Q:

How to write out all the integers used to calculate the factorial

Right now I have 4! = 24 being output into a message box. 
What I want to do is add all the integers multiplied in calculating for n. So if I enter 4, I want to get 4! = 1 x 2 x 3 x 4 = 24, instead of 4! = 24.
Code:
private void calculateButton_Click(object sender, EventArgs e)
{
    int userNumber = 0;
    double factorialNumber = 1;

    int.TryParse(numberInputTextBox.Text, out userNumber);

    if (userNumber > 0)
    {
        for (int countFactorial = 1; countFactorial <= userNumber; countFactorial++)
        {
            factorialNumber *= countFactorial;
        }

        MessageBox.Show(userNumber + "! = " + factorialNumber.ToString("n0"));
        resetButton.Focus();
    }
    else
    {
        MessageBox.Show("The factorial value entered must be an integer.");
        numberInputTextBox.Text = "";
        numberInputTextBox.Focus();
    }
}

A:

You can put the numbers from 1 through userNumber into a List<int> nums (for example) and then use var numsStr = string.Join(" x ", nums); to get the string 1 x 2 x 3 x 4 
Example:
if (userNumber > 0)
    {
        List<int> nums = new List<int>();
        for (int countFactorial = 1; countFactorial <= userNumber; countFactorial++)
        {
            factorialNumber *= countFactorial;
            nums.Add(countFactorial);
        }
        
        var numsStr = string.Join(" x ", nums);            
        var result = string.Format("{0} ! = {1} = {2}", userNumber, numsStr, factorialNumber.ToString("n0"));
        MessageBox.Show(result);
        resetButton.Focus();
    }

