Understanding Assemblies

An assembly is the actual .dll file on your hard drive where the classes in the .NET Framework are stored. For example, all the classes contained in the ASP.NET Framework are located in an assembly named System.Web.dll.

More accurately, an assembly is the primary unit of deployment, security, and version control in the .NET Framework. Because an assembly can span multiple files, an assembly is often referred to as a "logical" dll.

Note

The .NET Framework (version 2.0) includes 51 assemblies.



There are two types of assemblies: private and shared. A private assembly can be used by only a single application. A shared assembly, on the other hand, can be used by all applications located on the same server.

Shared assemblies are located in the Global Assembly Cache (GAC). For example, the System.Web.dll assembly and all the other assemblies included with the .NET Framework are located in the Global Assembly Cache.

Note

The Global Assembly Cache is located physically in your computer's \WINDOWS\Assembly folder. There is a separate copy of every assembly in your \WINDOWS\Microsoft.NET\Framework\v2.0.50727 folder. The first set of assemblies is used at runtime and the second set is used at compile time.



Before you can use a class contained in an assembly in your application, you must add a reference to the assembly. By default, an ASP.NET application references the most common assemblies contained in the Global Assembly Cache:

mscorlib.dll

System.dll

System.Configuration.dll

System.Web.dll

System.Data.dll

System.Web.Services.dll

System.Xml.dll

System.Drawing.dll

System.EnterpriseServices.dll

System.Web.Mobile.dll

To use any particular class in the .NET Framework, you must do two things. First, your application must reference the assembly that contains the class. Second, your application must import the namespace associated with the class.

In most cases, you won't worry about referencing the necessary assembly because the most common assemblies are referenced automatically. However, if you need to use a specialized assembly, you need to add a reference explicitly to the assembly. For example, if you need to interact with Active Directory by using the classes in the System.DirectoryServices namespace then you will need to add a reference to the System.DirectoryServices.dll assembly to your application.

Each class entry in the .NET Framework SDK documentation lists the assembly and namespace associated with the class. For example, if you look up the MessageQueue class in the documentation, you'll discover that this class is located in the System.Messaging namespace located in the System.Messaging.dll assembly.