How Does .NET/Link Compare to J/Link?
J/Link is an existing Wolfram Research product that integrates Java and Mathematica in almost exactly the same way that .NET/Link integrates .NET and Mathematica. You will find the J/Link User Guide right above this .NET/Link User Guide in the Add-ons & Links category of the Mathematica Help Browser. You can use J/Link to do many of the same things you can do with .NET/Link and vice versa. Because it is based on Java, J/Link has the advantage of being cross platform. If you are creating a user interface, like a dialog box for a Mathematica program, and you want it to work on any platform on which Mathematica runs, you should use J/Link. On the other hand, .NET integrates more tightly with the Windows operating system than Java does, so if you want to do Windows-specific things, or you want a very native Windows look and feel, you should use .NET/Link. On Windows, .NET/Link also does some things that J/Link cannot, such as allowing you to call C-style DLL functions directly from Mathematica or controlling COM objects.
Because this .NET/Link User Guide is so incomplete at this stage, some familiarity with J/Link will be very useful for programmers.