Could not load type ‘WebApp1._Default’ in ASP.NET

When I am running my ASP.NET application I will get an error

Server Error in '/' Application.

Parser Error
Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately.

Parser Error Message: Could not load type 'WebApp1._Default'.

Source Error:

Line 1: <%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApp1._Default" %>
Line 2:
Line 3:

Source File: /Default.aspx Line: 1

Version Information: Microsoft .NET Framework Version:2.0.50727.3053; ASP.NET Version:2.0.50727.3053

How can I solve this problem?