Question

I have a form and there is no control.I am trying get the controls from Database so my project is slow i thinks i can use threading but I get a error.

    void Form_Load(object sender,EventArgs e)
    {

    SqlDataAdapter adap=new SqlDataAdapter("Select * from MyControls");
    DataTable dt=new DataTable();

    adap.Fiil(dt);

     foreach(DataRow dr in dt.Rows) 
     {
      ThreadStart ts=delegate{ Sample1(dr) };
Thread th=new Thread(ts);
th.start();

     }

    }

    public void Sample1(DataRow dr)
    {
    this.Invoke(new AddControlsDelegate(AddControls),new object[] {dr } );
    }
    public void AddControls(DataRow dr)
    {
    TextBox tx=new TextBox();
    tx.Name=dr["Id"].ToString();
    this.Controls.Add(tx);

    }

public delegate void AddControlsDelegate(DataRow dr);

I am tryin with this code .But it dont work.It adding same control twice time,3 time,4 time

Where is the my wrong?
thanks

Answer 1

You are closing over the loop variable:

 foreach(DataRow dr in dt.Rows) 
 {
   ThreadStart ts=delegate{ Sample1(dr) };
   ...
 }

You are creating a closure of the loop variable, not its value at the time – since the thread will be only started a little time later, the loop has completed and each thread will use the value of the loop variable at that time, which will be the last row.

Instead create a local variable within the loop and use it in your delegate:

 foreach(DataRow dr in dt.Rows) 
 {
    DataRow currentRow = dr;
    ThreadStart ts=delegate{ Sample1(currentRow) };
    Thread th=new Thread(ts);
    th.start();
 }

Also see “Closing over the loop variable considered harmful”