Input string was not in a correct format.

This is true way
protected void Page_Load(object sender, EventArgs e)
    {
      
       
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string sFee = TextBox1.Text.ToString();
      
        double ssFee = Convert.ToDouble(sFee);
      
    }
ANd If You want to Put this code in Page_load Do that way.


protected void Page_Load(object sender, EventArgs e)
    {
         string sFee = TextBox1.Text.ToString();
       if(sFee !="")
        {
        double ssFee = Convert.ToDouble(sFee);
        }     
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
     
      
    }
Empty String Can not convert into double.That is why u got this error.

I hope It will Help you