“Object reference not set to an instance of an object” when filtering dataGridView

Question

I have this search function, in order to flag comments from MySQL database that matched a list of keywords, flagged comments will be displayed on the dataGridView_flaggedComments , then follow by populating comboBox_stockIndex with the involved share price symbol (eg BARC, LLOY, TSCO).

private void button_Search1_Click(object sender, EventArgs e)
{
    commentCount = 0;
    dataGridView_flaggedComments.Refresh();
    DataTable flaggedcomments = new DataTable("flaggedcomments");
    using (MySqlConnection sqlConn = new MySqlConnection(strProvider))
    {
        using (MySqlDataAdapter da = new MySqlDataAdapter("SELECT Comment_ID, Comments_Date, Comments_Time, Author, Title, Comments_Comment, Tickers_Ticker_ID FROM comments ORDER BY Comments_Date ASC, Comments_Time ASC", sqlConn))
        {
            da.Fill(flaggedcomments);
        }
    }
    StringBuilder sb = new StringBuilder();
    string[] words = File.ReadAllLines(sourceDirTemp + comboBox_crimeKeywords.SelectedItem.ToString() + ".txt");
    var query = flaggedcomments.AsEnumerable().Where(r => words.Any(wordOrPhrase => Regex.IsMatch(r.Field<string>("Comments_Comment"), @"\b" + Regex.Escape wordOrPhrase) + @"\b", RegexOptions.IgnoreCase)));

    dataGridView_flaggedComments.DataSource = query.AsDataView();

    PopulateStockIndex();
}


private void PopulateStockIndex()
{
    comboBox_stockIndex.Items.Clear();
    comboBox_stockIndex.Items.Add("Choose to Filter");
    DataTable link_stockIndex = new DataTable("link_stockIndex");
    using (MySqlConnection sqlConn = new MySqlConnection(strProvider))
    {
        using (MySqlDataAdapter da = new MySqlDataAdapter("SELECT Ticker_ID, Symbol FROM tickers", sqlConn))
        {
            da.Fill(link_stockIndex);
        }
    }
    foreach (DataRow da in link_stockIndex.Rows)
    {
        for (int i = 0; i < dataGridView_flaggedComments.Rows.Count - 1; i++)
        {
            if (dataGridView_flaggedComments.Rows[i].Cells["Tickers_Ticker_ID"].Value.ToString() != "" && dataGridView_flaggedComments.Rows[i].Cells["Tickers_Ticker_ID"].Value.ToString() == da["Ticker_ID"].ToString())
            {
                if (!comboBox_stockIndex.Items.Contains(da[1].ToString()))
                {
                    comboBox_stockIndex.Items.Add(da[1].ToString());
                }
                comboBox_stockIndex.SelectedIndex = 0;
            }
        }
    }            
}

Next, if I select a symbol from the comboBox_stockIndex , the dataGridView_flaggedComments should be filtered to show only the comments relevant to the selected symbol (once symbol is selected, it will look for the symbol's Tickers_TTicker_ID, then filter by the Tickers_Ticker_ID). But the below code wouldn't work. There's an error saying "Object reference not set to an instance of an object." for this line (dataGridView_flaggedComments.DataSource as DataTable).DefaultView.RowFilter = string.Format("Tickers_Ticker_ID = '{0}'", da["Ticker_ID"]); . I tried to debug, but I don't understand what went wrong.

private void comboBox_stockIndex_SelectedIndexChanged(object sender, EventArgs e)
{
    DataTable link_stockIndex = new DataTable("link_stockIndex");
    using (MySqlConnection sqlConn = new MySqlConnection(strProvider))
    {
        using (MySqlDataAdapter da = new MySqlDataAdapter("SELECT Ticker_ID, Symbol FROM tickers", sqlConn))
        {
            da.Fill(link_stockIndex);
        }
    }
    foreach (DataRow da in link_stockIndex.Rows)
    {
        for (int i = 0; i < dataGridView_flaggedComments.Rows.Count - 1; i++)
        {
            if (dataGridView_flaggedComments.Rows[i].Cells["Tickers_Ticker_ID"].Value.ToString() != "" && comboBox_stockIndex.SelectedItem.ToString() == da["Symbol"].ToString())
            {
                (dataGridView_flaggedComments.DataSource as DataTable).DefaultView.RowFilter = string.Format("Tickers_Ticker_ID = '{0}'", da["Ticker_ID"]);
            }
        }
    }
}

I've been spending two days looking into this, SOF is my last resort. Any help would be very very much appreciated! Thank you very much!

Answer 1

DataView is not DataTable .

Here you setting DataSoure as DataView :

dataGridView_flaggedComments.DataSource = query.AsDataView();

Here you casting it as DataTable :

(dataGridView_flaggedComments.DataSource as DataTable).DefaultView.RowFilter =...

So if I inderstood quickly your problem, solution would be:

(dataGridView_flaggedComments.DataSource as DataView).RowFilter =...

Next time add breakpoint to the place where you getting this kind of error. Look at what boxed type you have and to what type you trying to unbox. Also I would not suggest to use as like you do. Better do like so:

DataView dv = dataGridView_flaggedComments.DataSource as DataView;
if(dv == null)
    throw new Exception("Bad Data Source type");
else
{
    //use dv here
}

Answer 2

As from your code: dataGridView_flaggedComments.DataSource = query.AsDataView();

So when you are trying to call this: (dataGridView_flaggedComments.DataSource as DataTable) you will get null , because your DataSource is not DataTable but DataView .

It looks like you should use something like (dataGridView_flaggedComments.DataSource as DataView).RowFilter instead.