Thursday, January 28, 2010

DataGridView control change Selection on Mouse Up

Level: Intermediate

Knowledge Required:
  • DataGridView Control
  • Windows Forms
Recently, one of our friend "Luc" asked me,

Is there an "easy" way to change the selection behavior in a datagridview so that a click on a selected row does not toggle the selection as long as the mouse button is not released, without resorting to a user-defined control with inheritence and method overrides ?

So I was about to paste the whole code in comments but that was looking a mess. So I decided to publish a new post, after a very long time :).

The solution "is" simple. We just need to look what is the event sequence. So whenever we press mouse button on a cell or row of DataGridView control, events trigger in this way,
  1. MouseDown
  2. SelectionChanged
  3. MouseUp
When MouseDown event occurs Selection does NOT change. So in this event handler we can save the current row in some variable and then in SelectionChanged event handler we can select previously saved row, in this way user will experience that selection never changed.

And here is the code,

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Me.DataGridView1.Columns.Add("Column1", "Column1")
        Me.DataGridView1.Columns.Add("Column2", "Column2")
        Me.DataGridView1.Columns.Add("Column3", "Column3")

        Me.DataGridView1.Rows.Add("laksjdf", "alksjfsdf", "aksldjf")
        Me.DataGridView1.Rows.Add("laksjdf", "alksjfsdf", "aksldjf")
        Me.DataGridView1.Rows.Add("laksjdf", "alksjfsdf", "aksldjf")
        Me.DataGridView1.Rows.Add("laksjdf", "alksjfsdf", "aksldjf")
        Me.DataGridView1.Rows.Add("laksjdf", "alksjfsdf", "aksldjf")
        Me.DataGridView1.Rows.Add("laksjdf", "alksjfsdf", "aksldjf")
        Me.DataGridView1.Rows.Add("laksjdf", "alksjfsdf", "aksldjf")

        ' ***************************************
        ' IMPORTANT: MultiSelect property of DataGridView control should be False
        '            The result maybe same but if we set it to true then you
        '            may experience the flickering
        ' ***************************************
        Me.DataGridView1.MultiSelect = False
    End Sub

    Private _MouseDown As Boolean
    Private _PreviousSelectedCell As DataGridViewCell
    Private _NewSelectedCell As DataGridViewCell

    Private Sub DataGridView1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles DataGridView1.MouseDown
        ' if left button is clicked
        If e.Button = Windows.Forms.MouseButtons.Left Then
            Me._MouseDown = True    ' we will mark a flag that mouse is down
            ' and will note the row before the selection changed
            Me._PreviousSelectedCell = Me.DataGridView1.CurrentCell
        End If
    End Sub

    Private Sub DataGridView1_SelectionChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DataGridView1.SelectionChanged
        If Me._MouseDown Then       ' if this event is triggerd after the mouse down
            ' we will first note the new row on which mouse was clicked
            Me._NewSelectedCell = Me.DataGridView1.CurrentCell
            ' then change the selection back to the one before mouse was down
            Me.DataGridView1.CurrentCell = Me._PreviousSelectedCell
        End If
    End Sub

    Private Sub DataGridView1_MouseUp(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles DataGridView1.MouseUp
        If Me._MouseDown Then       ' if this event is triggered after the mouse down
            ' we will set the selection to the new row which was clicked
            Me.DataGridView1.CurrentCell = Me._NewSelectedCell
            Me._MouseDown = False   ' reset the flag
        End If
    End Sub

End Class

1 comment:

Anonymous said...

Dear Arsalan,

I really thank you for your post ! You are the only one that came up with that solution, which was right on point: MouseDown and SelectionChanged are the events to use (clearly not the MouseClick event), Thanks again ! You helped me much.