How to return value with anonymous method?

Question

This fails

string temp = () => {return "test";};

with the error

Cannot convert lambda expression to type 'string' because it is not a delegate type

What does the error mean and how can I resolve it?

Answer 1

The problem here is that you've defined an anonymous method which returns a string but are trying to assign it directly to a string. It's an expression which when invoked produces a string it's not directly a string. It needs to be assigned to a compatible delegate type. In this case the easiest choice is Func<string>

Func<string> temp = () => {return "test";};

This can be done in one line by a bit of casting or using the delegate constructor to establish the type of the lambda followed by an invocation.

string temp = ((Func<string>)(() => { return "test"; }))();
string temp = new Func<string>(() => { return "test"; })();

Note: Both samples could be shorted to the expression form which lacks the { return ... }

Func<string> temp = () => "test";
string temp = ((Func<string>)(() => "test"))();
string temp = new Func<string>(() => "test")();

Answer 2

You are attempting to assign a function delegate to a string type. Try this:

Func<string> temp = () => {return "test";};

You can now execute the function thusly:

string s = temp();

The "s" variable will now have the value "test".

Answer 3

Using a little helper function and generics you can let the compiler infer the type, and shorten it a little bit:

public static TOut FuncInvoke<TOut>(Func<TOut> func)
{
    return func();
}

var temp = FuncInvoke(()=>"test");

Side note: this is also nice as you then are able to return an anonymous type:

var temp = FuncInvoke(()=>new {foo=1,bar=2});

Answer 4

you can use anonymous method with argument :

int arg = 5;

string temp = ((Func<int, string>)((a) => { return a == 5 ? "correct" : "not correct"; }))(arg);

Answer 5

An anonymous method can return a value using a func delegate. Here is an example where I have shown how to return a value using an anonymous method.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {


        static void Main(string[] args)
        {
            Func<int, int> del = delegate (int x)
              {
                  return x * x;

              };

            int p= del(4);
            Console.WriteLine(p);
            Console.ReadLine();
        }
    }
}

Answer 6

This is another example using C# 8 (could also work with other .NET versions supporting parallel tasks)

using System;
using System.Threading.Tasks;

namespace Exercise_1_Creating_and_Sharing_Tasks
{
    internal static class Program
    {
        private static int TextLength(object o)
        {
            Console.WriteLine($"Task with id {Task.CurrentId} processing object {o}");
            return o.ToString().Length;
        }

        private static void Main()
        {
            const string text1 = "Welcome";
            const string text2 = "Hello";

            var task1 = new Task<int>(() => TextLength(text1));
            task1.Start();

            var task2 = Task.Factory.StartNew(TextLength, text2);

            Console.WriteLine($"Length of '{text1}' is {task1.Result}");
            Console.WriteLine($"Length of '{text2}' is {task2.Result}");

            Console.WriteLine("Main program done");
            Console.ReadKey();
        }
    }
}