Input string was not in a correct format in double.Parse

Question

I am new to C#. I\'m trying to make a calculator, but the following error occurred:

Input string was not in a correct format.

T

Answer 1

Since you have cleared the textbox on the previous line, the Parse conversion fails.

textBox1.Text = String.Empty;
num2 = double.Parse(textBox1.Text);

How will it convert String.Empty to Double? The way of doing it is not right. For example, if the "+" button is clicked, you have to check whether there was already a number. If so,, add the numbers and display the result:

Double num;
private void Add_Click(object sender, EventArgs e)
{

   If (num != null)
   {
    num == num + Convert.ToDouble(textBox1.Text);
   }
   else
   {
    num1 == Convert.ToDouble(textBox1.Text);
   } 
   textBox1.Text = num;
}