Question

I just ran into a problem involving Expressions.

In my class<T> have a field

Dictionary<Expression, ProjectedCollection> mCache;

where both Expression and ProjectedCollection cannot be specified as Expression<T, S> and ProjectedCollection<S> because the S will be different things at runtime:

void AddSomething<S>(Expression<Func<T, S>> projection)
{
  if (!mCache.ContainsKey(projection))
  {
    var runnable = projection.Compile();
    var allProjected = from elm in mList
                       select runnable(elm);

    mCache.Add(projection, new ProjectedCollection<S>(allProjected));
  }
}

---

Now at some point where I don’t know S, I want to iterate over everything in my cache and apply the expression to a new thing.

foreach (KeyValuePair<Expression, ProjectedCollection> keyValuePair in mCache)
{
  // Want something like
  var func = keyValuePair.Key.Compile();
  keyValuePair.Value.SignalAdd(func(newThing));
}

But the Compile() method is not available for the un-typed Expression. And casting is also not possible without knowing S.

Does anybody have an idea how to tackle this?

Answer 1

Cast to a LambdaExpression and call Compile on it. It will return an untyped delegate. You can…

• …cast this delegate to one of the Func/Action types
• …inspect its structure using reflection
• …use Delegate.DynamicInvoke to call it