Question

I was thinking about the difference between Expression<Func<>> and Func<>, and wondered if you could convert a static method to an expression tree as follows:

class Program
{
    static void Main(string[] args)
    {
        Func<int, int> t = x => hrm(x);
        Func<int, int> t2 = new Func<int, int>(hrm);

        // Works as expected:
        Expression<Func<int, int>> et = x => hrm(x);
        // Brokenness:
        Expression<Func<int, int>> et2 = new Func<int, int>(hrm);
    }

    static int hrm(int x)
    {
        return x + 9;
    }
}

What’s so special about the second “Func<>” that it can’t be converted to an Expression, when the first one can?

Answer 1

I think your confusion comes from the fact that lambdas can represent expressions or delegates (with the very same syntax) in C#. So this code:

x => hrm(x)

means something different depending on where it’s written. When assigned to Func<int, int>, it’s compiled as normal to create a Func<int, int> delegate. However, when assigned to an expression, the C# compiler defers compilation and the snippet is interpreted as an expression. Contrast this with new Func<int, int>(hrm), which always returns a Func<int, int> delegate.