Instantiating a class using Reflection

问题:

Suppose I have three projects in my sln.

(1) xyz.a{Class Lib}{no reference added}
(2) yzx.b{Class Lib}{added the reference of xyz.a}
(3) zxy.c{Console App}{added the reference of xyz.a}

Now, I need to create the instance of a class residing in yzx.b from within xyz.a using reflection.

And also this should be independent of the folder/directory-names.

I.e. even If I change the name of the directory of yzx.b, it should work.

Does anyone have any idea?

回答1:

First of all, Activator.CreateInstance() is a right way.

But, there is a more interesting way that is:

• 10 times faster
• Don't wrap exceptions in TargetInvocationException

Just create expression that calls constructor:

public static Func<object[], object> CreateConstructorDelegate(ConstructorInfo method)
{
        var args = Expression.Parameter(typeof(object[]), "args");

        var parameters = new List<Expression>();

        var methodParameters = method.GetParameters().ToList();
        for (var i = 0; i < methodParameters.Count; i++)
        {
            parameters.Add(Expression.Convert(
                               Expression.ArrayIndex(args, Expression.Constant(i)),
                               methodParameters[i].ParameterType));
        }

        var call = Expression.Convert(Expression.New(method, parameters), typeof(object));

        Expression body = call;

        var callExpression = Expression.Lambda<Func<object[], object>>(body, args);
        var result = callExpression.Compile();

        return result;
}

Performance test:

    public void activator()
    {
        var stopwatch = new Stopwatch();
        const int times = 10000000;

        stopwatch.Start();
        for (int i = 0; i < times; i++)
        {
            var v = Activator.CreateInstance(typeof (C));
        }
        stopwatch.Stop();

        Console.WriteLine(stopwatch.ElapsedMilliseconds + "ms with activator");

        var del = CreateConstructorDelegate(typeof(C).GetConstructor(new Type[0]));

        stopwatch = new Stopwatch();
        stopwatch.Start();

        var args = new object[0];

        for (int i = 0; i < times; i++)
        {
            var v = del(args);
        }

        stopwatch.Stop();

        Console.WriteLine(stopwatch.ElapsedMilliseconds + "ms with expression");
    }

Output:

1569ms with activator
134ms with expression

But:

• C# 3.0 only
• Complile() is long running operation

Just for curious.

回答2:

You might want to check out the Activator.CreateInstance() methods. Just pass it the name of the assembly and type.

If you don't have a compile-time reference to the assembly, you can still reference it at runtime with Assembly.Load().

回答3:

You can use Activator.CreateInstance to create an instance easily (this also does various caching of reflection information to make repeated calls faster), or Type.GetConstructor if you want to reflect over the constructor itself as well as directly running it (via ConstructorInfo.Invoke)