Library tutorials & articles
Winforms Data Binding Lessons Learned
- Displaying Many-to-Many Relationships
- Reacting to Illegal Row Editing in a DataGrid
- Reacting to Illegal Row Deletion in a DataGrid
- How to Make a Non-DataGrid Control Act as the Mast
Reacting to Illegal Row Deletion in a DataGrid
This part is a little trickier than before, since you can't just throw an exception which is handled by the DataGrid. For some reason you have to go through some hoops to create this kind of functionality yourself. The first trick to understanding how to accomplish this is to understand how the DataGrid displays its data. You should notice that the DataGrid does not have any properties, such as AllowDelete or AllowAddNew. In fact, you can't control how your users should interact with the grid using any of the grid's properties (except Enabled). The trick is that the DataGrid is actually basing its interactivity level on an underlying DataView object to which it is bound. If you look at a DataView object, you should see that it does contain these kinds of properties. So, setting AllowDelete on the DataGrid's underlying DataView object to false disables delete functionality from the grid.
Now, the technique becomes pretty simple: we disable the AllowDelete property of the DataView and catch the KeyDown event of the DataGrid. If the KeyCode is that of the Keys.Delete key, we enable deletion on the DataView, delete the current data row, and disable deletion again until next time. Here's the code:
Private Sub grd_KeyDown(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles grd.KeyDown
If e.KeyCode = Keys.Delete Then
'check whether the current DataRow Course
'is in Progress and cancel deletion if it is
If IsInProgress(CType(GetGridCurrency().Current, DataRowView).Row) Then
MsgBox("Already in progress")
Else
'Delete the current row
EnableDelete(True)
CType(GetGridCurrency().Current, DataRowView).Delete()
EnableDelete(False)
End If
End If
End Sub
Private Function GetGridCurrency() As CurrencyManager
'return the currency manager associated with the DataGrid
Dim cm As CurrencyManager = BindingContext(grd.DataSource, grd.DataMember)
Return cm
End Function
Private Sub EnableDelete(ByVal value As Boolean)
'enable or disable AllowDelete on the
'DataGrid() 's underlying DataView
Dim cm As CurrencyManager = GetGridCurrency()
Dim view As DataView = CType(cm.List, DataView)
view.AllowDelete = value
End Sub
Notice that what I'm actually deleting is a DataRowView object, not a DataRow. This is because just like a data table has data rows, a DataView has DataRowViews corresponding to each DataTable row. Each DataRowView holds a property reference to the data row associated with it. Performing actions on a DataRowView is just like performing them on a DataRow object.
That's it. There's no way around this that I could find, but I have seen one other way to accomplish this functionality, by deriving a custom grid from the DataGrid and pre-processing WM_KEYDOWN messages before they arrive.
Roy Osherove has spent the past 6+ years developing data driven applications for various companies in Israel. He's acquired several MCP titles, written a number of articles on various .NET topics, ...
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The solution is to bind the entire Child table to a bindingSource and that to a dataGridView. In this case the child table would be the courses that the soldier registered for.
Then, whenever a soldier is selected, store the selected row in a global variable, such as lastSelectedSoldierRow. Then, when that changes, you do a select on the table that connects the two. The registers table in this case. For example registerDataTable.Select("soldier_ID = " + lastSelectedSoldierRow.ID.ToString());
Take all those rows and add the course Ids to a globally declared stringBuilder. Whenever a different soldier is selected, add -1 to the stringBuilder as an initializer. Then, for each additional course ID added to the stringBuilder, add a comma to the stringBuilder before it. You'll end up with something like "1,2,3,4,5,6". Next, set the child table's bindingSource.Filter to a that string like so: Filter = "ID in (1,2,3,4,5)".
That will filter all the courses, down to the ones which belong to the soldier. Whenever a new course is added, append the ID to the stringBuilder and reset the filter.
Here's an example scrap of source code from my own project. It's messy, but may be helpful to somebody if the above wasn't enough.
StringBuilder settingsFilterSB = new StringBuilder();
private void dataGridViewComputers_SelectionChanged(object sender, EventArgs e)
{
if (this.dataGridViewComputers.SelectedRows.Count == 1 || this.dataGridViewComputers.SelectedCells.Count > 0)
{
object dataBoundItem;
if (this.dataGridViewComputers.SelectedRows.Count == 1)
dataBoundItem = this.dataGridViewComputers.SelectedRows[0].DataBoundItem;
else
dataBoundItem = this.dataGridViewComputers.Rows[this.dataGridViewComputers.SelectedCells[0].RowIndex].DataBoundItem;
lastSelectedPcRow = (CallistoDataSet.ComputerOrGroupRow)((DataRowView)dataBoundItem).Row;
settingsFilterSB = new StringBuilder();
settingsFilterSB.Append("-1");
int selectedPcId = lastSelectedPcRow.ID;
foreach(CallistoDataSet.ComputerOrGroup2SettingsRow row in this.callistoDataSet.ComputerOrGroup2Settings.Rows)
{
if(row.ComputerOrGroup_ID == selectedPcId)
settingsFilterSB.Append("," + row.ComputerSettings_ID.ToString());
}
this.computerSettingsBindingSource.Filter = "ID in (" + settingsFilterSB.ToString() + ")";
Use a BindingSource control then use its Find method to get the index of the row.
I'm getting this error message: "Object reference not set to an instance of an object." when I try to run the code below. I've spent a couple days trying to figure out what it is, but I'm kind of new to vb.net and programming. I have two tables that look something like below (not all fields are shown). They should be related via the userID fields as a "many tasks to each user" relation. Any help on how to do this or why there is an error when I run the code would be greatly appreciated.
Table: Users Table:Tasks
userID userID
userName taskID
password taskName
Dim
connection As New OleDbConnection("Provider=Microsoft.JET.OLEDB.4.0; data source=" & Application.StartupPath & "\blueteam2.mdb") Dim adpUsers As New OleDbDataAdapter("SELECT * FROM Users,Tasks,Projects", connection) Dim ds As New DataSet TryadpUsers.Fill(ds, "Users")
ds.Relations.Add("UserTasks", ds.Tables("Users").Columns("userID"), ds.Tables("Tasks").Columns("userID"))
dgUsers.SetDataBinding(ds, "Users.userID")
dgTasks.SetDataBinding(ds, "Tasks.userID")
Catch ex As ExceptionMessageBox.Show(ex.Message, "Admin Main Form Load")
End TryAnyway, I just want to thank you for this nice and realy usefull article.
I not only got the relationships shown in a DataGrid, but also I could
modify them through the DataSet.
try
Private view As DataView = m_ds.Tables("Stuff").DefaultView
But anyway I think that the trick is that the field ID is PK
I tried your code but the dv index is out of sync with the bm index. Please explain. You can email me at glenn_r@shaw.ca.
Here my challenge. I have reference to a datarow in a datatable. I want the set the bindingcontext position to that datarow. How can I do this? When I sort the dataview it goes out of sync with the datatable index returned when I use the dv find method.
Thanks,
Glenn
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