title: C# Exception thrown: 'System.FormatException' in mscorlib.dll
body: <p>Exception thrown: <code>System.FormatException</code> in <code>mscorlib.dll</code></p>
  
  <blockquote>
    <p>{"Input string was not in a correct format."}</p>
  </blockquote>
  
  <p>I have been at this all day. Please help! Those above were the error messages btw. I am a bit new to C# so I could use a bit more detailed answers (I don't get the jargon so well)</p>
  
  <pre><code>public partial class Form1 : Form
  {
      double a, b, d, f, g, t;
      string z;
      int c;
  
  
      public Form1()
      {
          InitializeComponent();
      }
  
  
      private void Form1_Load(object sender, System.EventArgs e)
      {
  
      }
  
  
  
      private void button7_Click(object sender, EventArgs e)
      {
          a = Convert.ToDouble(textBox18.Text);
          z = Convert.ToString(comboBox1);
          d = Convert.ToDouble(checkBox1);
          f = Convert.ToDouble(textBox13.Text);
          g = Convert.ToDouble(textBox14.Text);
          t = Convert.ToDouble(textBox19.Text);
  
  
          if (z == "a")
          {
              textBox15.Text = Convert.ToString(a * g * d * t);
          }
          else if (z == "b")
          {
              textBox15.Text = Convert.ToString(c);
          }
      }
  </code></pre>