member names cannot be the same as their enclosing type C#

Method names which are same as the class name are called constructors. Constructors do not have a return type. So correct as:

private Flow()
{
   X = x;
   Y = y;
}

Or rename the function as:

private void DoFlow()
{
   X = x;
   Y = y;
}

Though the whole code does not make any sense to me.

The problem is with the method:

private void Flow()
{
    X = x;
    Y = y;
}

Your class is named Flow so this method can't also be named Flow. You will have to change the name of the Flow method to something else to make this code compile.

Or did you mean to create a private constructor to initialize your class? If that's the case, you will have to remove the void keyword to let the compiler know that your declaring a constructor.

Constructors don't return a type , just remove the return type which is void in your case. It would run fine then.