.NET Assembly Internals: Part 1

Single file and Multifile Assembly Technically speaking, an assembly can be formed from a single file and multiple files. A single-file assembly contains all the necessary elements such as CIL code, header files, manifest in a single *.exe or *.dll package. A multifile assembly, on the other hand, is a set of .NET modules that are deployed and versioned as a single unit. Formally speaking, these modules are called primary and secondary modules. The primary module contains an assembly-level manifest and the secondary modules have a *.netmodule extension containing a module-level manifest. The major benefit of a multifile assembly is that they provide a very efficient way to download content. Assembly Structure An assembly is comprised of assembly metadata describing the complete assembly, type metadata unfolding the exported type and methods, MSIL code and resources. All these fragments can be inside one file or spread across several files. Structurally speaking, an assembly is composed of the following elements: CIL code The CIL code is a CPU and platform-agnostic intermediate language. It can be considered to be the core backbone of an assembly. Given this design, the .NET assemblies can indeed execute on a variety of devices, architectures, and operating systems. At the runtime, the internal CIL is compiled using the Just-In-Time (JIT) compiler for the platform and CPU specific instructions. Understanding the grammar of CIL code can be helpful when you are building a complex application but unfortunately, most .NET developers are not deeply concerned with the details of CIL code.

Windows File Header The Windows file header determines how the Windows family of operating systems can load and manipulate an assembly. The headers also identify the kind of application, such as DLL, console, or GUI application, to be hosted by Windows. You can view the assembly header information using the dumpbin.exe utility as in the following: Dumpbin /headers *.dll/*.exe

CLR File Header The CLR header is a block of data that all .NET assemblies must support in order to be hosted by the CLR. It typically defines numerous flags that enable the runtime to understand the layout of the managed code. We can view such diverse flags, again by using the dumpbin.exe /clrheader flag as in the following: Metadata The .NET runtimes practice metadata to resolve the location of types within the binary. Assembly metadata comprehensively describes the format of the contained types, as well as the format of external type references. If you press the Ctrl +M keystroke combination, idasm.exe displays the metadata for each type within the DLL file assembly as in the following: Manifest The assembly manifest documents each module within the assembly established the version and acknowledges the external reference assemblies with their dependencies. The Assembly manifest is a significant part of an assembly and can be composed of the following parts:

• Identity
  It includes version, name, culture, and public key details.
• Set of Permissions
  This portion displays the necessary permissions to run an assembly.
• List of Files
  It lists all the files belonging to a single-file or multiple-file assembly.
• External Reference Assemblies
  The manifest also documents the external reference files that are needed to run an assembly.

Resources Finally, a .NET assembly may contain multiple embedded resources, such as picture files, application icons, sound files, and culture information (satellite assemblies for building international software).

Summary

This article drilled down into the details of how the CLR resolves the location of external reference assemblies. You began with the disadvantages of existing COM technology and examined the content within an assembly such as CIL code, header, metadata, manifest, and resources. You have also come to an understanding of the distinction between the single-file and multi-file assembly. This article also focuses on the benefits of modules and assemblies in depth. Later, you will also explore the more advanced topics related to assemblies.