CRM 4.0 Deploying a custom assembly

The most common and suggested deployment scenario for custom CRM web extensions (i.e. custom web service or web application) is to deploy them under the ISV folder (e.g. ISV / MyApp) and put the application assembly inside the CRMWeb \ bin folder (or wwwroot / bin when deployed on the default website).

As of rollup2 MS also recommends putting the product assemblies of your custom extensions in their own bin folder (i.e. ISV / MyApp / bin). This option also requires you to add an assembly directive

<%assembly name=”MyApp” %>

to all your pages so the asp.net will be able to bind to your server side code behind.

Here is an example:
Test.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Test.aspx.cs" Inherits="MyApp.Test" %>
<%@ Assembly Name="MyApp" %>

Deployment Tree:

CRMWeb \ wwwroot
    |-- ISV 
          |-- MyApp
                  |-- bin
                        |-- MyApp.dll

Looks simple enough but many find that this does not work and produces the error

[HttpException: Could not load type 'MyApp.Test'].

The reason this does not work is that in order for asp.net to recognize and load your assembly the assembly directive must be the first directive in the page e.g.

<%@ Assembly Name="MyApp" %>
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Test.aspx.cs" Inherits="MyApp.Test" %>

And lastly a few tips to ensure successful deployment for both IFD and On-Premise environments:
Ensure that the MyApp folder is a regular Virtual Directory (not an application) – in other word your app will run under CRMAppPool.
Remove any application level nodes from your application web.config e.g.