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C# ref, out, in

last modified December 13, 2021

C# ref, out, in keywords tutorial explains the differences between the ref/out/in keywords and shows how to use them.

In C#, the method parameters are passed by copy by default. We use the ref keyword to pass a value by reference. When we pass values by reference, the method receives a reference to the actual values. The original values are affected when modified. The ref requires that the variable be initialized before it is passed. The ref modifier must be used on the method definition and in a method call. The out and in keywords are some modifications of ref.

The out keyword causes arguments to be passed by reference as well. Unlike ref, it does not require the passed variable to be initialized in advance. To use an out parameter, both the method definition and the calling method must explicitly use the out keyword.

The in keyword causes arguments to be passed by reference but ensures the argument is not modified. Variables passed as in arguments must be initialized before being passed in a method call.

When overloading methods, they cannot differ in signature solely by in, ref, and out. Overloading is possible when one method does not have one of modifiers and the other has.

The in, ref, and out modifiers have the following restrictions:

C# ref modifier example

The following example uses the ref keyword to change the values of two variables.

Program.cs
int a = 4;
int b = 7;

Console.WriteLine("Outside Swap method");
Console.WriteLine($"a is {a}");
Console.WriteLine($"b is {b}");

Swap(ref a, ref b);

Console.WriteLine("Outside Swap method");
Console.WriteLine($"a is {a}");
Console.WriteLine($"b is {b}");

void Swap(ref int a, ref int b)
{
    int temp = a;
    a = b;
    b = temp;

    Console.WriteLine("Inside Swap method");
    Console.WriteLine($"a is {a}");
    Console.WriteLine($"b is {b}");
}

The original variables are changed inside the Swap method.

Swap(ref a, ref b); 
...
void Swap(ref int a, ref int b)

Notice that both the method call and the method definition use the ref keyword.

$ dotnet run 
Outside Swap method
a is 4
b is 7
Inside Swap method
a is 7
b is 4
Outside Swap method
a is 7
b is 4

C# out modifier example

The following example uses the out modifier.

Program.cs
ReadName(out string? name);
Console.WriteLine(name);

void ReadName(out string? name) {

    Console.Write("Enter your name: ");
    name = Console.ReadLine();
}

In the example, we read an input from the user. The input is stored in a variable, which has the out modifier. The name variable is not defined before it is being used.

ReadName(out string? name);
...
void ReadName(out string? name) {

The out keyword is used with the method call and definition.

$ dotnet run 
Enter your name: Peter
Peter

C# in modifier example

In the following example, we use the in modifier.

Program.cs
string msg = "an old falcon";
Modify(in msg);

void Modify(in string msg) {

    msg = "a young eagle";
}    

We try to modify the msg variable, but the compilation ends with an error message: error CS8331: Cannot assign to variable 'in string' because it is a readonly variable

In this tutorial we have presented the C# ref, out, & in keywords.

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