String is not in correct format

Hai @Emmmanuel FIADUFE 

Check the textbox value is null or not.

if(txtQuantity.Text!=null

{

int NewQty = Stock - Convert.ToInt32(txtQuantity.Text);

}

Thank Amit, I just notice that

Please do confirm whether the control txtQuantity.Text is a required field in UI. 

In your code, there are several places where you are performing conversions from string to other data types (Convert.ToInt32, Convert.ToDecimal). If the string is not a valid representation of an integer or decimal number, this error will occur.

One potential issue in your code could be in the dataGridView1_CellContentClick method. The error might occur if dataGridView1.CurrentRow.Cells[0].Value is not a valid integer string.

here is the modified code for dataGridView1_CellContentClick

private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
{
    txtProductName.Text = Convert.ToString(dataGridView1.CurrentRow.Cells["dgvName"].Value);

    if (dataGridView1.SelectedRows.Count > 0 && dataGridView1.SelectedRows[0].Cells[3].Value != null)
    {
        if (!int.TryParse(dataGridView1.SelectedRows[0].Cells[3].Value.ToString(), out Stock))
        {
            // Handle the case where the value cannot be parsed to an integer
            Stock = 0; // Set default value or perform necessary error handling
        }
    }
    else
    {
        Stock = 0; // Set default value if no value is present
    }

    decimal sellingPrice;
    if (decimal.TryParse(dataGridView1.CurrentRow.Cells["dgvSellingPrice"].Value?.ToString(), out sellingPrice))
    {
        txtSellingPrice.Text = sellingPrice.ToString("#0.00");
    }
    else
    {
        txtSellingPrice.Text = "0.00"; // Set default value or perform necessary error handling
    }

    if (string.IsNullOrEmpty(txtProductName.Text))
    {
        ProdKey = 0;
    }
    else
    {
        if (!int.TryParse(dataGridView1.SelectedRows[0].Cells[0].Value?.ToString(), out ProdKey))
        {
            // Handle the case where the value cannot be parsed to an integer
            ProdKey = 0; // Set default value or perform necessary error handling
        }
    }
}

By using int.TryParse and decimal.TryParse methods, the code attempts to convert strings to integers or decimals safely without throwing exceptions if the conversion fails. This helps in preventing the "input string was not in a correct format" error.

Hello Amit, Please when I used your format as you said it is not working because I load the data from product dataGridView into the textboxes and then to the Billing dataGridView.

This code seems to be working alright because when I click the error popup ok buttong and it goes away,  the ("update tblItem set ItQty = ItQty - " + Int32.Parse(txtQuantity.Text) +  function work fine but when I run the project before the product dataGridView Load the first message that popup is the input string is not in the correct format.

 public void UpdateItem()
        {
          
            try
            {

                con.Open();              
                cmd = new SqlCommand("update tblItem set ItQty = ItQty - " + Int32.Parse(txtQuantity.Text) + "where ItId like '" +  Int32.Parse(dataGridView1.SelectedRows[0].Cells[1].Value.ToString()) + "%'", con);
                cmd.Parameters.AddWithValue("@ItQty", Int32.Parse(dataGridView1.CurrentRow.Cells["dgvQty"].Value.ToString()));
                cmd.ExecuteNonQuery();
                con.Close();
                MessageBox.Show("This Product has been successfully addedd!", "POS", MessageBoxButtons.OK);
                LoadProducts();
            }
            catch (Exception ex)
            {
                con.Close();
                MessageBox.Show(ex.Message);

            }
        }

To resolve this issue, consider implementing the following modifications in your code.

• Ensure the quantity text box initializes with a value of zero:
  • Set the default value of the quantity text box to zero.
• Restrict the quantity text box to accept only integer values:
  • Implement input validation to allow only whole numbers in the quantity text box.

    private void txtQuantity_KeyPress(object sender, KeyPressEventArgs e)
    {
        if (e.KeyChar >= 48 && e.KeyChar <= 57 || e.KeyChar == 8)
        {
            e.Handled = false;
        }
        else
        {
            e.Handled = true;
        }
    }

     

• You should use int.TryParse to check if the conversion is valid before performing the subtraction operation.

  private void UpdateItem()
  {
      int.TryParse(txtQuantity.Text, out int quantityValue);
      int NewQty = Stock - quantityValue;
  
      // rest of your code
  }

   

It's saying why it is saying?Google how to use and purpose of int32.TryParse(), learn to mouse over on value and see what value it contains, check value has any white space etc while debugging code.

Thank you Sam and Rajanikant 

I have remodify the code this way but it says input string was not in a correct format

 public void UpdateItem()
        {
           
            try
            {
                            
                    con.Open();          
                    cmd = new SqlCommand("update tblItem set ItQty = ItQty - " + int.Parse(txtQuantity.Text) + "where ItId like '" +                            Convert.ToInt32(dataGridView1.SelectedRows[0].Cells[0].Value.ToString()) + "'", con);
                    cmd.Parameters.AddWithValue("@ItId", Convert.ToInt32(dataGridView1.CurrentRow.Cells["dgvID"].Value.ToString()));                                   
                    cmd.ExecuteNonQuery();
                    con.Close();
                    MessageBox.Show("This Product has been successfully addedd!", "POS", MessageBoxButtons.OK, MessageBoxIcon.Information);
                                   
            }
            catch (Exception ex)
            {
                con.Close();
                MessageBox.Show(ex.Message);

            }
        }

You can use the debugger to see what the data actually is that is not valid. In the image you posted, when you have that error, you can just hover over txtQuantity.Text to see what is there. Or you can select it and press Shift-F9.

It is better to use Int32.TryParse instead of Convert.ToInt32 and then if the conversion fails you can issue an error for the user.

Your reading from data reader which is already closed.

In the diagram below, I have two dataGridView the product dataGridView and ClientBilling dataGridView, so when I select product from  the product dataGridView to the client dataGridView the quantity on the product dataGridView should be updated, that is what am trying to do and am getting this error

I use a different approach here but still,