.NET, Programming, Visual Web Developer

ASP.NET: Handling Custom Update Commands with ItemCommand

04.03.08 | 3 Comments

When programming an ASP.NET application, it’s very simple to use the built-in Button and LinkButton controls to perform actions like adding a record to a database or saving changes. When you create one of these buttons, the HTML looks something like this:

<asp:LinkButton ID="EditButton" runat="server" CommandName="Edit">Edit</asp:LinkButton>

Notice the CommandName attribute. Built-in commands include Insert, Update, Delete, Edit, and Cancel. If you are using a container such as a DataGrid or a DataList, you can customize these commands by adding an event handler:

<asp:DataList ID="AppList" runat="server" DataSourceID="AppDataSource"_
DataKeyField="AppID" OnEditCommand="AppList_EditCommand">

But what if, instead of redefining the standard command, you want to create your own? This is very, very simple, and you implement it using the OnItemCommand event handler. The ItemCommand also applies to objects like the FormView, and adds a lot of flexibility to your applications. First add it to the container definition (a FormView in this example):

<asp:FormView ID="AppFormView" runat="server" DataSourceID="AppDataSource"_
DataKeyNames="AppID" OnItemCommand="AppFormView_ItemCommand">

You can do a lot of cool stuff in the Item Command procedure, but here we’re going to focus on a common problem: calculating fields before saving to the database. For example, if you are uploading a file using the FileUpload control, you can modify the Update command to save it in the database, or you may want to record the time each record is modified and the username of the person who modified it.

To implement the various commands, edit the <name>_ItemCommand procedure in your code and add a Select statement:

Protected Sub AppFormView_ItemCommand(ByVal sender As Object,_
ByVal e As System.Web.UI.WebControls.FormViewCommandEventArgs)

   Select Case e.CommandName

      Case "Cancel"

Let’s look at how to handle some of the standard commands, using the FormView as an example. These will be different for a DataList or other objects – that code can generally be found in the MSDN Library.

Most of the commands have a built-in method – for example, if the command is “Update”, the FormView comes with a UpdateItem method. Cancel is an exception; to implement this command, run FormView.DataBind to replace any changed fields with the values retrieved from the database.

Now we’re ready to create our own custom command. Let’s take a look at it first, then I’ll explain later:

<asp:LinkButton ID="RetireButton" runat="server"_

   Case "Retire"

      AppDatasource.UpdateParameters("AppRetired").DefaultValue = True

So – the first step is the create a button or link button invoking your custom command, in this case a command to retire an application. Once the button is created, you can add a Case to the ItemCommand procedure with the name of the new command, and then fill in your code. For this example we are using an update parameter to store the new value. You can also place this value in a bound control, as we’ll see in a minute.

There are a couple of limitations here. First, you can only use the parameter method if there are no controls bound to this field. If there are any, the parameter’s default value will be replaced by the value in the control. Second, if you will not be setting this parameter every time you update this record, then you must use a bound field. Otherwise when you update without specifying a value, the data in the database will be overwritten by the default. In practical terms this means that you’ll probably be writing this value to a bound control.

Since we only want to mark an application as retired once, we’ll instead stick this value in a bound field, and it will be “remembered” even when we don’t supply it:

   Case "Retire"

      AppRetiredCtrl = AppFormView.FindControl("AppRetired")
      AppRetiredCtrl.Value = True

All we had to do here is find the control (a HiddenField) and stuff in the value. When we call UpdateItem this new value will be saved to the database.

Customizing the built-in commands and adding your own new ones is a powerful way to build your application without writing too much extra code. If these examples help you out, please leave a note in the comments and let me know.

Clarification: I didn’t cover handling the standard FormView commands such as “Insert”, but to be clear: You should not call a FormView.InsertItem command (for example) from inside the ItemCommand handler if the CommandName actually is “Insert”. A FormView control always executes the built-in actions for the standard events, even if you have a custom handler defined inside the ItemCommand procedure. So you don’t have to call FormView.ItemUpdate for the “Update” command, or FormView.DeleteItem for the “Delete” command. Your code is in addition to the default actions of these commands, and takes place before they are executed.

If you are creating a custom command, then you will need to invoke the appropriate method, InsertItem or UpdateItem or whatever.

Things are a bit different for a DataList. It isn’t as tightly integrated with its data source, and doesn’t even have methods such as InsertItem – the programmer is responsible for handling these commands himself inside ItemCommand, and will usually implement something like DataSource.Update (or .Insert, or whatever).

I wasn’t too clear about this in my own mind and as a result I wasn’t clear in this post. Sorry about that.


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