Error checking so that I don't divide by 10 everytime

Question

How can I check to see if the textbox is empty and then only divide by that number of int. As you can see now I will divide by 10 everytime so now I need help with error checking.

namespace Assignment1_White
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            int num1;
            int num2;
            int num3;
            int num4;
            int num5;
            int num6;
            int num7;
            int num8;
            int num9;
            int num10;
            int answer;

            num1 = int.Parse(num100.Text);
            num2 = int.Parse(num200.Text);
            num3 = int.Parse(num300.Text);
            num4 = int.Parse(num400.Text);
            num5 = int.Parse(num500.Text);
            num6 = int.Parse(num600.Text);
            num7 = int.Parse(num700.Text);
            num8 = int.Parse(num800.Text);
            num9 = int.Parse(num900.Text);
            num10 = int.Parse(num1000.Text);



            answer = (num1 + num2 + num3 + num4 + num5 + num6 + num7 + num8 + num9 +num10)/10;
            MessageBox.Show(answer.ToString());

Answer 1

string.IsNullOrEmpty(num100.Text)

This will return true if it is empty, and false if it isn't.

You can also use int.TryParse instead of int.Parse. If you use TryParse, you will also implicitly handle non-int values in your text boxes:

if(!int.TryParse(num100.Text, out num1))
    num1 = 0;
// And for the rest...

Edit

After Brian's comment, I realized you shouldn't divide by 10 if you have empty/invalid input for some of the boxes. Here is the block you should use instead:

int count = 0;
int num1 = 0;
// And the rest...

if(int.TryParse(num100.Text, out num1))
    count++;
// And the rest...

int answer = 0;
if(count > 0)
    answer = (num1 + num2 + num3 + num4 + num5 + num6 + num7 + num8 + num9 + num10)
        / count;

Answer 2

You could do something like:

int count = 0;

//For each textbox
if(num100.Text != "")
{
     num1 = int.Parse(num100.Text);
     count++;
}

Then divide by count

Answer 3

private void button1_Click(object sender, EventArgs e)
{
    int answer;

    int num1 = !string.IsNullOrEmpty(num100.Text)? int.Parse(num100.Text):0;
    int num2 = !string.IsNullOrEmpty(num200.Text)? int.Parse(num200.Text):0;
    //etc...
    answer = (num1 + num2 + num3 + num4 + num5 + num6 + num7 + num8 + num9 +num10)/10;
    MessageBox.Show(answer.ToString());

Answer 4

int num1 = 0;

if(!String.IsNullEmpty(num1.Text) && (new Regex("^/d$").Match(num1.Text)))
{
     num1 = Int32.Parse(num1.Text);
}

do this for each text box, this will check for both Empty Text boxes and also non-numeric entries.