Can you create private classes in C#?

Question

This is a question for the .NET philosophers:

It is my understanding that Microsoft consciously denied use of private classes in C#. Why did they do this and what are their arguments for doing so?

I, for example, am building a large application that includes a reporting tool. This tool uses a lot of business objects that are used only within the reporting tool and not in other parts of the project. I want to encapsulate them for use only within the reporting tool itself.

Great decision is creating separate project in VS for this tool, and I'll do like that, but I'm interesting, what if I can't do this - for exmple our architecture wasn`t good enough, and we have big single project.

Behind "private class" I mean a class that can't be used in any other namespace, except its own.

My question was not - how can I simulate this, or do in another way. I'm just wondering, why not use private keyword with class keyword without any parent classes. I`m thinking there should be some reason, and I want to know it

Answer 1

There's a workaround for this, but you might not like it.

Instead of using a namespace to scope your classes, use a public static partial class:

Before:

namespace MyCompany.Foo {
  class Bar { }
  public class Baz { }
}

After:

namespace MyCompany {
  public static partial class Foo {
    private class Bar { }
    public class Baz { }
  }
}

This construct, like a namespace, can span multiple files in the same project. But unlike a namespace, it cannot "escape" from your project (other projects cannot define other members inside Foo).

There's an added advantage that you can have utility methods that seem to have no class for code inside Foo.

The disadvantage is that, to use your non-private classes outside of your fake namespace, you have to reference them inside Foo:

using MyCompany;

// ...

var baz = new Foo.Baz();

This can be mitigated by using an alias for the class:

using Baz = MyCompany.Foo.Baz;

// ...

var baz = new Baz();

But you'd have to do it for each non-private class that you want to use.

UPDATE

It's interesting to note that C# 6 will have static using statements, which could effectively improve this proposal to use a public static partial class as a "module". You would just "use" the "module" to access its types directly.

Hopefully, it will work like this:

using MyCompany.Foo;

// ...

var baz = new Baz();

Just as if Foo was a namespace.