Question

I have this code that multiplies 32 bit * 32 bit.

public static void RunSnippet()
{
    System.Int32 x, y;
    System.Int64 z;

    System.Random rand = new System.Random(DateTime.Now.Millisecond);
    for (int i = 0; i < 6; i++)
    {
        x = rand.Next(int.MinValue, int.MaxValue);
        y = rand.Next(int.MinValue, int.MaxValue);
        z = (x * y);
        Console.WriteLine("{0} * {1} = {2}", x, y, z);
    }

However, the result is not exactly what I expected.

What’s wrong with this?

Answer 1

Overflow. The result is calculated as a 32-bit integer and after that promoted to 64 bits. To avoid that, convert the factors to 64 bits before the multiplication.

System.Int32 x, y;
System.Int64 z;

System.Random rand = new System.Random(DateTime.Now.Millisecond);
for (int i = 0; i < 6; i++)
{
    x = rand.Next(int.MinValue, int.MaxValue);
    y = rand.Next(int.MinValue, int.MaxValue);
    z = ((Int64)x * y); //by casting x, we "promote" the entire expression to 64-bit.
    Console.WriteLine("{0} * {1} = {2}", x, y, z);
}