NET: Generic Constraint Type - Default Constructor Constraint Type

Default Constructor constraint allow you to specify that the type parameter must have a default/parameterless constructor. This is useful if you want to create a new object instance of the type argument.

public class DefaultConstructorGenericType<T> where T : new()
{
    public T Value;
 
    public DefaultConstructorGenericType()
    {
        Value = new T();
    }
}
 
public class ClassWithDefaultCon
{
    public ClassWithDefaultCon() {}
}

public class ClassB
{
    public int a;
    public ClassB(int v) { a = v;}
}  
 
static void InstantiateDefault()
{
    // This code will compile fine.
    DefaultConstructorGenericType<ClassWithDefaultCon> vt =
        new DefaultConstructorGenericType<ClassWithDefaultCon>();
 
    // This code will give compile error 
    // because ClassB does not have default constructor
    DefaultConstructorGenericType<ClassB> vt =
        new DefaultConstructorGenericType<ClassB>();
}

In the above sample, DefaultConstructorGenericType can only accept a type argument that has a default constructor. where keyword is used to specify a generic constraint in C#. In the above code sample, I specify that type parameter T has to have default constructor (indicate by where T : new()).