Question

Going from a lambda to an Expression is easy using a method call…

public void GimmeExpression(Expression<Func<T>> expression)
{
    ((MemberExpression)expression.Body).Member.Name; // "DoStuff"
}

public void SomewhereElse()
{
    GimmeExpression(() => thing.DoStuff());
}

But I would like to turn the Func in to an expression, only in rare cases…

public void ContainTheDanger(Func<T> dangerousCall)
{
    try 
    {
        dangerousCall();
    }
    catch (Exception e)
    {
        // This next line does not work...
        Expression<Func<T>> DangerousExpression = dangerousCall;
        var nameOfDanger = 
            ((MemberExpression)dangerousCall.Body).Member.Name;
        throw new DangerContainer(
            "Danger manifested while " + nameOfDanger, e);
    }
}

public void SomewhereElse()
{
    ContainTheDanger(() => thing.CrossTheStreams());
}

The line that does not work gives me the compile-time error Cannot implicitly convert type 'System.Func<T>' to 'System.Linq.Expressions.Expression<System.Func<T>>'. An explicit cast does not resolve the situation. Is there a facility to do this that I am overlooking?

Answer 1

Ooh, it’s not easy at all. Func<T> represents a generic delegate and not an expression. If there’s any way you could do so (due to optimizations and other things done by the compiler, some data might be thrown away, so it might be impossible to get the original expression back), it’d be disassembling the IL on the fly and inferring the expression (which is by no means easy). Treating lambda expressions as data (Expression<Func<T>>) is a magic done by the compiler (basically the compiler builds an expression tree in code instead of compiling it to IL).

Related fact

This is why languages that push lambdas to the extreme (like Lisp) are often easier to implement as interpreters. In those languages, code and data are essentially the same thing (even at run time), but our chip cannot understand that form of code, so we have to emulate such a machine by building an interpreter on top of it that understands it (the choice made by Lisp like languages) or sacrificing the power (code will no longer be exactly equal to data) to some extent (the choice made by C#). In C#, the compiler gives the illusion of treating code as data by allowing lambdas to be interpreted as code (Func<T>) and data (Expression<Func<T>>) at compile time.