Create custom columns for ASP.NET DATAGRID

Before studying how to create a new column type, let's take a look at how to use DropDownList to use DropDownList directly to use DropDownList to solve the drop-down list without custom columns. ItemTemplate will only contain plain text representative of the current value, and EditItemTemplate contains a control that needs to be managed at runtime.

< asp: BoundColumn DataField = "OrderID" ReadOnly = "True" HeaderText = "Order ID" /> <% # Container.DataItem (" ShipVia ")%> Binding DataGrid code:

Sub BindGrid () Dim SQL As String = "SELECT OrderID, ShipName, ShipCountry, ShipVia FROM Orders" Dim DA As SqlDataAdapter = New SqlDataAdapter (SQL, ConnStr) Dim DS As New DataSet DA.Fill (DS, "Orders") DataGrid1. DataSource = DS.TABLES ("Orders"). DefaultView DataGrid1.Database () End Sub

The currently edited item is bound to DropDownlist when triggering a DataGrid ItemDatabase. When using the itemDatabase event, check the listitemType of the current project, otherwise you may find that you are using the HeaderItem or other non-applicable project types. Quote the DropDownList control for EditItem. In the following code, I use the cell control set directly (in order to keep the following examples), however, you can use a simple method to specify ID directly to the DROPDOWNLIST control, and use the DataGrid project's FindControl method locate control Quote. Since the DataGrid is bound to the DataTable default view, the element of the view is the DataRowView type, so you can convert the DataItem property of the current project to a DataRowView instance. This way, you can directly reference the fields in DataItem directly by field name. Using this method, save the current value of "ShipVia" into this record and use it to select the corresponding drop-down list item. Private Sub DataGrid1_ItemDataBound (ByVal sender As Object, _ ByVal e As System.Web.UI.WebControls.DataGridItemEventArgs) _ Handles DataGrid1.ItemDataBound If e.Item.ItemType = ListItemType.EditItem Then Dim DRV As DataRowView = CType (e.Item. DataItem, DataRowView) Dim CurrentShip As String = DRV ( "ShipVia") Dim DDL As DropDownList = _ CType (e.Item.Cells (4) .Controls (1), DropDownList) Dim SQL As String = _ "SELECT ShipperID, CompanyName FROM Shippers ORDER BY ShipperID "Dim DA As SqlDataAdapter = New SqlDataAdapter (SQL, ConnStr) Dim DS As New DataSet Dim item As ListItem dA.Fill (DS," Shippers ") DDL.DataSource = dS.Tables (" Shippers "). DefaultView DDL.DataTextField = "CompanyName" DDL.DatavalueField = "ShipperID" DDL.DataBind () item = DDL.Items.FindByvalue (CurrentShip) If Not item Is Nothing Then item.Selected = True End IfEnd Sub

Finally, write the code that retrieves the currently selected value from the DropDownList, and executes the database update:

Private Sub DataGrid1_UpdateCommand (ByVal source As Object, _ ByVal e As System.Web.UI.WebControls.DataGridCommandEventArgs) _ Handles DataGrid1.UpdateCommand Dim DDL As DropDownList = _ CType (e.Item.Cells (4) .Controls (1), DropDownList) Dim NewShip As Integer = DDL.Selectedvalue Dim OrderID As Integer = Int32.Parse (e.Item.Cells (1) .Text) Dim SQL As String = _ "UPDATE Orders SET ShipVia = @ Ship WHERE OrderID = @ ID" Dim Conn As SqlConnection = New SqlConnection (ConnStr) Dim Cmd As New SqlCommand (SQL, Conn) cmd.Parameters.Add (New SqlParameter ( "@ Ship", NewShip)) cmd.Parameters.Add (New SqlParameter ( "@ ID" , OrderID) Conn.open () cmd.executenononquery () CONN.Close () DataGrid1.edititeMindex = -1 BindGrid () End Sub All columns: DataGridColumn

We review all the parent types of the built-in type type DataGridColumn. (See Figure 1.) It contains properties and methods commonly used in all DataGrid columns. Types with star indicate that you want to use the type of custom column type.

DataGridColumn properties FooterStyle (TableItemStyle) FooterText (string) HeaderImageUrl (string) HeaderStyle (TableItemStyle) HeaderText (string) ItemStyle (TableItemStyle) SortExpression (string) the Visible (Boolean) DataGridColumn method Initialize InitializeCell LoadViewState OnColumnChanged SaveViewState TrackViewState

Reusable methods: Create DropDownColumn

First create a new category library in Microsoft® Visual Studio® .NET and name it MycustomColumn. Add a new class DROPDOWNCOLUMN and make sure you add a namespace in your class definition so your initial code is as follows:

Namespace MyCustomColumn Public Class DropDownColumn Inherits DataGridColumn Public DataSource As ICollection Public DataField As String Public DataTextField As String Public DatavalueField As String End ClassEnd Namespace I also declare four public properties, as follows:

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• DataSource . It is used to populate the DropDownList data set. Can be anything that implements the ICollection interface. In the examples of this article, I use ArrayList and dataview .

• DataField . It is the field in the parent DataGrid data source, which corresponds to the data selected from the drop-down list. For example, if DataSource contains a status set, DataField will be similar to "Statecode", or you can use the status in the form.

• DataTextField . This is the text to be displayed in the drop-down list, or it is not.

• data

  Valuefield . This is the value indicating the special drop-down option. data

  Valuefield is usually an integer value or other code, and DataTextField is a text description of the user's more meaningful.

