C# Multiple Return Values

Methods return only one value. This value can be a type instance with multiple fields. If you need to return multiple values from a method, you can use out parameters or return a type instance containing all the values. These approaches require different syntax forms. They result in different performance metrics.

This C# article shows how to return multiple values from a method.

Example

We first show an example program that demonstrates ways you can use multiple return values. The two ways of returning multiple values are using output parameters, which are formal parameters modified by the out keyword, and allocating a KeyValuePair instance to store the result values. Instead of KeyValuePair, you could a custom class or the Tuple type from the .NET Framework 4.0.

Program that uses multiple return values [C#]

using System;
using System.Collections.Generic;

class Program
{
    static void GetTwoNumbers(out int number1, out int number2)
    {
	number1 = (int)Math.Pow(2, 2);
	number2 = (int)Math.Pow(3, 2);
    }

    static KeyValuePair<int, int> GetTwoNumbers()
    {
	return new KeyValuePair<int, int>((int)Math.Pow(2, 2), (int)Math.Pow(3, 2));
    }

    static void Main()
    {
	// Use out parameters for multiple return values.
	int value1;
	int value2;
	GetTwoNumbers(out value1, out value2);
	Console.WriteLine(value1);
	Console.WriteLine(value2);

	// Use struct for multiple return values.
	var pair = GetTwoNumbers();
	Console.WriteLine(pair.Key);
	Console.WriteLine(pair.Value);
    }
}

Output

4
9
4
9

Out modifier. The program contains three methods: two alternate implementations of the GetTwoNumbers method, and the Main entry point, where control flow begins. The first version of the GetTwoNumbers method uses two output parameters, which are described with the out modifier. This version assigns the storage locations passed in by the out parameters in the method body.

KeyValuePair. The program also describes an alternate GetTwoNumbers method, which is ordered second in the lexical source. This version returns a KeyValuePair generic struct instance instead of using output parameters. The two numbers are specified as the key and value of the struct and then this struct is returned.

Performance

We next consider the performance aspects of returning multiple values. The first implementation, which uses out parameters, was found to be somewhat slower than the second, which uses the KeyValuePair. In this benchmark, inlining might account for why out parameters were slower: the compiler might be less likely to inline a method that uses out parameters. The KeyValuePair is allocated on the stack, so it ends up being faster. The Tuple, meanwhile, is allocated on the heap which ends up being much slower.

Program that benchmarks multiple return values [C#]

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 Tuple<int, int> GetTwoNumbersC()
    {
	return new Tuple<int, int>(1, 2);
    }

    static void Main()
    {
	const int max = 100000000;
	var s1 = Stopwatch.StartNew();
	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();
	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();
	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"));
	Console.Read();
    }
}

Result

1.64 ns [out]
0.32 ns [KeyValuePair]
5.40 ns [Tuple]

Summary

There are several ways you can return multiple values from a method invocation in the C# language. We described the use of the out modifier to pass parameters by reference, which allows you to modify the original storage location. We also explored the use of the KeyValuePair struct as a way of returning multiple values from a single method call.