a constructor as a delegate – is it possible in C#?

I’m assuming you would normally do something like this as part of a factory implementation, where the actual types aren’t known at compile-time…

First, note that an easier approach may be a post-create init step, then you can use generics:

static T Create<T>({args}) where T : class, ISomeInitInterface, new() {
    T t = new T();
    return t;

You can then use MakeGenericMethod and/or CreateDelegate.

Otherwise; you can do this with on the fly with Expression (3.5) or DynamicMethod (2.0).

The Expression approach is easier to code:

    var param = Expression.Parameter(typeof(int), "val");
    var ctor = typeof(Foo).GetConstructor(new[] { typeof(int) });
    var lambda = Expression.Lambda<Func<int, Foo>>(
        Expression.New(ctor, param), param);
    var func = lambda.Compile();
    Foo foo = func(123);
    string s = foo.ToString(); // proof

or (using DynamicMethod):

    ConstructorInfo ctor = typeof(Foo).GetConstructor(new[] { typeof(int) });
    DynamicMethod dm = new DynamicMethod("Create", typeof(Foo),
            new Type[] { typeof(int) }, typeof(Foo), true);
    ILGenerator il = dm.GetILGenerator();
    il.Emit(OpCodes.Newobj, ctor);
    Converter<int, Foo> func = (Converter<int, Foo>)
        dm.CreateDelegate(typeof(Converter<int, Foo>));        
    Foo foo = func(123);
    string s = foo.ToString(); // proof

Leave a Comment