Getting to Know Mono

Mono is useful for more than simply getting Linux to work with Microsoft's .NET. It offers you a chance to use libraries in one language from another without writing a wrapper.
Using Mono

The first step in taking Mono out for a test spin is to visit the project web site ( and download the latest source tarballs or platform binaries. Currently, Mono has been ported only to Linux and Windows, but work is being done on Mac OS X, FreeBSD and other platforms. Binaries are available for a variety of Linux distributions including Debian, Red Hat, SuSE and Mandrake. If you use Ximian Red Carpet, the files also are available in the Mono Channel. For this article, we are using Mono version 0.20. You'll notice that in addition to the Mono packages providing the runtime, C# compiler and class libraries, there are a few other goodies to play with such as the Mono debugger, XSP web server and Monodoc documentation browser.

If you have trouble installing Mono, check out the tutorials offered on the web site.

Mono currently comes packaged with the following components:

  • C# and Basic language compilers.

  • VES consisting of a JIT compiler and associated garbage collector, security system, class loader, verifier and threading system. An interpreter is also included.

  • A set of class libraries written in C# that implements the classes defined in the CLI standard, classes that are part of the .NET FCL, and other Mono-specific classes.

  • Various utilities.

The Mono C# language compiler is mcs. In an interesting programming feat, mcs is written in C#. Since Mono 0.10, mcs even has been able to compile itself. If you are interested in the details of the command-line options, which are compatible with the command-line options provided by Microsoft's C# compiler, a thorough man page is available.

The compiler for Mono's equivalent of Visual Basic.NET, MonoBasic, is mbas. Although not as far in development as the C# compiler, mbas provides enough functionality to experiment a little in Basic.

Two execution environments are included with Mono, mono and mint. mono is a JIT compiler compatible with the CLI's definition of the VES. mint on the other hand, is an interpreter. It is provided as an easy-to-port alternative to mono, which currently runs only on the x86 platform. For the greatest code execution speed, use mono.

A couple interesting utilities also provided with Mono are monodis and pedump. monodis is used to disassemble a compiled assembly and output the corresponding CIL code. It was used to display the sample CIL code for Listing 1. If you are curious to see more of what CIL looks like or to take a peek at what makes up a portable executable, play around with these.

Now that we are familiar with the components of Mono, it is time to try them out. To experiment with the language interaction of Mono, we write a simple class with a single method in C# and call it from a MonoBasic program.

Listing 2 shows the C# library ljlib.cs, and Listing 3 shows the MonoBasic program hello.vb.

Listing 2. ljlib.cs

Listing 3. hello.vb

The first step is to compile the ljlib.cs into a library. Compiled libraries have the .dll extension, and compiled executables have the .exe extension. To compile to a library, use the -target:library switch in mcs:

[jdq@newton]$ mcs -target:library ljlib.cs
Compilation succeeded

This creates the ljlib.dll file, which contains the LJlib namespace and Output class. Now we need to compile the hello.vb program. In order to use the ljlib.dll file we just created, we need to tell the MonoBasic compiler to use it as a reference. We do that with the -r switch:

[jdq@newton]$ mbas -r ./ljlib.dll hello.vb
Compilation succeeded
The output of mbas is the PE hello.exe. It can be executed with mono:
[jdq@newton]$ mono hello.exe
Hello Linux Journal!
And there you have it—two languages, C# and MonoBasic, executing on the same runtime and working together. This is a trivial example; however, it does demonstrate the language independence and interoperability of the CLI and hints at the power of Mono as a development platform.


Though still in development, Mono shows promise for greatly benefiting Linux development. With the progress of the last two years as a gauge, the future of Mono should prove to be quite exciting.



Julio David Quintana is an electrical engineer who stumbled upon Linux in 1997 and has been hooked ever since. If you find grammatical or factual errors, he is unable to be contacted. However, for praises and compliments, works just fine.