如何传递对象类型以调用泛型方法

Question

My generic method is this

public static class AppContainerInstaller
{
    public static IMessageHandler<T> Resolve<T>() 
    {
        return _container.Resolve<IMessageHandler<T>>();
    }
}

I am trying to call this generic method from a different class

public static string ProcessMessage(IMessage message)
{
    messageHandler = AppContainerInstaller.Resolve<message.GetType()>();
    // THe previous line throws an error.
    return messageHandler.Handle(message);
}

Can anyone tell me how to pass the type of message to a generic method?

Answer 1

You have to do this:

messageHandler = AppContainerInstaller.Resolve<IMessage>();

Resolve method is the one who should get the type being passed in T. You see, when using generics, you must provide an actual type.And you probably want to change Resolve to get a T parameter, so you can pass message . Something like this:

public static IMessageHandler<T> Resolve<T>(T param) 

and you call it like this:

messageHandler = AppContainerInstaller.Resolve<IMessage>(message);

Answer 2

There are two ways to get the runtime type: pass the message as a parameter or use reflection.

Modifying the method:

public static IMessageHandler<T> Resolve<T>(T unused) 
{
    return _container.Resolve<IMessageHandler<T>>();
}

Which will allow runtime type lookup:

// Cast to dynamic to force runtime type
messageHandler = AppContainerInstaller.Resolve((dynamic) message);

You can also to use reflection although there is a performance penalty.

private static readonly MethodInfo ResolveMethodInfo
    = typeof(AppContainerInstaller).GetMethod("Resolve", BindingFlags.Public | BindingFlags.Static);

// Assuming there exists a non-generic interface IMessageHandler
private readonly ConcurrentDictionary<Type, Func<IMessageHandler>> _methodCache
    = new ConcurrentDictionary<Type, Func<IMessageHandler>>();

public static string ProcessMessage(IMessage message)
{
    messageHandler = Resolve(message.GetType());
    return messageHandler.Handle(message);
}

private IMessageHandler Resolve(Type type)
{
    // Cache the generated method to avoid repeated reflection penalty.
    var resolve = _methodCache.GetOrAdd(type, () => ResolveMethodInfo.MakeGenericMethod(type));
    return resolve();
}