Multiple return values

Methods in C# return only one value. This value can be an object with multiple fields. Several options are available for returning multiple values from a method.

If we need to return multiple values, we can use out parameters or return a type instance (like a tuple) containing all the values. We benchmark and compare approaches.

Example

There are many ways to return multiple parameters in the C# language. Modern versions of C# provide tuple syntax which are probably the most elegant.

using System;
using System.Collections.Generic;

class Program
{
    static void GetTwoNumbersA(out int number1, out int number2)
    {
        // Part 1: assign out parameters.
        number1 = 1;
        number2 = 2;
    }

    static KeyValuePair<int, int> GetTwoNumbersB()
    {
        // Part 2: return KeyValuePair.
        return new KeyValuePair<int, int>(1, 2);
    }

    static (int, int) GetTwoNumbersC()
    {
        // Part 3: return tuple.
        return (1, 2);
    }

    static Tuple<int, int> GetTwoNumbersD()
    {
        // Part 4: return Tuple class.
        return new Tuple<int, int>(1, 2);
    }

    static void Main()
    {
        // Part 5: test all the methods and ensure their output is identical.
        GetTwoNumbersA(out int number1, out int number2);
        Console.WriteLine($"{number1} {number2}");

        var resultB = GetTwoNumbersB();
        Console.WriteLine($"{resultB.Key} {resultB.Value}");

        var resultC = GetTwoNumbersC();
        Console.WriteLine($"{resultC.Item1} {resultC.Item2}");

        var resultD = GetTwoNumbersD();
        Console.WriteLine($"{resultD.Item1} {resultD.Item2}");
    }
}
1 2
1 2
1 2
1 2

Benchmark

There are multiple ways to return multiple values from a method. The most common ones are probably out-parameters and KeyValuePair and tuples.

using System;
using System.Collections.Generic;
using System.Diagnostics;

class Program
{
    static void GetTwoNumbersA(out int number1, out int number2)
    {
        number1 = 1;
        number2 = 2;
    }

    static KeyValuePair<int, int> GetTwoNumbersB()
    {
        return new KeyValuePair<int, int>(1, 2);
    }

    static (int, int) GetTwoNumbersC()
    {
        return (1, 2);
    }

    static void Main()
    {
        const int max = 100000000;
        var s1 = Stopwatch.StartNew();
        // Version 1: use 2 out parameters.
        for (int i = 0; i < max; i++)
        {
            int a;
            int b;
            GetTwoNumbersA(out a, out b);
            if (a + b != 3)
            {
                throw new Exception();
            }
        }
        s1.Stop();
        var s2 = Stopwatch.StartNew();
        // Version 2: return KeyValuePair.
        for (int i = 0; i < max; i++)
        {
            var pair = GetTwoNumbersB();
            int a = pair.Key;
            int b = pair.Value;
            if (a + b != 3)
            {
                throw new Exception();
            }
        }
        s2.Stop();
        var s3 = Stopwatch.StartNew();
        // Version 3: return tuple.
        for (int i = 0; i < max; i++)
        {
            var tuple = GetTwoNumbersC();
            int a = tuple.Item1;
            int b = tuple.Item2;
            if (a + b != 3)
            {
                throw new Exception();
            }
        }
        s3.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s3.Elapsed.TotalMilliseconds * 1000000) / max).ToString("0.00 ns"));
    }
}
1.90 ns    out
7.59 ns    KeyValuePair
7.64 ns    tuple

Summary

Multiple values can be returned from a method invocation. We used out to pass parameters by reference. With KeyValuePair and Tuple we returned multiple values in a container.