Why am I getting an Argument exception when creating event handler dynamically?

Question

Good day all

I wrote the following method:

private void RegisterEvent(object targetObject, string eventName, string methodName)
{
    EventInfo eventInfo = targetObject.GetType().GetEvent(eventName);
    MethodInfo method = eventInfo.EventHandlerType.GetMethod("Invoke");
    IEnumerable<Type> types = method.GetParameters().Select(param => param.ParameterType);

    DynamicMethod dynamicMethod = new DynamicMethod(eventInfo.EventHandlerType.Name, typeof (void), types.ToArray(), typeof (QueryWindow));
    MethodInfo methodInfo = typeof (QueryWindow).GetMethod(methodName, new[] { typeof (object) });

    ILGenerator ilGenerator = dynamicMethod.GetILGenerator(256);
    ilGenerator.Emit(OpCodes.Ldarg_1);
    ilGenerator.EmitCall(OpCodes.Call, methodInfo, null);

    dynamicMethod.DefineParameter(1, ParameterAttributes.In, "sender");
    dynamicMethod.DefineParameter(2, ParameterAttributes.In, "e");

    // Get an argument exception here
    Delegate methodDelegate = dynamicMethod.CreateDelegate(eventInfo.EventHandlerType, this);
    eventInfo.AddEventHandler(targetObject, methodDelegate);
}

I get ArgumentException with the message

Error binding to target method.

in the line

Delegate methodDelegate = dynamicMethod.CreateDelegate(eventInfo.EventHandlerType, this); 

Could anyone point out on my mistake?

Thanks in advance.

Answer 1

When calling DynamicMethod.CreateDelegate, you should not pass a target parameter.

Edit:

I think you would also have to make the first parameter = 0, and change the codegen accordingly.

Answer 2

dynamicMethod.CreateDelegate(eventInfo.EventHandlerType, this);

The this argument cannot be correct. That references your class, the one that generates the dynamic type. Surely you need targetObject instead.

Answer 3

Assuming that methodName is a static method of QueryWindow, this should work:

private static void RegisterEvent(object targetObject, string eventName, string methodName)
{
    var eventInfo = targetObject.GetType().GetEvent(eventName);
    var method = eventInfo.EventHandlerType.GetMethod("Invoke");
    var types = method.GetParameters().Select(param => param.ParameterType);

    var methodInfo = typeof(QueryWindow).GetMethod(methodName, new[] { typeof(object) });

    // replaced typeof(void) by null      
    var dynamicMethod = new DynamicMethod(eventInfo.EventHandlerType.Name, null, types.ToArray(), typeof(QueryWindow));

    ILGenerator ilGenerator = dynamicMethod.GetILGenerator(256);

    // Using Ldarg_0 to pass the sender to methodName ; Ldarg_1 to pass the event args
    ilGenerator.Emit(OpCodes.Ldarg_0);
    ilGenerator.EmitCall(OpCodes.Call, methodInfo, null);

    // Added return
    ilGenerator.Emit(OpCodes.Ret); 

    // Removed parameter definition (implicit from DynamicMethod constructor)

    // Removed the target argument
    var methodDelegate = dynamicMethod.CreateDelegate(eventInfo.EventHandlerType);
    eventInfo.AddEventHandler(targetObject, methodDelegate);
}

Edit :

Since you can use .NET 3.5, you should create an expression tree. Here is another solution:

public class QueryWindow
{
    public void RegisterEvent(object targetObject, string eventName, string methodName)
    {
        var eventInfo = targetObject.GetType().GetEvent(eventName);
        var sender = Expression.Parameter(typeof (object), "sender");
        var e = Expression.Parameter(typeof (EventArgs), "e");
        var body = Expression.Call(Expression.Constant(this), methodName, null, e);
        var lambda = Expression.Lambda(eventInfo.EventHandlerType, body, sender, e);
        eventInfo.AddEventHandler(targetObject, lambda.Compile() );
    }

    public void OnEvent(object o)
    {
        Console.WriteLine(o);
    }
}

Notice that the OnEvent method is no longer static. I also assume that events you are trying to subscribe to are events that follow .NET conventions (sender + event args). This way, we can leverage contravariance and always pass a lambda of type :

(object sender, EventArgs e) => { /* */ }