C# How to Fix Error: 'Input string was not in a correct format.' in Textbox

Question

In the code below, I'm trying to validate the Textbox (txt_quantity and txt_discount)

But instead of getting this MessageBox.Show("Cannot be empty"); I'm getting the error of

('Input string was not in a correct format.')

Did i forgot something here?

• txt_quantity(INTEGER)

• txt_discount(DECIMAL)

  decimal Discount, DiscountTotal, Discountgiven, Total;
  int Cost, Quantity, ID;       
  byte[] data;
  public void Imagedisplay()
  {
      using (var con = SQLConnection.GetConnection())
      {
          using (var selects = new SqlCommand("Select * from employee_product where Codeitem =@Codeitem ", con))
          {
              selects.Parameters.Add("@Codeitem", SqlDbType.VarChar).Value = _view.txt_code.Text;
              using (var reader = selects.ExecuteReader())
              {
                  while (reader.Read())
                  {
                      data = (byte[])reader["Image"];
  
                      Cost = Convert.ToInt32(reader["Unitcost"]);
                      Convert.ToInt32(DiscountTotal);
                  //  This is where i'm getting the error at  
                      Quantity = Convert.ToInt32(txt_quantity.Text);
                      Discount = Convert.ToDecimal(txt_discount.Text); //
                      Discountgiven = Cost * (Discount / Convert.ToDecimal(100));
                      DiscountTotal = Cost - Discountgiven;
                      Total = DiscountTotal * Quantity;
                  }
  
              }
          }
      }
  }
  
  private void btn_ok_Click(object sender, EventArgs e)
  {
      Imagedisplay();
      using (var con = SQLConnection.GetConnection())
      {
          if (string.IsNullOrEmpty(txt_quantity.Text) || string.IsNullOrEmpty(txt_discount.Text))
          {
              MessageBox.Show("Cannot be empty");
          }
          else
          {                    
                  using (var command = new SqlCommand("Insert into product_result (Date, Image, Code, Name, Price, Discount, Quantity, Total) Values (@Date, @Image, @Code, @Name, @Price, @Discount, @Quantity, @Total)", con))
                  {                         
                      command.Parameters.Add("@Date", SqlDbType.VarChar).Value = date;
                      command.Parameters.Add("@Image", SqlDbType.VarBinary).Value = data;
                      command.Parameters.Add("@Code", SqlDbType.VarChar).Value = _view.txt_code.Text.Trim();
                      command.Parameters.Add("@Name", SqlDbType.VarChar).Value = _view.txt_name.Text.Trim();
                      command.Parameters.Add("@Price", SqlDbType.VarChar).Value = _view.txt_price.Text;
                      command.Parameters.Add("@Discount", SqlDbType.Decimal).Value = txt_discount.Text.Trim();
                      command.Parameters.Add("@Quantity", SqlDbType.Int).Value = txt_quantity.Text.Trim();
                      command.Parameters.Add("@Total", SqlDbType.Decimal).Value = Total;
                      command.ExecuteNonQuery();
                      Totals();                      
                  }
                  using (var selects = new SqlCommand("Update employee_product set quantity = quantity - @Quantity where Codeitem= @Codeitem", con))
                  {
                      selects.Parameters.Add("@Codeitem", SqlDbType.VarChar).Value = _view.txt_code.Text;
                      selects.Parameters.Add("@Quantity", SqlDbType.Int).Value = txt_quantity.Text;
                      selects.ExecuteNonQuery();
                      this.Close();                          
                  }                   
              _view.Display();
          }
      }          
  }
  

Answer 1

Use a NumericUpDown instead of a textbox to capture integer/decimal values. It handles all the validation for you stopping users from entering non-numeric values. You can set the maximum and minimum values you require and you don't have to worry about no values being entered as the NumericUpDown will always have a default value.

If you're using integers then you just need to cast to an int when you retrieve the value, otherwise it returns a decimal. So your code would be:

Quantity = Convert.ToInt32(numericupdown1.Value);
Discount = numericupdown2.Value;

If you're hellbent on using textboxes then you need to remove whitespace with .Trim()

Quantity = Convert.ToInt32(txt_quantity.Text.Trim());

And use int.TryParse instead;

int value = 0;
if (int.TryParse(txt_quantity.Text.Trim(), out value)
{
    // Successful conversion so value now contains your integer
}

You can do the same with decimals.