Out

This C# keyword signifies a reference parameter. Sometimes methods must return more than one value and not store class state.

Keyword info

Out fills these requirements. With it we pass parameters whose changes are realized in their calling methods. In a method, an out argument must be assigned.

An example

The C# language has robust support for object-oriented logic. But it also provides a way to return many values from a method without using objects.

using System;

class Program
{
    static void Main()
    {
        // Part 1: variables used as out parameter.
        bool period;
        bool comma;
        bool semicolon;
        const string value = "has period, comma.";

        TestString(value, out period, out comma, out semicolon);

        Console.WriteLine(value);
        Console.Write("period: ");
        Console.WriteLine(period);
        Console.Write("comma: ");
        Console.WriteLine(comma);
        Console.Write("semicolon: ");
        Console.WriteLine(semicolon);
    }

    static void TestString(string value,
        out bool period,
        out bool comma,
        out bool semicolon)
    {
        // Part 2: assign all out parameters to false.
        period = comma = semicolon = false;

        for (int i = 0; i < value.Length; i++)
        {
            switch (value[i])
            {
                case '.':
                    {
                        period = true; // Set out parameter.
                        break;
                    }
                case ',':
                    {
                        comma = true; // Set out parameter.
                        break;
                    }
                case ';':
                    {
                        semicolon = true; // Set out parameter.
                        break;
                    }
            }
        }
    }
}
has period, comma.
period: True
comma: True
semicolon: False

Inline out parameters

We can combine statements when calling a method that requires an out parameter. We can declare the variable directly inline with the method call.

using System;

class Program
{
    static bool Test(out int size)
    {
        // This method has an out parameter.
        size = 1000;
        return size >= 1000;
    }

    static void Main()
    {
        // Declare out parameter directly inside method call.
        if (Test(out int size))
        {
            Console.WriteLine(size);
        }
    }
}
1000

Discard name syntax

Sometimes we need to call a method with "out" parameters, but do not need to use those parameters. We can avoid using a "dummy" name for those locals.

using System;

class Program
{
    static void Test(out int size, out string name)
    {
        size = 10;
        name = "bird";
    }

    static void Main()
    {
        // We do not need the out int, so use an underscore name (discard name).
        Test(out _, out string name);
        Console.WriteLine("NAME: {0}", name);
    }
}
NAME: bird

A discussion

The out descriptor tells the intermediate language compiler to copy the actual variable storage location, not just the value at a storage location.

Definite assignment

The C# compiler applies definite assignment analysis. The out keyword ensures that variables are always assigned in the body of a method.

A summary

With out, a method can signal to the calling location that the variable location's were initialized and written. Out simplifies code that returns values.