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There is more than one cause of this error, for example, you will sometimes see it when attempting to use the App_Code folder within an ASP.NET 2.0 Web Application project.
On this particular occassion, we are building an ASP.NET 2.0 Web Application project in C# using VS 2005. The folder containing the code file is just a regular folder, but the same error message appears.
Here's a fix that might work for you. Right-click the file you are trying to access in Solution Explorer.
Select Properties which will open the Properties window. Go to the Build Action DropDown menu.
Select 'Compile' for the build action. In this case, it had been to be set to 'Content'. Now compile as normal and you should be able to reference the file.
Also, make sure to using the namespace at top like this.