Both ref and out parameters are used to pass arguments within a method. These ref and out parameters are useful whenever your method wants to return more than one value. Regarding ref parameter you need to initialize it before passing to the method and out parameter you don’t need to initialize before passing to function. 

Ref parameter:

If you want to pass a variable as ref parameter you need to initialize it before you pass it as ref parameter to method. Ref keyword will pass parameter as a reference this means when the value of parameter is changed in called method it get reflected in calling method also.

Declaration:

Generally we will use ref parameters like as shown below

int i=3; // variable need to be initialized
RefMethod(ref i);

If you observe above code first we declared variable and initialized with value 3 before it pass a ref parameter to RefMethod. 

Example:

class Program
{
    static void Main()
    {
        int i; // variable need to be initialized
        i = 3;
        RefMethod(ref i);
        Console.WriteLine(i);
    }
    public static void RefMethod(ref int val1)
    {
        val1 += 10;
    }
}

Output:

13

As we discussed if ref parameter value changed in called method that parameter value reflected in calling method also

Out Parameter:

If you want to pass a variable as out parameter you don’t need to initialize it before you pass it as out parameter to method. Out keyword also will pass parameter as a reference but here out parameter must be initialized in called method before it return value to calling method.

Declaration:

Generally we will use out parameters like as shown below

int i,j; // No need to initialize variable
OutMethod(out i, out j);

If you observe above code first we declared variable and we it pass a out parameter to OutMethod without initialize the values to variables

class Program
{
    static void Main()
    {
        int i, j; // No need to initialize variable
        OutMethod(out i, out j);
        Console.WriteLine(i);
        Console.WriteLine(j);
    }
    public static int OutMethod(out int val1, out int val2)
    {
        val1 = 5;
        val2 = 10;
        return 0;
    }
}

If we observe code we implemented as per our discussion like out parameter values must be initialized in called method before it return values to calling method. When we run above code we will get like as shown below

Output:

5

10