c# - Fast creation of objects instead of Activator.CreateInstance(type)

Question

I'm trying to improve the performance of our application. We have a lot of Activator.CreateInstance calls that are causing some grief.

We instantiate a lot of classes based on an interface (ITabDocument) and after looking around I thought of using this code:

The code is no better (infact marginally slower) than using the Activator.CreateInstance code we had.

    public static Func<T> CreateInstance<T>(Type objType) where T : class, new()
    {
        var dynMethod = new DynamicMethod("DM$OBJ_FACTORY_" + objType.Name, objType, null, objType);
        ILGenerator ilGen = dynMethod.GetILGenerator();
        ilGen.Emit(OpCodes.Newobj, objType.GetConstructor(Type.EmptyTypes));
        ilGen.Emit(OpCodes.Ret);
        return (Func<T>)dynMethod.CreateDelegate(typeof(Func<T>));
    }

I'm wondering why this is, all I'm doing is:

ITabDocument document = CreateInstance<ITabDocument>(Type.GetType("[Company].Something"));

Is there a better way of creating objects which would assist with the above? Its a little hard when you're not sure of the concrete type.

Answer 1

I did some benchmarking between these (I would write down the bare minimum details):

public static T Instance() //~1800 ms
{
    return new T();
}

public static T Instance() //~1800 ms
{
    return new Activator.CreateInstance<T>();
}

public static readonly Func<T> Instance = () => new T(); //~1800 ms

public static readonly Func<T> Instance = () => 
                                 Activator.CreateInstance<T>(); //~1800 ms

//works for types with no default constructor as well
public static readonly Func<T> Instance = () => 
               (T)FormatterServices.GetUninitializedObject(typeof(T)); //~2000 ms


public static readonly Func<T> Instance = 
     Expression.Lambda<Func<T>>(Expression.New(typeof(T))).Compile();  
     //~50 ms for classes and ~100 ms for structs

As CD says compiled expression is the fastest, and by a big margin. All the methods except (T)FormatterServices.GetUninitializedObject(typeof(T)) work only for types with default constructor.

And caching the compiled resultant delegate is trivial when you have a static class per generic type. Like:

public static class New<T> where T : new()
{
    public static readonly Func<T> Instance = Expression.Lambda<Func<T>>
                                              (
                                               Expression.New(typeof(T))
                                              ).Compile();
}

Note the new constraint. Call anything

MyType me = New<MyType>.Instance();

Except for the first time the class is being loaded in memory, the execution is going to be fastest.

To have a class that handles both types with default constructor and without, I took a hybrid approach, from here:

public static class New<T>
{
    public static readonly Func<T> Instance = Creator();

    static Func<T> Creator()
    {
        Type t = typeof(T);
        if (t == typeof(string))
            return Expression.Lambda<Func<T>>(Expression.Constant(string.Empty)).Compile();

        if (t.HasDefaultConstructor())
            return Expression.Lambda<Func<T>>(Expression.New(t)).Compile();

        return () => (T)FormatterServices.GetUninitializedObject(t);
    }
}

public static bool HasDefaultConstructor(this Type t)
{
    return t.IsValueType || t.GetConstructor(Type.EmptyTypes) != null;
}

Will handle value types too in an efficient manner.

Note that (T)FormatterServices.GetUninitializedObject(t) will fail for string. Hence special handling for string is in place to return empty string.