Reasons for Learning the Grammar of CIL

CIL is the true mother tongue of the .NET platform. When you build a .NET assembly using your managed language of choice (C#, VB, F#, COBOL.NET, etc.), the associated compiler translates your source code into terms of CIL. Like any programming language, CIL provides numerous structural and implementation-centric tokens. Given that CIL is just another .NET programming language, it should come as no surprise that it is possible to build your .NET assemblies directly using CIL and the CIL compiler (ilasm.exe) that ships with the .NET Framework 4.0 SDK.

Now while it is true that few programmers would choose to build an entire .NET application directly with CIL, CIL is still an extremely interesting intellectual pursuit. Simply put, the more you understand the grammar of CIL, the better able you are to move into the realm of advanced .NET development. By way of some concrete examples, individuals who possess an understanding of CIL are capable of the following:

• Talking intelligently about how different .NET programming languages map their respective keywords to CIL tokens.
• Disassembling an existing .NET assembly, editing the CIL code, and recompiling the updated code base into a modified .NET binary. For example, there are some scenarios where you might need to modify CIL in order to interoperate with some advanced COM features.
• Building dynamic assemblies using the System.Reflection.Emit namespace. This API allows you to generate an in-memory .NET assembly, which can optionally be persisted to disk.
• Leveraging aspects of the CTS that are not supported by higher-level managed languages, but do exist at the level of CIL. To be sure, CIL is the only .NET language that allows you to access each and every aspect of the CTS. For example, using raw CIL, you are able to define global-level members and fields (which are not permissible in C#).

Again, to be perfectly clear, if you choose not to concern yourself with the details of CIL code, you are absolutely able to gain mastery of C# and the .NET base class libraries. In many ways, knowledge of CIL is analogous to a C(++) programmer’s understanding of assembly language. Those who know the ins and outs of the low-level “goo” are able to create rather advanced solutions for the task at hand and gain a deeper understanding of the underlying programming (and runtime) environment. So, if you are up for the challenge, let’s begin to examine the details of CIL.

Note Understand that this chapter is not intended to be a comprehensive treatment of the syntax and semantics of CIL. If you require a full examination of the topic, I'd recommend downloading the official ECMA specification (ecma-335.pdf) from the ECMA International web site (http://www.ecma-international.org).