Question

In following code snippet, AddRow() is called from a non-UI thread:

  public partial class Form1 : Form
  {
    public delegate void InvokeDelegate();

    ...
    SqlConnection mSqlConnection = new SqlConnection("Data Source=" + Environment.MachineName + "\\SQLEXPRESS; Initial Catalog=orderDB; Integrated Security=TRUE; MultipleActiveResultSets=True;");

    DataSet mDataSet = new DataSet();
    SqlDataAdapter mSqlDataAdapter = new SqlDataAdapter();


    ...
    private void UpdateGridView()
    {
      if (mSqlConnection.State == ConnectionState.Closed)
        mSqlConnection.Open();

      mSqlDataAdapter.SelectCommand = new SqlCommand("SELECT * FROM customerTable", mSqlConnection);
      mDataSet.Clear();
      mSqlDataAdapter.Fill(mDataSet);
      dataGridView1.DataSource = mDataSet.Tables[0];

      if (mSqlConnection.State == ConnectionState.Open)
        mSqlConnection.Close();
    }


    public void AddRow(int field1, int field2, int field3)
    {
      mSqlDataAdapter.InsertCommand = new SqlCommand("INSERT INTO customerTable VALUES(@field1, @field2, @field3)", mSqlConnection);

      mSqlDataAdapter.InsertCommand.Parameters.Add("@field1", SqlDbType.Int).Value = field1;
      mSqlDataAdapter.InsertCommand.Parameters.Add("@field2", SqlDbType.Int).Value = field2;
      mSqlDataAdapter.InsertCommand.Parameters.Add("@field3", SqlDbType.Int).Value = field3;

      mSqlConnection.Open();
      mSqlDataAdapter.InsertCommand.ExecuteNonQuery();
      dataGridView1.BeginInvoke(new InvokeDelegate(UpdateGridView)); // UpdateGridView() won't work from a non-UI thread
      mSqlConnection.Close();

    }
}

Before having to call AddRow() from a non-UI thread, I had UpdateGridView() being called directly and it worked flawlessly. But now AddRow() is no longer guaranteed to be called from a UI thread and so I replaced the direct call with dataGridView1.BeginInvoke().

As soon as I did that, my form-based application started throwing a System.InvalidOperationException every several AddRow() calls, breaking on the mSqlDataAdapter.Fill(mDataSet); statement (!) with the following message:

Invalid attempt to call Read when reader is closed

My question is why?

1. What reader? the DataAdapter’s? The SqlConnection’s? The DataGridView’s data source?
2. I am taking care of surrounding BeginInvoke() with mSqlConnection’s Open() and Close() and I even open mSqlConnection (again!) if it isn’t open, so how come I’m receiving this “closed” error?
3. What is the correct way to solve this problem? (i.e. updating DataGridView from a non-UI thread)

Answer 1

The issue is certainly due to the race condition.

Remove these two lines from UpdateGridView,as it is not the right place to close your connection.

 if (mSqlConnection.State == ConnectionState.Open)
    mSqlConnection.Close();

use IAsyncResult to retrieve a waithandle and wait for the thread to complete the GridUpdate.

 IAsyncResult Result = dataGridView1.BeginInvoke(new InvokeDelegate(UpdateGridView));
 Result.AsyncWaitHandle.WaitOne();
 mSqlConnection.Close();