c# - Expression Trees and Invoking a Delegate

Question

So I have a delegate which points to some function which I don't actually know about when I first create the delegate object. The object is set to some function later.

I also then want to make an expression tree that invokes the delegate with an argument (for this question's sake the argument can be 5). This is the bit I'm struggling with; the code below shows what I want but it doesn't compile.

Func<int, int> func = null;
Expression expr = Expression.Invoke(func, Expression.Constant(5));

For this example I could do (this is practical since I need to build the expression trees at runtime):

Func<int, int> func = null;
Expression<Func<int>> expr = () => func(5);

This makes expr become:

() => Invoke(value(Test.Program+<>c__DisplayClass0).func, 5)

Which seems to mean that to use the delegate func, I need to produce the value(Test.Program+<>c__DisplayClass0).func bit.

So, how can I make an expression tree which invokes a delegate?

Answer 1

I think what you want to do is use the Target and Method properties of the delegate to pass to create a Call expression. Building on JulianR's sample, this is what it would look like:

Action<int> func = i => Console.WriteLine(i * i);

var callExpr = Expression.Call(Expression.Constant(func.Target), func.Method, Expression.Constant(5));

var lambdaExpr = Expression.Lambda<Action>(callExpr);
var fn = lambdaExpr.Compile();
fn();    //  Prints 25