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default Keyword in Generic Code (C# Programming Guide)

In generic classes and methods, one issue that arises is how to assign a default value to a parameterized type T when you do not know the following in advance:

  • Whether T will be a reference type or a value type.

  • If T is a value type, whether it will be a numeric value or a struct.

Given a variable t of a parameterized type T, the statement t = null is only valid if T is a reference type and t = 0 will only work for numeric value types but not for structs. The solution is to use the default keyword, which will return null for reference types and zero for numeric value types. For structs, it will return each member of the struct initialized to zero or null depending on whether they are value or reference types. The following example from the GenericList<T> class shows how to use the default keyword. For more information, see Generics Overview.

public class GenericList<T>
{
    private class Node
    {
        //...

        public Node Next;
        public T Data;
    }

    private Node head;

    //...

    public T GetNext()
    {
        T temp = default(T);

        Node current = head;
        if (current != null)
        {
            temp = current.Data;
            current = current.Next;
        }
        return temp;
    }
}

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