Difference Between Ref and Out Keywords in C#?

What is the difference between "ref" and "out" keywords?

By default, the parameters are passed to a method by the value.
By using ref and out, we can pass a parameter by reference.

The ref Modifier

Used to state that the parameter passed may be modified by the method.
It is necessary the parameters should initialize before it passes to ref.
Useful when the called method also needs to change the value of a passed parameter.

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello World!");
        int id = 1;
        Console.WriteLine("id = " + id.ToString());  // id = 1
        string newString = GetString(ref id);
        Console.WriteLine("id = " + id.ToString());  // id = 2
    }

    public static string GetString(ref int id)
    {
        id += 1;
        return "String";
    }
}

The out Modifier

Used to state that the parameter passed must be modified by the method.
It is not necessary to initialize parameters before they pass to an out.
Useful when a method returns multiple values.

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello World!");
        int id =1;
        Console.WriteLine("id = " + id.ToString());  // id = 1
        string newString = GetString(out id);
        Console.WriteLine("id = " + id.ToString());  // id = 2
    }

    public static string GetString(out int id)
    {
        id = 2;
        return "String";
    }
}

Note: Both ref and out parameters are treated the same at compile-time but different at run-time.

ref	out
The parameter or argument must be initialized first before it is passed to the ref	Not required to initialize the parameter or argument before passing it to an out
Not required to assign or initialize the value of a parameter before returning to the calling method.	A called method is required to assign or initialize a value of a parameter before returning to the calling method.
Data can be passed bi-directionally	Data is passed only unidirectional way ( from the called method to the caller method)
Useful when the called method is also needed to modify the passed parameter	Useful when multiple values need to be returned from a function or method.