C# error on displaying string on textbox

Question

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this is my error : Object reference not set to an instance of an object.

C#

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
          
            if (conn.State == ConnectionState.Closed)
            {
                conn.Open();

            }
                              
                
                string query = "SELECT DESCRIPTIO FROM stock ORDER by DESCRIPTIO ASC";
                da = new MySqlDataAdapter(query, conn);

                DataSet ds = new DataSet();

                da.Fill(ds, "stock");

                comboBox_Supp1.DataSource = ds.DefaultViewManager;
                comboBox_Supp1.DisplayMember = "stock.descriptio";
                MySqlCommand cmd = new MySqlCommand("SELECT ITEM_CODE FROM stock WHERE DESCRIPTIO = '" + comboBox_Supp1.Text + "'", conn);

                var varTemp = cmd.ExecuteScalar().ToString();
                if (varTemp != null)
                {
                     textBox1.Text = varTemp.ToString();
                }
                                  

            conn.Close();
      
        }

===EDIT: Please use code tags===
CodingK

Posted 24-Nov-14 14:15pm

Member 10124914

Updated 24-Nov-14 14:28pm

Sheepings

v2

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Comments

[no name] 24-Nov-14 20:31pm

Please use code tags when posting code, as this helps keep readable alignment of your code.

I suggest you also read up on ToString and when to use it. Search MSDN for ToString and cmd.ExecuteScalar()

syed shanu 24-Nov-14 21:14pm

Which line did you get the error.use break point and check for each line of code and find where exactly you get the error.

PIEBALDconsult 24-Nov-14 21:38pm

That is a total mish-mash of concepts; you really need to study the classes you are trying to use before you hurt yourself.

4 solutions

Solution 4

C#

private void comboBox_Supp1_SelectedIndexChanged(object sender, EventArgs e)
       {

           if (Classes.OpenConn.conn.State == ConnectionState.Closed)
           {
               Classes.OpenConn.conn.Open();
           }

           Com.CommandText = "SELECT ITEM_CODE FROM stock WHERE DESCRIPTIO = '" + comboBox_Supp1.Text + "'";
           reader = Com.ExecuteReader();
           while (reader.Read())
           {
               textBox1.Text = reader["ITEM_CODE"].ToString();
           }
           reader.Close();
           Com.Dispose();
       }

Posted 24-Nov-14 18:50pm

Member 10124914

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BacchusBeale · Accepted Answer · 2014-11-24T15:48:00

cmd.ExecuteScalar() returns a null Object if there are no rows returned: "The first column of the first row in the result set, or a null reference ". So you need to check the if this returns null before using toString().
Also ExecuteScalar only gives you the first column, sure you don't need ExecuteReader?

PIEBALDconsult · Accepted Answer · 2014-11-24T15:54:00

0) Do not use string concatenation to form an SQL statement; use a parameterized statement.
1) You are overusing ToString; learn to cast.
2) I suspect that ExecuteScalar is returnnig null, and then ToString is causing the Exception
"or a null reference (Nothing in Visual Basic) if the result set is empty"

var varTemp = cmd.ExecuteScalar().ToString();

DamithSL · Accepted Answer · 2014-11-24T17:15:00

apart from the answers provided by others, I think you are doing extra work to get ITEM_CODE, you can simply change the sql statement as

C#

string query = "SELECT DESCRIPTIO, ITEM_CODE FROM stock ORDER by DESCRIPTIO ASC";

then set both DisplayMember and ValueMember

C#

comboBox_Supp1.DisplayMember = "stock.descriptio";
comboBox_Supp1.ValueMember  = "stock.item_code";

then you can set get the combobox selected item value(ITEM_CODE) or value(ITEM_CODE) of any item by index

C#

textBox1.Text = comboBox_Supp1.Items[0].Value.ToString();

or

C#

textBox1.Text = comboBox_Supp1.SelectedValue.ToString();
//for above you need to select item

C# error on displaying string on textbox

4 solutions

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Solution 2

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