How to exit sub-function in C#?

You cannot exit the function without returning a value

you can use the default value (or) some defined value of the return type to exit the function

private int f(int x)
       {
           if (x == 1)
           {
               return 2 * x;
           }
           else
           {
               return default(int);
               // exit this function???
           }
       }

If it is a value type you can use default(Type);
if it is referece type you can use return null

There are couple of ways you can do that

Method 1 - Return NULL (you will need to change the return type int to int?)

private int? f(int x)
{
     if (x==1)
     {
           return 2*x;
     }
     else
     {
           return null;// exit this function
     }
}

Method 2 - You can set a default value that will represent incorrect condition.

private int f(int x)
{
     if (x==1)
     {
           return 2*x;
     }
     else
     {
           return  -1; // exit this function with a default value and in caller function you will check if it is -1.
     }
}

Method 3 - You can create TryParse kind functions where return type is bool and one parameter is out.