C# Multiple Return ValuesReturn multiple values from a method. Use out and ref parameters and KeyValuePair.
Multiple return values. Methods return only one value. This value can be an object with multiple fields. Many options are available for returning multiple values from a method.
Out keyword. If you need to return multiple values, you can use out parameters or return a type instance containing all the values. We benchmark and compare approaches.
An example. There are 2 ways of returning multiple values using output parameters. We can use formal parameters modified by the out keyword.
Or We can allocate a KeyValuePair instance to store the result values. A Tuple (or ValueTuple) can also be used.
Info The first GetTwoNumbers() uses 2 output parameters. It assigns the storage locations (for the out parameters) in the method body.
Next The second GetTwoNumbers() returns a KeyValuePair generic struct instance instead of using output parameters.
KeyValuePair The 2 numbers are specified as the key and value of the struct. And this struct is returned.
C# program that uses multiple return values
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); } }
4 9 4 9
ValueTuple example. We can use a ValueTuple to return multiple values from a method. Consider this program. We must ensure the ValueTuple package is included before using it.
Performance The ValueTuple has excellent performance. When available, this type should be used—it is used in a similar way to Tuple.
C# program that uses ValueTuple
using System; class Program { static (int, int) GetTwoNumbers() { // Return 2 numbers in a ValueTuple. return (10, 20); } static void Main() { var result = GetTwoNumbers(); Console.WriteLine(result.Item1); Console.WriteLine(result.Item2); } }
10 20
Benchmark. There are multiple ways to return multiple values from a method. The most common ones are probably out-parameters and KeyValuePair and tuples.
Version 1 This version of the code uses out parameters to return 2 values from a method.
Version 2 Here we use a KeyValuePair and read in its Key and Value properties from the returned method.
Version 3 A tuple can be used to return 2 values from a method. We access Item1 and Item2.
Result The first implementation, which uses out parameters, was found to be somewhat slower than the second, which uses the KeyValuePair.
C# program that benchmarks multiple return values
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(); // 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")); Console.Read(); } }
1.64 ns, out 0.32 ns, KeyValuePair 5.40 ns, Tuple
A summary. Multiple values can be returned from a method invocation. We used out to pass parameters by deference. With KeyValuePair and Tuple we returned multiple values in a container.
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