属性值在回发时被清除

Question

I have to Get and Set the BackColur of Current Page. So user can use this property from any other page

And this is my code

  Private _BackgroundColour As System.Drawing.Color
  Public Property MenuBackColour() As System.Drawing.Color
     Get
       Return _BackgroundColour
     End Get
     Set(ByVal value As System.Drawing.Color)
       _BackgroundColour = value
     End Set
  End Property

  Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
  Try
    NavigationMenu.BackColor = MenuBackColour
  Catch ex As Exception                 
  End Try
  End Sub

But the problem is Property value gets cleared on PostBack so I'm not able to get the BackColor value from other page

Answer 1

On post back only the input elements are posted.

So they are the same as before the post back, what they have before the post back, have and now. So actually you do not "get" this parameters on code behind, you only "set" them and on server controls with viewstate on, you can remember this parametres on post back - but you can not change them on client side and expect to read this change on server.

Here the workaround is. Either use the viewstate of the page to save some values and keep them on post back, ether use input hidden elements to have them on post back.

Answer 2

If your main purpose is to retain or share the backcolor of some control between different pages, there are many ways to do it in ASP.NET. You can keep the value in cookies or sessions, or cache.

As per request, if Property must be used, I create a Default.aspx as the following:

Public Class _Default
    Inherits Page

    Private Shared _BackgroundColour As System.Drawing.Color = Drawing.Color.Azure
    Public Shared Property MenuBackColour() As System.Drawing.Color
        Get
            Return _BackgroundColour
        End Get
        Set(ByVal value As System.Drawing.Color)
            _BackgroundColour = value
        End Set
    End Property

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
        MenuBackColour = Drawing.Color.Red
        Response.Redirect("Default1")
    End Sub
End Class

It will automatically be redirected to Default1.aspx, where there is a Label control in that page. It will use _Default.MenuBackColour as the label's backcolor:

Public Class Default1
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Label1.BackColor = _Default.MenuBackColour
    End Sub

End Class

Again, this approach is not recommended. If Default.aspx has never been run (at least once), MenuBackColour's value may not be what you think it is. I won't encourage people to retain any static variable or property in an .aspx page for sharing.

Answer 3

In web applications, variable values simply get erased. But it is very simple to persist these values. They may be persisted using the Viewstate object. Before the postback is invoked, the variable's value is saved in a viewstate object. In the recieving page, the viewstate's value may be retrieved back.

//Save the value in ViewState object before PostBack

ViewState("myColour")="Black";

//Retrive the value from ViewState after the PostBack

myColourProperty=ViewState("myColour").ToString();