Addition using a single TextBox

move n2 = Convert.Int32(textbox1.Text); to your result button's click event

Add button click event:

n1=convert.Int32(textbox1.text);   
textbox1.text=""; 

result button's click event:

n2=convert.Int32(textbox1.text);
result=n1+n2;
textbox1.text=result.tostring(); 

try something like this:

private int result=0;

addMethod()
{
    //here you can use also int.TryParse
    if(string.isnullorempty(textbox1.text.trim()))
    textbox1.text="0";

    result+=convert.toint32(textbox1.text);

    textbox1.text=string.empty;
}

resultMethod()
{
    textbox1.text=result.tostring();
}

look at the folloing code

      private int i = 0;
    private int[] a = new int[2];
    private void button1_Click(object sender, EventArgs e)
    {
        int b;
        if(Int32.TryParse(textBox1.Text, out b))
        {
            a[i] = b;
            i++;
            textBox1.Text = "";
        }
        else
        {
            MessageBox.Show(@"Incorrect number");
        }


    }

    private void resultbutton2_Click(object sender, EventArgs e)
    {
        int sum = a[0] + a[1];
        MessageBox.Show("Sum: " + sum);
    }
}

You code contains a couple of errors:

1. You are using Int32 Convert.ToInt32(String) which is bad, because will throw an exception if the user enters something that can't be cast to an Int32, you should use Boolean Int32.TryParse(String, out Int32) instead, and check if the conversion is successful.

2. You are calling n2 = Convert.ToInt32(textbox1.Text); right after you set textbox1.Text = ""; and this is also bad, because it won't wait for a user input, it will try to convert an empty string to an integer right away, which will yield an exception.

3. You can't know when the user has finished inserting his number, so you have to rely to something external to the TextBox to let you know you should perform the addition.

Try the following:

• Place a TextBox.
• Place a Button.
• Initialize a "grandTotal" integer variable to zero.
• When the user press the button, parse the content of the TextBox in a temporary "submittedValue" integer variable, then sum this temporary variable to your "grandTotal" variable, and display the resulting "grandTotal" variable value.

This way you'll be able to continuously perform the addition operation on the previous total.

Example

The global variable:

protected Int32 GrandTotal = 0;

The Button click event handler:

Int32 currentValue = 0;

if (Int32.TryParse(userValue.Text, out currentValue))
{
    GrandTotal += currentValue;

    MessageBox.Show(String.Format("The new total is: {0}", GrandTotal));
}
else
{
    MessageBox.Show("Invalid value!");
}

I have no idea why you are doing it in this fashion.

Still you can do this as Declare n1, n2 as int? at the form level

Then in your Add click event do it in this fashion

if (! n1.HasValue)
{
    n1 = int.Parse(textBox1.Text);                
}
n2 = int.Parse(textBox1.Text);
textBox1.Text = "";

if you are going to do this for lotz of number then use List<int?> myIntList = new List<int?>() and your add event would be

myIntList.Add(int.Parse(textBox1.Text));
textBox1.Text = "";

and the result is int? result = myIntList.Sum();