How to get value of Expression<Action<T>> action quickly?

Question

Suppose we have the following code:

ExpressionHelper.GetRouteValuesFromExpression<AccountController>(ax => ax.MyAction("a", "b"));

(from ASP.NET MVC Futures assembly). Method is reasonably fast - it executes 10k iterations in 150ms.

Now, we change code to this:

string a = "a";
string b = "b";
ExpressionHelper.GetRouteValuesFromExpression<AccountController>(ax => ax.MyAction(a, b));

This code will execute 10k iterations in 15 seconds

The problem is the following code:

Expression<Func<object>> lambdaExpression = Expression.Lambda<Func<object>>(Expression.Convert(arg, typeof (object)));

Func<object> func = lambdaExpression.Compile();

value = func()

Is there a better way to get value from expression than compiling expression every time? This can greatly affect ASP.NET MVC link generation speed.

Answer 1

Why don't you just cache the value of the expression and its compiled value locally if this is such a bottleneck? I imagine a simply Dictionary could do the trick:

Dictionary<Expression<Action<T>>, Action<T>> m_Cache =
    new Dictionary<Expression<Action<T>>, Action<T>>();

public void GetRouteValuesFromExpression<T>(Expression<Action<T>> expr) {
    Action<T> compiled = null;
    if (!m_Cache.TryGetValue(expr, ref compiled)) {
        compiled = expr.Compile();
        m_Cached.Add(expr, compiled);
    }
    // execute …
}

Answer 2

Does it have to be a Func<object>? You could probably manually craft a "capture" - i.e. have a type that declares a & b; have a Func<Whatever, object>, and compile this to a delegate. Then all you do at runtime is:

Foo foo = new Foo {A = a, B = b};
return cachedFunc(foo);

I'm not quite sure what the Convert(blah, typeof(object)) is doing - can you clarify this? I've got quite a bit of experience with expressions, but this seems... unusual...

Answer 3

I've fiddled for a bit and came up with the following:

var body = (MethodCallExpression)expr.Body;
var arg1 = (MemberExpression)body.Arguments[0];
var contextType = arg1.Member.DeclaringType;
var field = contextType.GetField(arg1.Member.Name);
Console.WriteLine(field.GetValue(…));

Assuming that expr is your Expression<Action<T>> argument, this gives you the reflected field which is passed as the first argument to your call (a in your case). However, I wasn't able to extract the context necessary to evaluate this field (last line, location marked by “…”). I believe this context cannot be accessed without compilation of the expression. As a consequence, what you want isn't possible.

Please prove me wrong. ;-)

(Actually, I'm not really sure because even using the Reflector I'm unable to find where the execution context is being stored so I might overlook something.)