The recommended way to design, develop, and test custom assemblies is to create a solution that contains both your test reports and your custom assembly. This is the way
In the Solution Explorer Right click on the Solution and Select
Select DebugLocal as the Active Solution Configuration
Make sure that the report project in your solution is set to DebugLocal
and that deploy is unchecked.
Right click on the project containing your reports
Set the Report project as the startup project.
Right click again and select Project Dependencies.
In the Project Dependencies dialog select the project you want to add as the
Click OK to save the changes, and close the Property Pages dialog.
Right click again and select Project Properties.
Select StartItem and set it to the report you want to debug.
In Solution Explorer, select the Employee custom assembly project.
Right click and select Properties.
Expand Configuration Properties, and click Build.
On the Build page, enter the path to the Report Designer folder (by
default, C:\Program Files\Microsoft SQL Server\80\Tools\Report
Designer) in the Output Path text box.
Now set breakpoints in your custom assembly code.
Make sure to set the Report as the startup project and then press F5 to
start the solution in debug mode. When the report uses the custom code
in our expression, the debugger will stop at any breakpoints that you
have set when they are executed. Now you can use the all the powerful
debugging features of Visual Studio.NET to debug your code.