No value given for one or more required parameters. C#

Question

I have a C# program and I made a code for subtracting the amount of sold products from the amount of these products in the stock (access datatable) so I used this code :

foreach (DataGridViewRow r in dataGridView1.Rows)
{
      OleDbCommand command = new OleDbCommand("Update BookInserting set Amount = Amount - '" + Convert.ToInt32(r.Cells[1].Value) + "' where BookName = " + r.Cells[0].Value.ToString() + "", connection);
      connection.Open();
      command.ExecuteNonQuery();
      connection.Close();
}

but when I run it gives me this error : (No value given for one or more required parameters) . I tried solving it several times but failed, I hope you can help me solve it. Thanks in advance.

Answer 1

Your problem is probably caused by Access unable to recognize some part of your query as an object of the underlying table (or the table itself).
This problem and a more serious one called Sql Injection could be avoided using parameters. (And a side benefit your code becomes a lot clearer without all those strings concatenations)

So let's try to change your code in this way:

// String sql with parameters placeholders 
string cmdText = @"Update BookInserting 
                   set Amount = Amount - @amt
                   where BookName = @book";

connection.Open();

// Just build the command just one time outside the loop and 
// add the two parameters required (without a value and in the exact order
// expected by the placeholders
OleDbCommand command = new OleDbCommand(cmdText, connection);
command.Parameters.Add("@amt", OleDbType.Integer);
command.Parameters.Add("@book", OleDbType.VarWChar);

// Inside the loop just change the parameters values and execute
foreach (DataGridViewRow r in dataGridView1.Rows)
{
    // If the cell with the parameter for the WHERE 
    // clause is invalid skip the update
    if(!r.IsNewRow && r.Cells[0].Value != null  
                   && r.Cells[0].Value.ToString() != "")
    {
       cmd.Parameters["@amt"].Value = Convert.ToInt32(r.Cells[1].Value);
       cmd.Parameters["@book"].Value = r.Cells[0].Value.ToString();
       command.ExecuteNonQuery();
    }

}
connection.Close();

Final note. A connection object should be created each time you require it. From your code it is not clear if this is the case. Use the following pattern. ( Using Statement )

using(OleDbConnection connection = new OleDbConnection(....))
{
    ... code that uses the connection ....
} // <- here the connection is closed and disposed