[c#]
//this example show how to start editing cell specified by the row #1 and column #0
flyGrid.ActivePort.BeginEdit(
        flyGrid.ActivePort.Rows.GetNodeFromRow(1),
        flyGrid.ActivePort.Columns.VisibleColumns[0]);

[c#]
private void beginEditMnu_Click(object sender, System.EventArgs e)
{
  if (treeviewGrid.Selected != null)
  {
    //get next node for editing
    NodeBase nextNode = treeviewGrid.Selected.GetNext() as NodeBase;
    if (nextNode != null)
    {
      treeviewGrid.ActivePort.BeginEdit(node, treeviewGrid.Columns.Items[0]);
    }
  }
}

private void endEditMnu_Click(object sender, System.EventArgs e)
{
  treeviewGrid.ActivePort.EndEdit(true);
}

[C#]
public class ExtGrid : FlyGrid
{
  protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
  {
    int lastRow = Rows.Items.Count -1;
    int row  = Selected.Index;
    int col = Columns.CurrentColumn;
    if(keyData == Keys.Down || keyData == Keys.Return)
    {
      if (row < lastRow)
        row++;
      else
        row = 0;
    }
    else if (keyData == Keys.Up)
    {
      if (row > 0)
        row--;
      else
        row = lastRow;
    }
    else
    {
      return base.ProcessCmdKey (ref msg, keyData);
    }
    ActivePort.EndEdit(true);
    ActivePort.BeginEdit(row, col);
    return true;
  }

  //this method uses a little trick to call SelectAll on internal editor.
  protected override bool OnMouseClick(FlyGridViewPort port, NodeBase node, int col, int x, int y, int count, System.Windows.Forms.MouseButtons button)
  {
    bool res =  base.OnMouseClick (port, node, col, x, y, count, button);
    if (res)
    {
      TextBox editor = (this as IServiceProvider).GetService(typeof(NineRays.Windows.Forms.Grids.IEditSelectionService)) as TextBox;
      if (editor != null && editor.Visible)
      {
        editor.SelectAll();
      }
    }
    return res;
  }
}

    Private Function FlyGrid1_IsReadOnly(ByVal sender As System.Object, ByVal node As NineRays.Windows.Forms.Data.NodeBase) As Boolean Handles FlyGrid1.IsReadOnly
        Dim row As DataRow = node.Value.Row

        Return DetermineWhetherARowIsReadOnly(row)
    End Function

    Dim res As Boolean = MyBase.OnMouseClick(port, node, col, x, y, count, Button)
    Dim editor As TextBox = GetService(GetType(NineRays.Windows.Forms.Grids.IEditSelectionService))

        If Not editor Is Nothing AndAlso editor.Visible Then
            editor.SelectAll()
            Return res
        End If

Is the following the equivalent Visual Basic syntax for your OnMouseClick() method? When I try it, the debugger shows that editor is Nothing.

Dim res As Boolean = MyBase.OnMouseClick(port, node, col, x, y, count, Button)
Dim isp As IServiceProvider = CType(Me, IServiceProvider))
Dim editor As TextBox = isp.GetService(GetType(NineRays.Windows.Forms.Grids.IEditSelectionService))

If Not editor Is Nothing AndAlso editor.Visible Then
  editor.SelectAll()
Return res
End If