Conversion failed when converting the varchar value '' to data type int

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C#

<pre lang="c#">
try
{
string txt = "select * from Products where id='" + txtSalesItemNo.Text + "'";
                    SqlCommand cmd = new SqlCommand(txt, con);
                    con.Open();
                    SqlDataReader re = cmd.ExecuteReader();

while (re.Read())
{
                        
int price = int.Parse(txtSalesQty.Text.ToString()) * int.Parse(re[4].ToString());
totalprice = totalprice + price;
dataGridView1.Rows.Add(dataGridView1.RowCount, re[0], re[1], txtSalesQty.Text.Trim(), re[4], price);   
                    }

lbTotalItem.Text = " " + (dataGridView1.RowCount - 1) + "";
lbTotal.Text = " " + totalprice + " ";

con.Close();

                }
                catch(Exception ee)
                {
                    MessageBox.Show(ee.Message);
                }
                finally
                {
                    con.Close();
                }

What I have tried:

I am struggling for a few days with this issue and I can't figure out how can I fix it.

Posted 18-Aug-18 2:12am

Member 11236638

Updated 18-Aug-18 2:25am

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Santosh kumar Pithani 18-Aug-18 8:26am

Hello , string datatypes only give under single quotation('value') not integers

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OriginalGriff · Accepted Answer · 2018-08-18T02:26:00

Two things:
1) Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'

The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'

Which SQL sees as three separate commands:

SQL

SELECT * FROM MyTable WHERE StreetAddress = 'x';

A perfectly valid SELECT

SQL

DROP TABLE MyTable;

A perfectly valid "delete the table" command

SQL

--'

And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

2) Never send user strings that should be numbers to anywhere unless you have fully validated them. Users make mistakes - and if you don't validate, you don't know.

So, at the top of your method, use int.TryParse to convert the user input value:

C#

int saleItemNo;
if (!int.TryParse(txtSaleItemNo.Text, out saleItemNo))
   {
   MessageBox("the Sale Item needs to be a number: " + txtSaleItemNo.Text);
   return;
   }

Then when you use that value to access SQL, use a parameterised query to pas the converted value:

C#

string txt = "SELECT * FROM Products WHERE id=@SIID";
SqlCommand cmd = new SqlCommand(txt, con);
cmd.Parameters.AddWithValue("@SIID", salesItemNo);

And go through the whole of your app replacing concatenation with parameters - miss one, and your DB is toast.