C# window form application

Additional information: roomID is neither a DataColumn nor a DataRelation for table .

at this line    string selectedValue = selectedRow["roomID"].ToString();

Try this:

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (comboBox1.SelectedIndex != -1)
    {
        ComboBox comboBox = (ComboBox)sender;
        DataRowView selectedRow = (DataRowView)comboBox.SelectedItem;

        if (selectedRow != null)
        {
            string selectedValue = selectedRow["roomID"].ToString();
            string sqlQuery = "SELECT rRate FROM tblRoom WHERE roomID = @selectedValue";
            using (SqlConnection sqlConnection = new SqlConnection(connectionString))
            {
                sqlConnection.Open();
                using (SqlCommand cmd = new SqlCommand(sqlQuery, sqlConnection))
                {
                    cmd.Parameters.AddWithValue("@selectedValue", selectedValue);

                    using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                    {
                        DataTable dt = new DataTable();
                        da.Fill(dt);
                        if (dt.Rows.Count > 0)
                        {
                            txtRate.Text = dt.Rows[0]["rRate"].ToString();
                        }
                    }
                }
            }
        }
    }
}

Hello Amit,

Please I enconter another error as shown in the picture.

da.Fill(dt);

On which line you got the error?

Hello Amit,

Please I enconter another error as shown in the picture.

public void BindComboBox(ComboBox comboBox)
{
    string sqlQuery = "SELECT [roomId], [rName] FROM tblRoom";
    SqlDataAdapter sqlDataAdapter = new SqlDataAdapter(sqlQuery, MainClass.con);
    DataTable dataTable = new DataTable();
    sqlDataAdapter.Fill(dataTable);
    comboBox.DataSource = dataTable;
    comboBox.DisplayMember = "rName";
    comboBox.ValueMember = "roomId";
}

Call the above method in your page load event to bind the combobox.

private void Form1_Load(object sender, EventArgs e)
{
    BindComboBox(cmbRoom);

    // your existing code
}

private void cmbRoom_SelectedIndexChanged(object sender, EventArgs e)
{
    if (cmbRoom.SelectedIndex != -1)
    {
        string qry = "SELECT rRate FROM tblRoom WHERE roomID = @selectedValue";
        using (SqlCommand cmd = new SqlCommand(qry, MainClass.con))
        {
            cmd.Parameters.AddWithValue("@selectedValue", cmbRoom.SelectedValue);
            using (SqlDataAdapter da = new SqlDataAdapter(cmd))
            {
                DataTable dt = new DataTable();
                da.Fill(dt);
                if (dt.Rows.Count > 0)
                {
                    txtRate.Text = dt.Rows[0]["rRate"].ToString();
                }
            }
        }
    }
}

Hello Amit, 

Thank you so much for your help,

Please I have enconter this error as I follow your formate,

Hey, some where in your form you have bind data to combobox(cmbRoom) right?. There you have to bind combobox like this:

cmbRoom.DataSource = yourDataSource; // Set your data source here
cmbRoom.DisplayMember = "RoomName"; // Replace with the actual display column name
cmbRoom.ValueMember = "RoomId"; // Replace with the actual value column name

As you can see in the picture when I select room number then it would automatically select the rate from the Room dataGridview which is on a diferent page as you can see below, though everything seems to be working and that is the only problem am having now, so when I select different room number I want the rate change as well.

Thank you so much Amit, I have applied your format and am a bit confuse about what you mean by you should ensure that the ComboBox is properly bound to a data source, and the ValueMember property is set to the column that you want to use as the selected value and how I should apply this, kindly provide further clarification on how to go about it. thank you in advance

 cmbRoom.DataSource = yourDataSource; // Set your data source here
cmbRoom.DisplayMember = "displayColumnName"; // Replace with the actual display column name
cmbRoom.ValueMember = "valueColumnName"; // Replace with the actual value column name