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c#.netgenericsextension-methodsfluent-interface

No type inference with generic extension method


I have the following method:

public static TEventInvocatorParameters Until
    <TEventInvocatorParameters, TEventArgs>(this TEventInvocatorParameters p,
                                            Func<TEventArgs, bool> breakCond)
    where TEventInvocatorParameters : EventInvocatorParameters<TEventArgs>
    where TEventArgs : EventArgs
{
    p.BreakCondition = breakCond;
    return p;
}

And this class

public class EventInvocatorParameters<T>
    where T : EventArgs
{
    public Func<T, bool> BreakCondition { get; set; }
    // Other properties used below omitted for brevity.
}

Now, I have the following problems:

  1. This extension method shows on all types, even string.
  2. I can't write new EventInvocatorParameters<EventArgs>(EventABC).Until(e => false); It is telling me "The type arguments for method ... cannot be inferred from the usage."

Can't I use generic type parameters like this? How would you resolve this problem?
Important point: I need both of those generic parameters, because I need to return the same type this extension method was called on.


Broader picture (not necessary for answering the question!):
I am trying to create a fluent interface to invoking events. The base is this static class:

public static class Fire
{
   public static void Event<TEventArgs>(
       ConfiguredEventInvocatorParameters<TEventArgs> parameters)
    where TEventArgs : EventArgs
    {
        if (parameters.EventHandler == null)
        {
            return;
        }

        var sender = parameters.Sender;
        var eventArgs = parameters.EventArgs;
        var breakCondition = parameters.BreakCondition;

        foreach (EventHandler<TEventArgs> @delegate in 
                 parameters.EventHandler.GetInvocationList())
        {
            try
            {
                @delegate(sender, eventArgs);
                if (breakCondition(eventArgs))
                {
                    break;
                }
            }
            catch (Exception e)
            {
                var exceptionHandler = parameters.ExceptionHandler;
                if (!exceptionHandler(e))
                {
                    throw;
                }
            }
        }
    }
}

To make sure this method can only be called with fully configured parameters, it only accepts a ConfiguredEventInvocatorParameters<T> which derives from EventInvocatorParameters<T>:

public class ConfiguredEventInvocatorParameters<T>
    : EventInvocatorParameters<T>
    where T : EventArgs
{
    public ConfiguredEventInvocatorParameters(
        EventInvocatorParameters<T> parameters, object sender, T eventArgs)
        : base(parameters)
    {
        EventArgs = eventArgs;
        Sender = sender;
    }

    public T EventArgs { get; private set; }
    public object Sender { get; private set; }

}

The following would be valid calls:

Fire.Event(EventName.With(sender, eventArgs));
Fire.Event(EventName.With(sender, eventArgs).Until(e => e.Cancel));
Fire.Event(EventName.Until(e => e.Cancel).With(sender, eventArgs));

The following would be invalid:

// no sender or eventArgs have been specified, i.e. missing call to With(...)
Fire.Event(EventName.Until(e => e.Cancel));

To make this work, there exist extension methods named With, that accept either a EventHandler<TEventArgs or a TEventInvocatorParameters and return a ConfiguredEventInvocatorParameters<TEventArgs>. All calls following the With now also need to return the type ConfiguredEventInvocatorParameters<TEventArgs>, otherwise the second example of a valid call (with the Until at the end) wouldn't work.
If you have any thoughts on the API in general, please let me know. However, I want to avoid the following three things:

  • Fail only at runtime if the parameters have not been configured fully
  • Creating an inverse syntax like EventName.With(...).Until(...).Fire()
  • Use the infamous Do method to start off things: Fire(EventName).With(...).Until(...).Do();

Solution

  • For anyone interested, for now, I solved the original problem (fluent event invocation API) with a generic class hierarchy. This is basically Hightechrider's answer on steroids.

    public abstract class EventInvocatorParametersBase
        <TEventInvocatorParameters, TEventArgs>
        where TEventArgs : EventArgs
        where TEventInvocatorParameters :
            EventInvocatorParametersBase<TEventInvocatorParameters, TEventArgs>
    
