Dynamically calling methods corresponding a parameter's type using expression trees in c#

Question

I\'m building an event handler which will work similarly to how aggregates behave in event sourced systems.

What I\'m trying to achieve can be done in ways as documented

Answer 1

You can treat lambda functions like a regular static methods. This mean you should pass additional parameter to it (Aggregate in your case). In other words you need to create lambda, that type looks like Action.

Change declaration of your _handlers to

private readonly IDictionary> _handlers
  = new Dictionary>();

Now you can write AggregateBase constructor like this:

var methods = this.GetType().GetMethods()
    .Where(p => p.Name == handleMethodName
                && p.GetParameters().Length == 1);

var runnerParameter = Expression.Parameter(typeof(AggregateBase), "r");
var commonEventParameter = Expression.Parameter(typeof(Event), "e");

foreach (var method in methods)
{
    var eventType = method.GetParameters().Single().ParameterType;

    var body = Expression.Call(
        Expression.Convert(runnerParameter, GetType()),
        method,
        Expression.Convert(commonEventParameter, eventType)
      );

    var lambda = Expression.Lambda>(
      body, runnerParameter, commonEventParameter);

    _handlers.Add(eventType, lambda.Compile());
}

EDIT: Also you need to change invocation in Apply method:

public void Apply(Event @event)
{
    var type = @event.GetType();
    if (_handlers.ContainsKey(type))
        _handlers[type](this, @event);
}