当尝试从数据库中检索数据时，我收到错误“InvalidOperationException未处理”

Question

I have a form named Form1 :

There is one ComboBox and one TextBox , when I select US$ from the ComboBox then it must retrieve data from the database and display 150 in the TextBox .

This is myform code:

For ComboBox ;

namespace PCJ_System
{
  public partial class Form1 : Form
  {
    SqlConnection conn;
    SqlCommand cmd;
    SqlDataReader dr;
    public Form1()
    {
        InitializeComponent();
    }

     private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        string str = "server = DESKTOP-LKEG8FM\\SQLEXPRESS;initial catalog= PCJ_DB ; Integrated Security=True;";
        SqlConnection conn = new SqlConnection(str);
        conn.Open();
        conn = new SqlConnection(str);
        string GetData = "Select [FC_Rate] from Forcur where FC_TYPE ='" + comboBox1.Text + "' ";
        cmd = new SqlCommand(GetData, conn);
        var returnValue = cmd.ExecuteScalar();

        textBox1.Text = returnValue.ToString();
        conn.Close();
    }

  }
}  

My database table Forcur:

ID |FC_TYPE |FC_RATE|
1   US$      150
2   UK#      210

What's wrong with my code?

Answer 1

This might not be the exact answer you are looking for, but you need to take care of following:

1) Assign DB connection string to SqlConnection object and open connection.

2) Since you are assigning one value to textbox, you need to use ExecuteScalar instead of ExecuteReader

Once you fix this, you should get the desired result.

Example:

conn=new SqlConnection(connectionStringHere);
conn.Open();
string GetData = "Select [FC_Rate] from Forcur where FC_TYPE ='" + comboBox1.Text + "' ";
cmd = new SqlCommand(GetData, conn);
var returnValue = cmd.ExecuteScalar();

textBox1.Text = returnValue.ToString();
conn.close();

Note: You still have SQL injection attack open in your SQL query. Try using varables instead to stop that.