当组合框选定索引更改时，填充文本框

Question

In the code below, i just want to fill my textbox based on the selected combo box changed. but i get the following error.

'Conversion failed when converting the varchar value 'System.Data.DataRowViewConvert.ToString()' to data type int.'

i'd appreciate it if you help me.

SqlConnection objConnection = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\UniversityDataBase.mdf;Integrated Security=True");
        private void comboBox1_Click(object sender, EventArgs e)
        {
            string query = "SELECT *FROM TutorTable";
            SqlDataAdapter SDA = new SqlDataAdapter(query, objConnection);
            DataTable dt = new DataTable();
            SDA.Fill(dt);
            comboBox1.DataSource = dt;
            comboBox1.DisplayMember = "Tid";
            objConnection.Close();
        }

    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        string sqlQuery = "SELECT *FROM TutorTable where Tid = '"+comboBox1.Text+ "Convert.ToString()'";
        SqlCommand objCommand = new SqlCommand(sqlQuery, objConnection);
        objConnection.Open();
        objCommand.ExecuteNonQuery();
        SqlDataReader dr;
        dr = objCommand.ExecuteReader();
        while (dr.Read())
        {
            string Tname = (string)dr["Tname"].ToString();
            textBox1.Text = Tname;
        }

    }

Answer 1

Why are you using the Convert.ToString() in this piece of code:

"SELECT * FROM TutorTable where Tid = '"+comboBox1.Text+ "Convert.ToString()'"

I think the correct way wolud be:

"SELECT * FROM TutorTable where Tid = '"+comboBox1.Text+ "'"

But consider using a store procedure to prevent sql injection or using an ORM.

I think that comboBox1.Text is already returning a string value. So if that, it is not necessary to put the Convert.ToString(comboBox.Text) , just put comboBox1.Text

According to documentation, the Text property is a string https://msdn.microsoft.com/en-us/library/system.windows.forms.combobox.text(v=vs.110).aspx

Answer 2

I found the Solution as follows:

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\UniversityDataBase.mdf;Integrated Security=True");
        string sqlQuery = "SELECT *FROM TutorTable WHERE Tid = '" + comboBox1.Text + "'";
        SqlCommand objCommand = new SqlCommand(sqlQuery, con);
        con.Open();
        SqlDataReader dr;
        dr = objCommand.ExecuteReader();
        while (dr.Read())
        {
            string name = (string)dr["Tname"].ToString();
            textBox1.Text = name;
        }
    }

Answer 3

You have got the answer already but few more things should be taken care of

Your code

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    string sqlQuery = "SELECT *FROM TutorTable where Tid = '"+comboBox1.Text+ "Convert.ToString()'";
    SqlCommand objCommand = new SqlCommand(sqlQuery, objConnection);
    objConnection.Open();
    objCommand.ExecuteNonQuery();
    SqlDataReader dr;
    dr = objCommand.ExecuteReader();
    while (dr.Read())
    {
        string Tname = (string)dr["Tname"].ToString();
        textBox1.Text = Tname;
    }

}

Things to notice:

1) Use varabled instead of manipulating sql eg SELECT *FROM TutorTable where Tid = @id and pass id into sqlCommand object

2) You dont need to call ExecuteNonQuery before SqlDataReader

3) you need to use if(dr.Read() ) instead of while

4) You can directly assign value textbox. eg texBox1.Text = dr["Tname"].ToString();

5) Close the objConnection