Real World Computing

Assembling assemblies

17th January 2006 [PC Pro]

Information on referenced assemblies: a list of other assemblies used by this assembly.

To look inside an assembly and view its manifest, use ILDASM, as shown in Figure 2. As you can see, the manifest lists the external assemblies the client program uses (for example, Mscorlib and maths.dll) and defines the content of the assembly, including the version of the assembly and details of the actual runtime module (client.exe). The manifest for the maths DLL looks similar, although since it's a DLL called by the client it contains no reference to the client, but it does contain a definition of the Maths class contained in maths.dll.

Identifying Assemblies

An important feature of any runtime framework is that it can find the various files that contain needed components. In traditional Win32 programming, a program is identified by name only, so when a client program calls an external DLL, that DLL is identified by only the name of its file. If your Win32 app calls a library routine that some other app's installer later overwrites, your app may or may not work correctly - this crude feature of Win32 is not so fondly referred to in the business as 'DLL Hell'.

The .NET Framework addresses this problem by introducing the concept of a 'strong name'. In Win32, and by default in .NET as shown in the example above, assemblies are weakly named and just use the filename as a way of identifying a unit of code to load and execute. A strong name consists of four components: filename, assembly version, an optional culture name and a public key and associated digital signature. To create an assembly with a strong name, you have to carry out a few simple steps. First, you generate a key-pair using the strong name program sn.exe, which gets installed with the .NET SDK. In the .NET 1.1 and 1 frameworks, the key size was fixed at 1,024 bits, but in .NET Framework 2 you can have longer keys.

To create a key-pair with sn.exe, use the -k switch, specifying an output filename and an optional key length, as follows:

[MSH] C:\demo> sn -k 2048 pcprokey.snk

Microsoft (R) .NET Framework Strong Name Utility Version 2.0.50727.42

Key pair written to pcprokey.snk

Once you've created the key-pair, you need to add attributes to the app's source. Attributes are special code statements placed in an app's source files that tell the .NET language compiler, among other things, how to create a strongly named assembly. For example, you can add the appropriate attribute for the version number into the maths.cs source file like this:

using System;

using System.Reflection;

[assembly:AssemblyVersionAttribute("1.2.3.4")]

public class Maths

{

public long Add(long a, long b)

{

Console.WriteLine("In strong named maths.dll");

return a + b;

}

Lastly, you need to recompile the application using the key you just generated, and then run it. This looks like:

[MSH] C:\demo>csc /t:library /out:maths.dll maths2.cs /keyfile:pcprokey.snk