[Implement the ASP. Net Control as day22] So that dropdownlist will no longer cause errors because the project list does not exist.

The dropdownlist control often has the following error message because there are no closed fields in the project list. Such an exception is caused by incomplete or abnormal data binding, which is quite troublesome in design, and the most troublesome thing is that the program code of this error on the page cannot use try... catch Method to skip the error. In fact, the simplest way is to directly modify the dropdownlist control, so that when the dropdownlist control binds data, even if the field value does not exist in the list item, do not release the error. This article describes how to inherit the dorpdownlist and modify it, to effectively solve this problem.

Download program code: ASP. NET Server Control-day22.rar
Download the northwnd Database: northwnd.rar

 

I. Overwriting SelectedvalueAttribute solves the problem of data binding

The reason for the dropdownlist control to fasten the error is that the error message probably indicates the error occurred when the selectedvalue attribute is written. Therefore, we inherit the dorpdownlist and name it tbdropdownlist, and overwrite the selectedvalue attribute to solve this problem. The solution is to determine whether the value to be written exists in the project list before writing the selectedvalue attribute when writing the selectedvalue attribute. If the value does not exist, set the selectedindex attribute to-1.

    Public Class TBDropDownList

        Inherits DropDownList

        ''' <summary>

''' Override selectedvalue adequacy.

        ''' </summary>

        Public Overrides Property SelectedValue() As String

            Get

                Return MyBase.SelectedValue

            End Get

            Set(ByVal value As String)

                Dim oItem As ListItem = Me.Items.FindByValue(value)

                If (oItem Is Nothing) Then

Me. selectedindex =-1 'When items does not exist

                    Exit Property

                Else

                    MyBase.SelectedValue = value

                End If

            End Set

        End Property

    End Class

 

We use the products data table of the northwnd database as the test data, and define the items content of dropdownlist in advance. The first project that is added to "not matched" binds the selectedvalue attribute to the categoryid field.

                <bee:TBDropDownList ID="DropDownList1" runat="server" 

                    SelectedValue='<%# Bind("CategoryID") %>'>

<Asp: listitem value = ""> not matched </ASP: listitem>

                    <asp:ListItem Value="2">Condiments</asp:ListItem>

                    <asp:ListItem Value="3">Confections</asp:ListItem>

                </bee:TBDropDownList>

 

When the categoryid field value of a data item does not exist in the items set attribute of dropdownlist, the first "unmapped" project is displayed.

 

II,TbdropdownlistSet DatasoruceidProblems

The above solution has been mentioned in the author's article "Removing dropdownlist databind from errors". However, some readers have found another problem, when the datasourceid is set in dropdownlist, data cannot be properly tied. Here we will solve this problem.

We set the datasoruceid of tbdropdownlist to get the content of the project list, and set the datasourceid to another sqldatasource control that gets the content of the categories data table.

 

                <bee:TBDropDownList ID="DropDownList1" runat="server" 

                    SelectedValue='<%# Bind("CategoryID") %>' DataSourceID="SqlDataSource2" 

                    DataTextField="CategoryName" DataValueField="CategoryID">

                </bee:TBDropDownList>

                <asp:SqlDataSource ID="SqlDataSource2" runat="server" 

                    ConnectionString="<%$ ConnectionStrings:Northwnd %>" 

                    SelectCommand="SELECT CategoryID, CategoryName, Description, Picture FROM Categories" 

                    ProviderName="<%$ ConnectionStrings:Northwnd.ProviderName %>" >

                </asp:SqlDataSource>

 

When the program is executed, the value of the categoryid field in formview is 7 (the categoryname should be product) in browsing mode ).

 

When you press Edit to switch to edititemtemplate, use tbdropdownlist to fasten the value of the categoryid field. You can disable the correct value.

 

Iii. Solution TbdropdownlistSet PerformanceidData cannot be closed

To solve the problem of setting the performanceid for tbdropdownlist, you need to set the selectedvalue attribute if items. count = 0 first use a fcachedselectedvalue variable to save the correct value first, and then overwrite the mongomdatabinding method. When dorpdownlist gets the project list content corresponding to datasoruceid, in this case, the items content will be completely retrieved, and then the selectedvalue attribute can be set again to properly fasten the data.

    Public Class TBDropDownList

        Inherits DropDownList

        Private FCachedSelectedValue As String

        ''' <summary>

''' Override selectedvalue adequacy.

        ''' </summary>

        Public Overrides Property SelectedValue() As String

            Get

                Return MyBase.SelectedValue

            End Get

            Set(ByVal value As String)

                If Me.Items.Count <> 0 Then

                    Dim oItem As ListItem = Me.Items.FindByValue(value)

                    If (oItem Is Nothing) Then

Me. selectedindex =-1 'When items does not exist

                    Else

                        MyBase.SelectedValue = value

                    End If

                Else

                    FCachedSelectedValue = value

                End If

            End Set

        End Property

        Protected Overrides Sub PerformDataBinding(ByVal data As System.Collections.IEnumerable)

            MyBase.PerformDataBinding(data)

'Datasoruceid: Specify the selectedvalue adequacy value after the data is saved.

            If (Not FCachedSelectedValue Is Nothing) Then

                Me.SelectedValue = FCachedSelectedValue

            End If

        End Sub

    End Class

 

 

Re-execute the program and switch to the editing mode. Then, tbdropdownlist can properly fasten the field value.

 

Note: This article is also published in "the first it state help tie Ren Competition". If you think this article is helpful to you, remember to repeat it to improve your popularity ^
Http://ithelp.ithome.com.tw/question/10012909
Http://ithelp.ithome.com.tw/question/10012915