    {
        protected EventInvocatorParametersBase(
            EventHandler<TEventArgs> eventHandler,
            Func<Exception, bool> exceptionHandler,
            Func<TEventArgs, bool> breakCondition)
        {
            EventHandler = eventHandler;
            ExceptionHandler = exceptionHandler;
            BreakCondition = breakCondition;
        }
    
        protected EventInvocatorParametersBase(
            EventHandler<TEventArgs> eventHandler)
            : this(eventHandler, e => false, e => false)
        {
        }
    
        public Func<TEventArgs, bool> BreakCondition { get; set; }
        public EventHandler<TEventArgs> EventHandler { get; set; }
        public Func<Exception, bool> ExceptionHandler { get; set; }
    
        public TEventInvocatorParameters Until(
            Func<TEventArgs, bool> breakCondition)
        {
            BreakCondition = breakCondition;
            return (TEventInvocatorParameters)this;
        }
    
        public TEventInvocatorParameters WithExceptionHandler(
            Func<Exception, bool> exceptionHandler)
        {
            ExceptionHandler = exceptionHandler;
            return (TEventInvocatorParameters)this;
        }
    
        public ConfiguredEventInvocatorParameters<TEventArgs> With(
            object sender, 
            TEventArgs eventArgs)
        {
            return new ConfiguredEventInvocatorParameters<TEventArgs>(
                EventHandler, ExceptionHandler, BreakCondition,
                sender, eventArgs);
        }
    }
    
    public class EventInvocatorParameters<T> :
        EventInvocatorParametersBase<EventInvocatorParameters<T>, T>
        where T : EventArgs
    {
        public EventInvocatorParameters(EventHandler<T> eventHandler)
            : base(eventHandler)
        {
        }
    }
    
    public class ConfiguredEventInvocatorParameters<T> :
        EventInvocatorParametersBase<ConfiguredEventInvocatorParameters<T>, T>
        where T : EventArgs
    {
        public ConfiguredEventInvocatorParameters(
            EventHandler<T> eventHandler,
            Func<Exception, bool> exceptionHandler,
            Func<T, bool> breakCondition, object sender,
            T eventArgs)
            : base(eventHandler, exceptionHandler, breakCondition)
        {
            EventArgs = eventArgs;
            Sender = sender;
        }
    
        public ConfiguredEventInvocatorParameters(EventHandler<T> eventHandler,
                                                  object sender,
                                                  T eventArgs)
            : this(eventHandler, e => false, e => false, sender, eventArgs)
        {
        }
    
        public T EventArgs { get; private set; }
        public object Sender { get; private set; }
    }
    
    public static class EventExtensions
    {
        public static EventInvocatorParameters<TEventArgs> Until<TEventArgs>(
            this EventHandler<TEventArgs> eventHandler,
            Func<TEventArgs, bool> breakCondition)
            where TEventArgs : EventArgs
        {
            return new EventInvocatorParameters<TEventArgs>(eventHandler).
                Until(breakCondition);
        }
    
        public static EventInvocatorParameters<TEventArgs> 
            WithExceptionHandler<TEventArgs>(
                this EventHandler<TEventArgs> eventHandler,
                Func<Exception, bool> exceptionHandler)
            where TEventArgs : EventArgs
        {
            return
                new EventInvocatorParameters<TEventArgs>(eventHandler).
                    WithExceptionHandler(exceptionHandler);
        }
    
        public static ConfiguredEventInvocatorParameters<TEventArgs>
            With<TEventArgs>(
                this EventHandler<TEventArgs> eventHandler, object sender,
                TEventArgs eventArgs)
            where TEventArgs : EventArgs
        {
            return new ConfiguredEventInvocatorParameters<TEventArgs>(
                eventHandler, sender, eventArgs);
        }
    }
    

    This allows you to write code like this:

    Fire.Event(EventName.WithExceptionHandler(e => false)
                        .Until(e => false).With(this, EventArgs.Empty));
    Fire.Event(EventName.With(this, EventArgs.Empty));
    Fire.Event(EventName.WithExceptionHandler(e => false)
                        .With(this, EventArgs.Empty).Until(e => false));
    Fire.Event(EventName.With(this, EventArgs.Empty)
                        .WithExceptionHandler(e => false).Until(e => false));
    

    But it doesn't allow you to write this, because not all necessary info (eventArgs and sender) has been provided:

    Fire.Event(EventName.Until(e => false));
    Fire.Event(EventName);