A small program for calculating prime numbers

The program is just beginning to learn C #, which is the first very small small program: calculating prime numbers. Do worse, don't laugh!

Http://true5460.muicc.com/other/calPN.zip

The following is the source code:

Using System;

Class Test
{
static void Main () {
ULONG I;
while (true) {
Console.Write ("Please enter the prime number to calculate (0 exit):");
I=convert.touint64 (Console.ReadLine ());
if (i==0) break;
DateTime T1 = DateTime.Now;
switch (i) {
Case 1:console.writeline ("1 is not prime!") ");
Case 2:console.writeline ("2 is prime!") ");
Default:cal (i);
}
DateTime t2 = DateTime.Now;
Console.WriteLine ("Time: {0} ms \ n", (T2-T1). TICKS/10000F);
}
}

The following are part of the function

static void Cal (ULONG x) {
ULONG Sum=1;
BYTE row=1;
Console.Write ("\ n");
for (ulong a=3;a<x+1;a++) {
BOOL Flag=true;
for (ULONG b=2;b< (A/2) +1;b++) {if (a%b!=0) continue;
Flag=false;
Break
}
if (flag) {
if (row==10) {Console.WriteLine (); row=0;}
if (sum==1) Console.Write ("{0,7}", 2);
Console.Write ("{0,7}", a);
sum++;row++;
}

}
Console.WriteLine ("\n\n{0} has a total of {1} prime numbers \ n", x,sum);

}

}