What is the difference between passing data to a method by reference and by value?

In C#, data can be passed to a method mainly in two ways: by value and by reference.

• By value (default): a copy of the variable's value is passed to the method. Changes inside the method do not affect the original variable outside.

• By reference (using ref or out keywords): a reference to the original variable is passed. Changes inside the method are reflected in the original variable.

Example:

void ChangeValue(int x) {
    x = 10; // changes the copy
}

void ChangeValueRef(ref int x) {
    x = 10; // changes the original
}

int a = 5;
ChangeValue(a);
Console.WriteLine(a); // Outputs 5

ChangeValueRef(ref a);
Console.WriteLine(a); // Outputs 10

Thus, passing by value protects the original data from changes inside the method, while passing by reference allows the method to modify the original data.