How to display a Tooltip on C1Combo rows?

You can enable the CellTips in C1Combo by setting the CellTips property to something other than NoCellTips and then you can handle the FetchCellTips event to provide the text to be shown in the tooltip.

Please refer to the sample attached and let us know if you face any issues.

private void Form1_Load(object sender, EventArgs e)
{
    DataTable dt = new DataTable();
    string connectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" +
                 System.IO.Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments),
                 @"ComponentOne Samples\Common\C1NWind.mdb") + ";";
    OleDbConnection conn = new OleDbConnection(connectionString);
    OleDbDataAdapter adapter = new OleDbDataAdapter("Select * from Data", conn);
    adapter.Fill(dt);

    c1Combo1.DataSource = dt;

    c1Combo1.CellTips = C1.Win.C1List.CellTipEnum.Floating;
    c1Combo1.FetchCellTips += C1Combo1_FetchCellTips;
}

private void C1Combo1_FetchCellTips(object sender, C1.Win.C1List.FetchCellTipsEventArgs e)
{
    e.CellTip = $"Row{e.Row}";
}