Why is My Namespace Invisible After Adding System.Web in Visual Studio 2010?

Namespace Visibility Error in Visual Studio 2010

This issue arises when working with a C# WinForms solution comprising multiple projects. After adding a reference to System.Web in the main DLL project, the namespace of that project becomes invisible to an executable WinForms project used for debugging purposes.

Possible Root Cause

The culprit behind this issue appears to be a bug in Visual Studio 2010, specifically related to the default selection of the Client Profile for ".NET Framework 4" applications.

Fix:

1. Open the Visual Studio project's properties window by right-clicking on the project in the Solution Explorer and selecting "Properties."
2. Navigate to the "Application" tab.
3. Under "Target Framework," change the value from ".NET Framework 4 Client Profile" to ".NET Framework 4."

This simple adjustment updates the application's configuration to use the full version of .NET 4.0, which includes System.Web and resolves the namespace visibility issue.

Context:

System.Web is part of the full version of .NET 4.0 but is not included in the Client Profile. The Client Profile's existence primarily serves to provide a slightly smaller footprint, but since it's only marginally smaller than the full version, its use has raised questions. The issue was eventually addressed in VS2012, which discontinued the Client Profile as the default option.

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