How to set default values for data bound controls

问题:

I have a vb.net 2010 form with 22 data bound controls from two tables held in a dataset which is navigated by a bindingnavigator. This successfully adds deletes and updates. However what I need is when adding a new record I need some of the fields to be pre filled out. More specifically I have points balance fields etc which could be any value but will normally be 0 on a new customer so I want to initialise them to 0 when adding new records.

I located an AddingNew event on my datasource but this is called before the new item is added and thus all my initialisation is lost.

any help on this would be appreciated. kind regards Feldoh

回答1:

Since you are using DataTables, you can manually set the DefaultValue property of the DataColumn:

Dim dt As New DataTable
Dim dc As New DataColumn("test", GetType(String))
dc.DefaultValue = "hello"
dt.Columns.Add(dc)
dt.Rows.Add()
Debug.WriteLine(dt.Rows(0)("test").ToString)

Result: hello

回答2:

Answer including multiple linked tables solution:

Like LarsTech said using

dataset.table.Columns.Item("someField").DefaultValue = someValue

works nicely if you have a standard default but it also works for generated defaults, if you reset this default on saving to the next value you want if a new addition is made. However if you are using a dataset as your data source where a child table record cannot feasibly be generated (like in my case where an account id will be generated only on saving using a database trigger to link all different kinds of accounts) you can still set defaults for the other fields, the ones from the sibling (or non-existent parent) table but

dataset.siblingTable.Columns.Item("someField").DefaultValue = someValue

WILL NOT WORK as the binding navigator only generates the first sibling and the parent is not generated until the trigger. However on the BindingSource.PositionChanged event of the binding source that IS generated by clicking add you can freely put defaults into the actual controls on the screen and they will not be overwritten by nulls, clearly you need to restrict this so it only happens if the user pressed add by adding a variable to the Clicked event of the add button to tell you when add has been clicked and resetting this variable on save or roll-back. It is also possible to modify the bindings themselves which have an if null or empty default value.

Hope this helps someone else :)

回答3:

Use the MouseUp event on the 'Add Row' button in the BindingNavigator instead of the "Click" event. This is becouse the new ROW does not exist in the datagridview until AFTER the click event is completed.

Private Sub BindingNavigatorAddNewItem_MouseUp(sender As Object, e As MouseEventArgs) Handles BindingNavigatorAddNewItem.MouseUp
    'works with add row from the binding navigator
    myDataGridView.Item(3, myDataGridView.CurrentRow.Index).Value = txtDefaultDescrip.Text.Trim
End Sub

to do this same thing when adding data directly in you DataGridView do this.

Private Sub myDataGridView_DefaultValuesNeeded(sender As Object, e As DataGridViewRowEventArgs) Handles myDataGridView.DefaultValuesNeeded
    'works with add row inside a datagridview
    e.Row.Cells(3).Value = txtDefaultDescrip.Text.Trim
End Sub

In this example I have a editable default value shown on my from. If you have a non-changing default value (e.g., is always 1 for column 3) then you should do that when setting up your datasource.