1.

What is the difference between ref and out keywords?

Answer»

  • ref keyword is used to pass an already initialized variable to a method as a reference type, facilitating bi-directional data passing.

Example 1:

// C# program to illustrate the concept of out parameter
using System;
class GFG {
// Main method
static public void Main()
{
// Lets declare a string
string str3 = "Scaler";
// Pass variable str3 to the method using ref keyword
initializeString(ref str3);
// Display the value str3 now
// Since initially we declared value "Scaler" in str3,
// normally below statement must print "Hello Scaler"
// But due to the use of ref keyword,
// the value will be overwritten in the initializeString() Method
// So now the value printed will be "Hello InterviewBit" instead
Console.WriteLine("Hello {0}", str3);
}


// Method in which ref parameter is passed and a value is written into this variable
public static void initializeString(ref string str1)
{

// Since ref keyword supports bi-directional data flow,
// This check statement will be validated and output will be printed
if (str1 == "Scaler") {
Console.WriteLine("Hello Scaler");
}

// Now lets try to change the value of ref parameter str1
// The value "InterviewBit" will be overwritten in reference variable str3
str1 = "InterviewBit";

}
}

The above program will generate the below output:

Hello Scaler
Hello InterviewBit
  • Whereas out keyword is used to pass a variable as an empty container that can store multiple values to a method as a reference type. out keyword allows uni-directional data passing, as the container passed using out keyword doesn’t need to be initialized beforehand.

Example 2: Let’s see if and how the data flows when out keyword is used:

// C# program to illustrate the concept of out parameter
using System;
class GFG {
// Main method
static public void Main()
{
// Declaring a string variable
// Since we are going to use out parameter
// We do not need to assign any value to it

string str1;
// Pass variable str1 to the method using out keyword
// Normally it should pass the default value, but due to use of out keyword
// An empty container will be passed instead, leading to error

checkIfScaler(out str1);
// Lets try again with another value

string str2 = "Scaler";
// Pass variable str2 to the method using out keyword
// Now you must think that the value "Scaler" will be passed
// But due to use of out keyword,
// again an empty container will be passed instead, leading to error

checkIfScaler(out str2);
}

// Method in which out parameter is passed and checked
// if the current string value is Scaler or not

public static void checkIfScaler(out string str1)
{

// Check parameter value
// Since we are using out parameter, this will lead to error
// as no data is passed when into this method when out keyword is used
// This is because out keyword onlu facilitates uni-direction data transfer

if (str1 == "Scaler") {
Console.WriteLine("Hello!!Scaler");
}

}

}

The above program will generate the below error:

prog.cs(39,13): error CS0269: Use of unassigned out parameter `str1'

Example 3: Now let us try to see what happens when we initialize some value to an out parameter.

// C# program to illustrate the concept of out parameter
using System;
class GFG {
// Main method
static public void Main()
{
// Lets declare a string
string str3 = "Scaler";
// Pass variable str3 to the method using out keyword
// Now you must think that the value "Scaler" will be passed
// But due to use of out keyword,
// again an empty container will be passed instead
initializeString(out str3);
// Display the value str1 now
// Since initially we declared value "Scaler" in str3,
// normally below statement must print "Hello Scaler"
// But due to the use of out keyword,
// the value will be overwritten in the initializeString() Method
// So now the value printed will be "Hello InterviewBit" instead
Console.WriteLine("Hello {0}", str3);
}


// Method in which out parameter is passed and a value is written into this variable
public static void initializeString(out string str1)
{

// Since out keyword supports uni-directional data flow,
// the value "InterviewBit" will be written in reference variable str3

str1 = "InterviewBit";

// Check parameter value
if (str1 == "InterviewBit") {
Console.WriteLine("InterviewBit value assigned successfully");
}

}
}

The above program will generate the below output:

InterviewBit value assigned successfully
Hello InterviewBit


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