How to insert a variable in a case if it gives an error: "A constant value is required"?

Question

C++ / C#

int d = Convert.ToInt32(Console.ReadLine());
int g = d % 2;
if (g == 0)
{
for (int s = g; s < 20; s++) ;
switch (s)
{
case s: s = 0;
break;

Answer 1

The switch/case construct implies that the value passed to the switch will be initially known.
In fact, each case is a constant.
Try to rewrite under if / else and you will immediately understand why this happens.
Your code

int d = Convert.ToInt32(Console.ReadLine());
            int g = d % 2;
            if (g == 0)
            {
                for(int s = g; s < 20; s++)
                {
                    if(s) // тут уже не понятно с чем сравнивать вашу s для выполнения условия.
                }
            }