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  Next, override Initializecell, it is a natural event of the DataGrid column. All cells in the column will happen to INITIALIZECELL, which is very similar to the itemcreated events directly using DataGrid. You can use it to manage cell content, such as setting Headertext, add the DropDownList control you will add data. I have added a handler to the cell's DataBinding event, which needs to take different processing based on whether the current is currently editing. Each system.web.ui.control has a DataBinding event. When the data is bind to the control, you can access the underlying data here, this example is the TableCell object in the DataGrid.

  Public Overrides Sub InitializeCell (ByVal cell As TableCell, _ ByVal columnIndex As Integer, _ ByVal itemType As ListItemType) MyBase.InitializeCell (cell, columnIndex, itemType) Select Case itemType Case ListItemType.Header cell.Text = HeaderText Case ListItemType.Item, ListItemType .AlternatingItem AddHandler cell.DataBinding, AddressOf ItemDataBinding Case ListItemType.EditItem AddHandler cell.DataBinding, AddressOf EditItemDataBinding Dim DDL as New DropDownList cell.Controls.Add (DDL) End Select End Sub followed ItemDataBinding routine, when the Item of Datagrid The routine will be triggered when the AlternatingItem is data. You need to reference back to TableCell, you can directly convert the "sender" object that is passed to the event, and then use the Tablecell's NamingContainer attribute to reference the current DataGridItem. This can only display the contents of Datafield in plain text format, just like displayed in BoundColumn. Finally, if the user specified by the user does not exist, I will display more friendly error messages for the user, not only the "index beyond range".

  Private Sub ItemDataBinding (ByVal sender As Object, ByVal e As EventArgs) Dim cell As TableCell = CType (sender, TableCell) Dim DGI As DataGridItem = CType (cell.NamingContainer, DataGridItem) Try cell.Text = DGI.DataItem (DataField) Catch Rangeex As IndexOutofrangeException Throw New Exception ("Specified Datafield Was Not Found.") Catch Otherex As Exception Throw new exception (Otherex.innerexception.toString) End Try End Sub

  Next, write the code of the EditItemDataBinding event. When a row enters the Edit mode, the event will be triggered on our custom column cell. Retrieve the current cell again and insert the DROPDOWNLIST control when the Initializecell method is called. Add an empty project in DropDown to the first option, if the current data in the column does not match any items you choose from the DataSource collection and placed in the list, select this option.

  Then, you need to determine the type of the incoming collection. In this case, I will handle two cases: Passing a set of strings through ArrayList, or DataView in the data table, which is constructed by the DATAROWVIEW project. For string data, I will enter a new ListItem and set the value of the pull-up item and the text. Since both cases are the same, only the text is required here. But I will select the corresponding item to make a selection according to the value so as to be consistent with the next example, and the next example will set a separate value attribute. For the DataRowView project, in the previous example, DataRowViewInstance ("FieldName

  ") Returns an object indicating data in the field. You can search DataTextField and Data using the same method.

  Valuefields needed.

  Finally, some exceptions are throwing with common errors encountered when the developer uses columns, such as send invalid field names to the DataField property, or incompatible DataSource type. I have made hardcodes to the abnormal message to be popped, but I hope that you save these messages to a more configurable location in the actual application. For example, if you want to use your application within a global scale, you can save it to your web.config file or resource file. Similarly, you don't have to throw "Data Datafield" exceptions again, because it may have been captured in the ItemDataBinding event before the DataGrid is placed in the "Edit" mode.

  Private Sub EditItemDataBinding (ByVal sender As Object, _ByVal e As EventArgs) Dim cell As TableCell = CType (sender, TableCell) Dim DDL As DropDownList = _CType (cell.Controls (0), DropDownList) Dim DataSourceItem As Object Dim item As ListItem Dim DGI As DataGridItem 'add an empty first option DDL.Items.Add (New ListItem ( "")) For Each DataSourceItem In DataSource Select Case DataSourceItem.GetType.ToString Case "System.String"' is applied to ArrayList exemplary item = New ListItem ( DataSourceItem, DataSourceItem) DDL.Items.Add (item) Case "System.Data.DataRowView" Dim DRV As DataRowView = _ CType (DataSourceItem, DataRowView) item = new_ ListItem (DRV (DataTextField), DRV (DatavalueField)) DDL.Items .Add (item) Case ELS e Throw New Exception ( "Invalid DataSource type.") End Select Next Try DGI = CType (cell.NamingContainer, DataGridItem) item = DDL.Items.FindByvalue (DGI.DataItem (DataField)) Catch RangeEx As IndexOutOfRangeException Throw New Exception ( " Specified Datafield Was Not Found. ") Catch Otherex As Exception Throw new exception (Otherex.innerexception.tostring) End Try if not item is nothing kiln = truend SUB Using DropdownColumn

  The above is all the code you need to create a DROPDOWNCOLUMN class. Let's take a look at how to use the control in your application. If you are studying at home, but have not yet started, please compile the namespace created above to MyCustomColumn.dll and copy it to the / bin folder of the application you want to experiment. In this example, I created a new web application UsecustomColumn, and add a reference to my / bin directory mycustomcolumn.dll. At the top of the ASPX file, add @register instruction: <% @ register tagprefix = "dgg" namespace = "mycustomcolumn" assembly = "mycustomcolumn"%>

  Note that the new DataGrid column type does not appear in the Visual Studio .Net property generator for DataGrid, so you need to enter the HTML view and add a column declaration. Make sure the DataGrid declaration is located in a group of

  "Server

  "> ... tags, these tags are used to deal with the rest of the postback.aspx files as follows:

  ! <% @ Page Language = "vb" AutoEventWireup = "false" Codebehind = "WebForm1.aspx.vb" Inherits = "UseCustomColumn.WebForm1" Trace = "False" Debug = "True"%>