The .NET Framework Class Libraries

The .NET Framework class libraries are monumentally important to providing language interoperability because they allow developers to use a single programming interface to all the functionality exposed by the CLR. If you've ever used more than one dissimilar language in development for Windows, you'll love this feature. In fact, the .NET Framework class libraries are forging a revolutionary trend in compiler development. Before .NET, most compiler writers developed a language with the ability to do most of its own work. Even a language such as C++, which was designed as a scaled-down grouping of functionality to be used in conjunction with a class library, has at least some functionality on its own. However, in the world of .NET, languages are becoming little more than syntactical interfaces to the .NET Framework class libraries.
As an example, let's first take a look at the standard "Hello, World" application in C++ and then compare it to an application that does the same thing in C#: -

#include 

int main(int argc, char* argv[])
{
    cout << "Hello, World!" << endl;
    return 0;
}

Notice that the application first includes a header file with the declaration of the cout function. The application's main function-every C/C++ application's entry point-uses the cout function to write the string "Hello, World" to the standard output device. However, what's important to note here is that you can't write this application in any .NET language without the .NET Framework class libraries. That's right: .NET languages don't even have the most basic compiler features, such as the ability to output a string to the console. Now I know that technically the cout function is implemented in the C/C++ runtime, which is itself a library. However, basic C++ tasks such as string formatting, file I/O, and screen I/O are at least logically considered part of the base language. With C#-or any .NET language for that matter-the language itself has almost no ability to do even the most menial task without the .NET Framework class library.
Let's look at the "Hello, World" example in C# to see what I mean: -

using System;

class Hello
{
    public static void Main()
    {
        Console.WriteLine("Hello, World");
    }
}

So, what does this common set of class libraries mean to you, and is it a good thing? Well, it depends on your vantage point. A common set of class libraries means that all languages, theoretically, have the same capabilities because they all have to use these class libraries to accomplish anything except declaring variables.
One gripe I've seen on discussion boards is, "Why have multiple languages if they all have the same capabilities?" For the life of me, I don't understand this complaint. As someone that has worked in many multilanguage environments, I can attest that there's a great benefit to not having to remember what language can do what with the system and how it does it. After all, our job as developers is to produce code, not to worry about whether a favorite language has this advantage or that advantage.
Another question I've seen frequently is, "If all these .NET languages can do the same thing, why do we need more than one?" The answer relates to the fact that programmers are creatures of habit. Microsoft certainly didn't want to pick one language out of the many available and force millions of programmers to toss out their years of experience in other languages. Not only might a programmer have to become familiar with a new API, he or she might have to master a completely different syntax. Instead, a developer can continue using the language that's best suited for the job. After all, the name of the game is productivity. Changing what doesn't need to be changed is not part of that equation.

NOTE

---

While in theory the .NET Framework class libraries enable compilers to make all the CLR's functionality available to a language's users, this is not always the case. One point of contention at Microsoft between the .NET Framework class libraries team and the different compiler teams is that although the .NET Framework class libraries team has attempted to expose all its functionality to the different languages, there's nothing-besides meeting minimal CLS standards-that requires the different compiler teams to implement every single feature. When I asked several Microsoft developers about this discrepancy, I was told that instead of each language having access to every exposed bit of .NET Framework functionality, each compiler team has decided to implement only the features that they feel are most applicable to their users. Luckily for us, however, C# happens to be the language that seems to have provided an interface to almost all of the .NET Framework functionality.