Data
In database application development, we know that the primary task of data manipulation is to "Browse, edit, delete, add records," and use the VB.net Data Form Wizard to help us quickly create Windows Forms and related code to implement the above features. This automatically generated code is fully functional and useful. However, the author found in the use of the "Data Form Wizard" in the course of operation, if we select a single record for individual control display style, if the field in the data source to which the control is bound is not allowed to be null, the automatically generated form is run and an error occurs when you click Add.
To avoid this error, the existing workaround is that the Add feature does not use the code generated by the Data Form Wizard, but instead creates a new form and then uses a non-data binding structure to implement the Add function separately. The author discovers that there is no need to build a new form at all, only by making appropriate improvements to the code generated by the Data Form Wizard, you can simultaneously implement the "Browse, edit, delete, add" feature in the same form, as illustrated by an example (note: The development tool is Microsoft Visual Studio.) NET 2003, the sample database is "pubs" in SQL Server 2000; The following code, which takes VB as an example, also applies to other development languages in Vs.net.
One, a field that is not allowed to be null values is bound to a non-TextBox class control
As shown in the following figure, this is the case where null values are not allowed for the column field "contract" that is bound to the check box control next to "contract" in the form. After you run the form and load, click Add to make the new record (the record numbered 24), but clicking the navigation button (such as the Last ">>") will not move the record location to the new record.
Figure one form and code are automatically generated by the Data Form Wizard (Menu action: "File/Add New item, then select Data Form Wizard, the name of the data form instance is" DEMO ", the DataSet name is" DS ", and the data connection is" pubs "in SQL Server 2000 Example database, select the Authors table, select Single records for individual controls display style, and others take the default value of the Data Form Wizard. A better way to do this is to first modify the click event handling code for the Add button, as shown below.
Private Sub btnAdd_Click (ByVal sender as System.Object, ByVal e as System.EventArgs) Handles Btnadd.click
Me.BindingContext (OBJDS, "authors"). SuspendBinding () temporary suspend data binding
Me.BindingContext (OBJDS, "authors"). AddNew () ' Add new record
' The following two-tier loop first finds the column fields in the authors table that are not allowed to be null, and then finds the binding object that is bound to the field.
' Assigns different initial values to fields that cannot be empty in new records, depending on the control properties that are bound to
Dim I as Integer
For i = 0 to objds.authors.columns.count-1 ' traverses all column fields in the authors table
' Find a column field in the authors table that is not allowed to be null
If objds.tables ("authors"). Columns (i). AllowDBNull = False Then
Dim J as Integer
For j = 0 to Me.BindingContext (OBJDS, "authors"). Bindings.count-1 ' Traverse all binding objects
' Find a binding object bound to a null field that is not allowed
If Me.BindingContext (OBJDS, "authors"). Bindings (j). Bindingmemberinfo.bindingfield = _
ObjDS.authors.Columns (i). ColumnName Then
' If the property of the control being bound to is text
If BindingContext (OBJDS, "authors"). Bindings (j). PropertyName () = "Text" Then
' Add an initial value ' to a field that cannot be null in the newly added record, which can also be replaced with a comment statement next to it!
Me.BindingContext (OBJDS, "authors"). Current (j) = ""
The essence of the above Btnadd.click event-handling code improvement is to temporarily suspend data binding, add a new record, and then assign a value to a column field in the record that is not allowed to be a null value (if you do not need to use the RowChanging event for a new record data test, You can also assign an initial value to a field that does not allow null values and cannot bind to a text box textbox, and finally restores data binding.
In addition, the Click event handlers for the four navigation buttons (first, Previous, Next, and last) are modified as follows (for simplicity, click event handlers for one of the < buttons above, for example, other buttons are modeled).
Private Sub Btnnavprev_click (ByVal sender as System.Object, ByVal e as System.EventArgs) Handles Btnnavprev.click
Try ' uses the Try statement for error handling (NULL and uniqueness)
Me.BindingContext (OBJDS, "authors"). Position = (Me.BindingContext (OBJDS, "authors"). POSITION-1)
Exit Sub ' If there is an error, don't move the position
End Try
Me.objds_positionchanged ()
End Sub
If you want to use the RowChanging event for new additions or edits ... Records for data validation (recommended), you also need to add the following RowChanging event handling code.
Private Sub row_changing (ByVal sender as Object, ByVal e as Ds.authorsrowchangeevent)
' Numeric validation of newly added rows
If e.action = Datarowaction.add Then
' Here for simplicity, the field au_id cannot be ' "for example
If editau_id. Text = "" Then
Throw (New Exception ("Data cannot be blank")
End If
End If
' You can also validate the edited value of a record
' If e.action = Datarowaction.change Then
' Here to fill in the Edit validation code
' End If
' Other verification, follow the above join it
End Sub
To start RowChanging event handling, add the following statement to the appropriate location of the program (I am appended to the statement Objds.merge (OBJDATASETTEMP) in the Loaddataset () method).
Two, although there are not allowed to null value of the field, but all can be bound to the TextBox class control
For the sake of convenience, we'll use the Data Form Wizard to create a new form (note: The new form can be built in the same project on the previous form, the name of the data form is "Demo2", the new dataset name is "Ds2", and the data form is the employee table bound to the pubs database. As shown in Figure II, all fields in this table that are not allowed to be null values can be bound to a TextBox class control.
After you run the form, you can add a new record (a record numbered 44) After you click Load, but if you click any navigation button (such as the previous "<") without entering a new record, the error appears as shown in Figure three.
This kind of mistake is lighter, you can use the first solution, of course, if you have too many, you can also use the following simpler way: In the Data Form Wizard automatically generated code, only four navigation buttons (first, previous, Next, The last one), click event handler code to make appropriate changes (note: or according to the first case of the four navigation buttons to modify the code to modify the method, the other code does not increase, everything on the 0k, and then try again, you will see as the figure four friendly interface, Tip You must enter a column field that does not allow null values before you can change the location of the record.
Third, summary
When a field with null values is bound to a data form, to avoid a new record error, the above article describes a solution that can be found in code, and we can also find a solution in the dataset: "In the schema of the dataset, set a default value for the column field that does not allow null values." Of course, the specific use of the solution or you are flexible to choose according to the actual situation.
The above gives the first case of the main solution code, the last place please refer to the complete code below.
Public Class DEMO
Inherits System.Windows.Forms.Form
' To simplify, the code generated by the Windows Forms designer has been removed, remembering that the Data Form Wizard executes with the following requirements.
' The name of the data form instance is ' DEMO ', the name of the dataset is ' DS ', and the data connection is in SQL Server 2000
' Pubs sample Database For example, select the authors table, single record for individual controls display style, and all other default values.
Private Sub Btncancel_click (ByVal sender as System.Object, ByVal e as System.EventArgs) Handles Btncancel.click
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