In order to use a component in your application, you must first add a reference to it. Visual Studio provides five options in the Add Reference dialog box:
.NET lists all .NET Framework components available for referencing.
COM lists all COM components available for referencing.
Projects lists all reusable components created from local projects.
Browse enables you to browse for a component in the file system.
Recent contains a list of components recently added to projects on your computer.
The number of tabs available at the top of the Add Reference dialog box can vary, depending on the type of project open and the resources it is using. C++ native projects contain only aProjects tab.
Some components in the list may not be shown, depending on the .NET Framework version of your project. This can occur under the following conditions:
A component that uses a recent version of the .NET Framework is incompatible with a project that targets an earlier version of the .NET Framework.
For information about how to change the target .NET Framework version for a project.
A component that uses .NET Framework 4 is incompatible with a project that targets the .NET Framework 4 Client Profile.
When you create a new application, some projects target the .NET Framework 4 Client Profile by default. For more information.
You cannot add references from the global assembly cache because it is strictly part of the run-time environment.