In order to write code against an external component, your project must first contain a reference to it. You can make a reference to the following types of components:
.NET Framework class libraries or assemblies
Other assemblies or class libraries of projects in the same solution
XML Web services
For more information about XML Web service references, see Web References in Visual Studio.
Adding References at Design Time
Use the Add Reference dialog box to add references to components at design time. In Solution Explorer, right-click on the project node and select Add Reference (in C# or VB) or References (in C++).
You can use the Add Reference dialog box to browse to assemblies not listed. You cannot add references from the Global Assembly Cache (GAC), as it is strictly part of the run-time environment.
When you make a reference to an assembly in your project, Visual Studio searches for the assembly in the following locations:
The current project directory. (You can find these assemblies by using the Browse tab.)
Other project directories in the same solution. (You can find these assemblies on the Projects tab.)
For more information about how to add references at design time, see How to: Add or Remove References in Visual Studio.
All projects contain an implied reference to mscorlib. Visual Basic projects contain an implied reference to Microsoft.VisualBasic.
In Visual Studio 2010, all projects contain an implied reference to System.Core, even if System.Core is removed from the list of references.
References to Shared Components at Run Time
At run time, components must be either in the output path of the project or in the Global Assembly Cache (GAC). If the project contains a reference to an object that is not in one of these locations, you must copy the reference to the output path of the project when you build the project. The CopyLocal property indicates whether this copy has to be made. If the value is True, the reference is copied to the project directory when you build the project. If the value is False, the reference is not copied.
If you deploy an application that contains a reference to a custom component that is registered in the GAC, the component will not be deployed with the application, regardless of the CopyLocal setting. In earlier versions of Visual Studio, you could set the CopyLocal property on a reference to ensure that the assembly was deployed. Now, you must manually add the assembly to the \Bin folder. This puts all custom code under scrutiny, reducing the risk of publishing custom code with which you are not familiar.
By default, the CopyLocal property is set to False if the assembly or component is in the global assembly cache or is a framework component. Otherwise, the value is set to True. Project-to-project references are always set to True.
Referencing a Project or Assembly That Targets a Different Version of the .NET Framework
You can create applications that reference projects or assemblies that target a different version of the .NET Framework. For example, you could create an application that targets the .NET Framework 4 Client Profile that references an assembly that targets .NET Framework version 2.0. If you create a project that targets an earlier version of the .NET Framework, you cannot set a reference in that project to a project or assembly that targets the .NET Framework 4 Client Profile or .NET Framework version 4.
For more information, see Targeting a Specific .NET Framework Version or Profile.
Project-to-Project References and File References
File references are direct references to assemblies; you create them by using the Browse tab of the Add Reference dialog box. Project-to-project references are references to projects that contain assemblies; you create them by using the Project tab of the Add Reference dialog box.
The advantage of a project-to-project reference is that it creates a dependency between the projects in the build system. Therefore, the dependent project will be built if it has changed since the last time the referencing project was built. A file reference does not create a build dependency, so it is possible to build the referencing project without building the dependent project, and the reference can become obsolete. (That is, the project can reference a previously built version of the project.) This can result in several versions of a single DLL being required in the bin directory, which is not possible. When this conflict occurs, you will see a message such as Warning: the dependency 'file' in project 'project' cannot be copied to the run directory because it would overwrite the reference 'file.'.
You should avoid adding file references to outputs of another project in the same solution, because doing this may cause compilation errors. Instead, use the Projects tab of the Add Reference dialog box to create project-to-project references within the same solution. This makes team development easier by allowing for better management of the class libraries you create in your projects. For more information, see Troubleshooting Broken References and How to: Create and Remove Project Dependencies.
In Visual Studio 2010, a file reference instead of a project reference is created if the target version of the .NET Framework of one project is version 4, and the target version of the other project is version 2, 3, or 3.5.
You can also add Web references by using the Add Web Reference dialog box. For more information, see How to: Add and Remove Web References.