Question

I seem to be having trouble executing a lambda expression that I’ve previously assigned to a variable. Here’s a small C# example program I’ve put together:

public class Program
{
    public static void Main(string[] args)
    {
        int[] notOrdered = { 3, 2, 5, 8, 1, 4, 7, 9, 6 };
        Print(notOrdered);
        
        IEnumerable<int> ascOrdered = Order(notOrdered, true);
        Print(ascOrdered);

        IEnumerable<int> descOrdered = Order(notOrdered, false);
        Print(descOrdered);
    }
    
    static IEnumerable<T> Order<T>(IEnumerable<T> enumerables, bool ascending)
    {
        Expression<Func<T, object>> selector = (z) => z; // simple for demo purposes; pretend it's complex
        if (ascending)
            return enumerables.OrderBy(z => selector);
        else
            return enumerables.OrderByDescending(z => selector);
    }
    
    static void Print<T>(IEnumerable<T> enumerables)
    {
        foreach(T enumerable in enumerables)
            Console.Write(enumerable.ToString() + " ");
        Console.WriteLine();
    }
}

I want it to produce this output:

3 2 5 8 1 4 7 9 6

1 2 3 4 5 6 7 8 9

9 8 7 6 5 4 3 2 1

But, confusingly, it produces this output:

3 2 5 8 1 4 7 9 6

3 2 5 8 1 4 7 9 6

3 2 5 8 1 4 7 9 6

Basically, I just want to be able to pass the same expression to the two different ordering operations without having to type it out twice, hence why I assign it to selector beforehand. I have a real-world use case where the lambda expression is really long/messy and I don’t want to duplicate the mess, I’d rather just refer to a variable like I have here.

So, a) what is causing the current output? b) how can I get the output that I want?

Answer 1

In your code you don’t use the expression.

Your code is like:

object selector = new object();
if (ascending)
    return enumerables.OrderBy(z => selector);
else
    return enumerables.OrderByDescending(z => selector);

Your code should be without expression:

Func<T, object> selector = (z) => z;
if (ascending)
    return objects.OrderBy(selector);
else
    return objects.OrderByDescending(selector);

However, if you really want an expression (for test purpose or anything else), compile it before:

Expression<Func<T, object>> selector = (z) => z;
var compiledExpression = selector.Compile();
if (ascending)
    return objects.OrderBy(compiledExpression);
else
    return objects.OrderByDescending(compiledExpression);