#2782

An alternative to the C# generic new() constraint

Here’s something I came up with other the weekend while playing with some code…

I wanted to write a method that ensured that some object of type T was always available.

ModelElement element = EnsureElementExists<ModelElement>(_store);

Unfortunately in this case the ModelElement class does not have a parameterless constructor. This means I can’t use the where T:New() constraint and then just create a new instance in my generic method. What I can do is use a delegate to provide a new instance of the object if I need it:

public delegate T ConstructorDelegate<T>() where T : class; 

public static T EnsureElementExists<T>(Store store, ConstructorDelegate<T> ctor)
    where T : ModelElement
{
    T element = store.GetElement<T>(); 

    if (element == null)
        element = ctor(); 

    return element;
}

This means I can now write code like this where DiagramElement (a class derived from ModelElement) is created by the delegate:

DiagramElement diagramElement = EnsureElementExists<DiagramElement>(_store,
    delegate()
    {
        return new DiagramElement(_store);
    });

Calls to EnsureElementExists can use the anonymous delegate to create a new DiagramElement with a parameterized constructor.