Hi IamAzhar,
In master page event should access page contents by FindControl method of ContentPlaceHolder1.
Check this example. Now please take its reference and correct your code.
HTML
Master Page
<body>
<form id="form1" runat="server">
<div>
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
<asp:Button Text="Save" runat="server" OnClick="Save" />
</div>
</form>
</body>
Default Page
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<div id="dvDetails" runat="server">
Welcome to ASPSnippets.com
</div>
</asp:Content>
Code
Master Page
C#
protected void Save(object sender, EventArgs e)
{
System.Web.UI.HtmlControls.HtmlGenericControl div1 = (System.Web.UI.HtmlControls.HtmlGenericControl)ContentPlaceHolder1.FindControl("dvDetails");
string message = div1.InnerText.Trim();
}
VB.Net
Protected Sub Save(ByVal sender As Object, ByVal e As EventArgs)
Dim div1 As System.Web.UI.HtmlControls.HtmlGenericControl = CType(ContentPlaceHolder1.FindControl("dvDetails"), System.Web.UI.HtmlControls.HtmlGenericControl)
Dim message As String = div1.InnerText.Trim()
End Sub
Screenshot
