Question

I have made a little CQRS API for our system
I tried to replace some reflection code by using the dynamic keyword but it does not work

Each command handler is Generic CommandHandler<TCommand> with a Method void Execute(TCommand Command)

With reflection it works

public void Invoke(Contracts.Commands.Command command)
{
    var handlerType = types[command.GetType()];

    var handler = kernel.Get(handlerType);
    var method = handlerType.GetMethod("Execute");
    method.Invoke(handler, new object[] { command });
}

Kernel.Get is the untyped version of kernel.Get<T> in our IoC (Ninject). This works and the generic method Execute of T fires

This code fails with a argument mismatch exception

public void Invoke(Contracts.Commands.Command command)
{
    var handlerType = types[command.GetType()];

    dynamic handler = kernel.Get(handlerType);
    handler.Execute(command);
}

If i statically declare the type it works with dynamic

dynamic handler = new TestCommandHandler();
handler.Execute(new TestCommand());

edit: Some more info to answer questions in comments

• handlerType is a concrete class that implements the abstract class CommandHandler<TCommand> where TCommand : Command
• The execute method is declared in the abstract class as public virtual void Execute(TCommand command)
• TestCommand implements the abstract Class Command

• Strange thing is that the same Handler and command works if I statically declare them strongly typed, last example (In the real world example I have dependencies in the constructor though

  edit2

• Stacktrace

at CallSite.Target(Closure , CallSite , Object , Command ) at
System.Dynamic.UpdateDelegates.UpdateAndExecuteVoid2[T0,T1](CallSite
site, T0 arg0, T1 arg1) at
XXX.Business.Command.CommandHandlerInvoker.Invoke(Command
command) in
C:\XXX.Business\Command\CommandHandlerInvoker.cs:line
29 at XXX.Web.XXXService.Execute(Command command) in
C:\XXX.Web\ExfiService.svc.cs:line 29 at
XXX.Web.Controllers.ComplianceController.XXX(XXXViewModel
viewModel) in
C:\XXX.Web\Controllers\XXXController.cs:line
52 at lambda_method(Closure , ControllerBase , Object[] ) at
System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase
controller, Object[] parameters) at
System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext
controllerContext, IDictionary2 parameters) at
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext
controllerContext, ActionDescriptor actionDescriptor, IDictionary2
parameters) at
System.Web.Mvc.ControllerActionInvoker.<>c_DisplayClass15.b_12()
at
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter
filter, ActionExecutingContext preContext, Func`1 continuation)

• Exception

The best overloaded method match for
XXX.Business.Command.CommandHandler<XXX.Contracts.Commands.TestCommand>.Execute(XXX.Contracts.Commands.TestCommand)' has some invalid arguments

Error message is stating the abstract class of T not the concrete class, but handlerType in above code is the concrete class

Answer 1

You need to change your code to look like this:

public void Invoke(Contracts.Commands.Command command)
{
    var handlerType = types[command.GetType()];

    dynamic handler = kernel.Get(handlerType);
    dynamic cmd = command;
    handler.Execute(cmd);
}

Notice that the command parameter is first assigned to a dynamic local variable (cmd). This allows the type of cmd to be evaluated at runtime and keeps the call to Execute truly dynamic; where as NOT doing it this way, the Execute method is assumed to have a fixed signature of Execute(Contracts.Commands.Command command)