ASP.NET Change Master page on Run time

As in my earlier post we looked at how we can change Theme of a page on click of a button, this similar principle is applied when we try to change master page on click of a button.

We again have the same question:

Can in Change master page on button click?
How to change master page on click of button?

The answer is again simple after reading the documentation of System.Web.UI.Page.MasterPageFile() property:

Property: Public Overridable Property MasterPageFile() As String
Member of: System.Web.UI.Page
Summary: Gets or sets the file name of the master page.
Exceptions:
System.InvalidOperationException: The System.Web.UI.Page.MasterPageFile
property is set after the System.Web.UI.Page.PreInit event is complete.
System.Web.HttpException: The file specified in does not exist or The page does not have a System.Web.UI.WebControls.Content control as the top level control.


The 'MasterPageFile' property can only be set in or before the 'Page_PreInit' event.

Same work around to change the MasterPageFile on run time on click of a button:

Partial Class Default
    Inherits System.Web.UI.Page

    Protected Sub btnChange_Click(ByVal sender As Object, _
        ByVal e As System.EventArgs) Handles btnChange.Click
        'Cannot change MasterPageFile on Click event
    End Sub

    Protected Sub Page_PreInit(ByVal sender As Object, _
            ByVal e As System.EventArgs) Handles Me.PreInit
        'If Page is postback
        If Me.IsPostBack = True Then
            If Not Request(btnChange.UniqueID) Is Nothing Then
                Me.MasterPageFile = "~/NewMasterpage.master"
            End If
        End If

    End Sub

End Class

We have changed the MasterPageFile in 'Page_PreInit' event but on click of a button.

UniqueID of a control gets the unique, hierarchically qualified identifier for the server control. The fully qualified identifier for the server control, this is the ID which is received as key when value is post back.

The output of button is like this:
<input type="submit" name="btnChange" value="Change Theme" id="btnChange" />

Here name is what we call btnChange.UniqueID and id is what we call btnChane.ClientID, in HTML input controls "id" is used for client side validation and other java script things, and "name" attribute is used to identify the field when it is post back to server.