In the MasterPage.cs file add the property of Label like this:
public string ErrorMessage
{
get
{
return lblMessage.Text;
}
set
{
lblMessage.Text = value;
}
}
On your aspx page, just below the Page Directive add this:
<%@ Page Title="" Language="C#" MasterPageFile="Master Path Name"..... %>
<%@ MasterType VirtualPath="Master Path Name" %> // Add this
And in your codebehind(aspx.cs) page you can then easily access the Label Property and set its text as required. Like this:
this.Master.ErrorMessage = "Your Error Message here";