Ref vs Out

Out keyword

The out is a keyword in C# which is used for passing the arguments to methods as a reference type. It is generally used when a method returns multiple values. The out parameter does not pass the property.

// C# program to illustrate the 
// concept of out parameter 
using System; 
  
class GFG { 
  
    // Main method 
    static public void Main() 
    { 
  
        // Declaring variable 
        // without assigning value 
        int G; 
  
        // Pass variable G to the method 
        // using out keyword 
        Sum(out G); 
  
        // Display the value G 
        Console.WriteLine("The sum of" +  
                " the value is: {0}", G); 
    } 
  
    // Method in which out parameter is passed 
    // and this method returns the value of 
    // the passed parameter 
    public static void Sum(out int G) 
    { 
        G = 80; 
        G += G; 
    } 
} 

// Output
The sum of the value is: 160

Another example can be:

int initializeInMethod;
OutArgExample(out initializeInMethod);
Console.WriteLine(initializeInMethod);     // value is now 44

void OutArgExample(out int number)
{
    number = 44;
}

You can't use the in, ref, and out keywords for the following kinds of methods:

• Async methods, which you define by using the async modifier.

• Iterator methods, which include a yield return or yield break statement.

In addition, extension methods have the following restrictions:

• The out keyword cannot be used on the first argument of an extension method.

• The ref keyword cannot be used on the first argument of an extension method when the argument is not a struct, or a generic type not constrained to be a struct.

• The in keyword cannot be used unless the first argument is a struct. The in keyword cannot be used on any generic type, even when constrained to be a struct.

Calling a method with an out argument

In C# 6 and earlier, you must declare a variable in a separate statement before you pass it as an out argument. The following example declares a variable named number before it is passed to the Int32.TryParse method, which attempts to convert a string to a number.

string numberAsString = "1640";

int number;
if (Int32.TryParse(numberAsString, out number))
    Console.WriteLine($"Converted '{numberAsString}' to {number}");
else
    Console.WriteLine($"Unable to convert '{numberAsString}'");
// The example displays the following output:
//       Converted '1640' to 1640

Starting with C# 7.0, you can declare the out variable in the argument list of the method call, rather than in a separate variable declaration. This produces more compact, readable code, and also prevents you from inadvertently assigning a value to the variable before the method call. The following example is like the previous example, except that it defines the number variable in the call to the Int32.TryParse method.

string numberAsString = "1640";

if (Int32.TryParse(numberAsString, out int number))
    Console.WriteLine($"Converted '{numberAsString}' to {number}");
else
    Console.WriteLine($"Unable to convert '{numberAsString}'");
// The example displays the following output:
//       Converted '1640' to 1640

Ref keywork

The ref is a keyword in C# which is used for passing the arguments by a reference. Or we can say that if any changes made in this argument in the method will reflect in that variable when the control return to the calling method. The ref parameter does not pass the property.

using System; 
  
class GFG { 
  
    // Main Method 
    public static void Main() 
    { 
  
        // Assign string value 
        string str = "Geek"; 
  
        // Pass as a reference parameter 
        SetValue(ref str); 
  
        // Display the given string 
        Console.WriteLine(str); 
    } 
  
    static void SetValue(ref string str1) 
    { 
  
        // Check parameter value 
        if (str1 == "Geek") { 
            Console.WriteLine("Hello!!Geek"); 
        } 
  
        // Assign the new value 
        // of the parameter 
        str1 = "GeeksforGeeks"; 
    } 
} 

// Output
Hello!!Geek
GeeksforGeeks

Difference between Ref and Out keywords

ref keyword	out keyword
It is necessary the parameters should initialize before it pass to ref.	It is not necessary to initialize parameters before it pass to out.
It is not necessary to initialize the value of a parameter before returning to the calling method.	It is necessary to initialize the value of a parameter before returning to the calling method.
The passing of value through ref parameter is useful when the called method also need to change the value of passed parameter.	The declaring of parameter through out parameter is useful when a method return multiple values.
When ref keyword is used the data may pass in bi-directional.	When out keyword is used the data only passed in unidirectional.