answers why you'd want the parameterless constructor constraint. Again, when do you want to call new T() within GenericList? — Jon Skeet

horgh horgh · Accepted Answer · 2012-11-30T07:04:13

String Class does not have a public parameterless constructor..that is why the new() constraint is not applicable to it.

Read Constraints on Type Parameters (C# Programming Guide):

where T : new()

The type argument must have a public parameterless constructor. When used together with other constraints, the new() constraint must be specified last.

Generic class and Constraint

2 Answers