Develop Reference

Generation of UML class diagram out of C# code

I have the following problem. I have some C# classes and I need an UML class diagram for that as documentation.
As I want to spare time and nerves it would be awesome if I can generate the diagram out of my code. (Reverse engineering tool)
I'm using Visual Studio 2015 Enterprise, but in the UML tool there I can only draw them by hand or am I missing something?
I know there's such a tool in eclipse named object aid uml.
Something similar would fulfill my needs.
EDIT:
View > Class view - is unfortunately not enough

A deeper research got me to this stackoverflow post from a Microsoft employee.
He clearly states that there's no intention to bring such a reverse engineering tool to Visual Studio 2015.
Reference: https://stackoverflow.com/a/32139082/3957817

Automatically add NuGet dependencies and using statements in Visual Studio Code?

I am using Visual Studio Code to develop an ASP.NET 5 application on Mac. In order to use new classes (framework or third-party) in my .cs file, I need to
Manually add a NuGet dependency in project.json and then
Manually add a using statement to my .cs file.
It seems that there should be a better way to import new functionality that doesn't involve searching for the right NuGet and the correct namespace. Any suggestions?

Well, once I got my IntelliSense issues figured out (Visual Studio Code on Mac), I don't have to type using statements anymore. OmniSharp-based IntelliSense is smart enough to suggest (Cmd + . on Mac) adding them for me:
I still have to add a NuGet dependency manually, but I think this is the default behavior in the full Visual Studio too and you need ReSharper to get smarter than that.

Unfortunately, as far as I know of, the short answer is no.
Visual Studio Code is meant to be a light weight editor, so does not have support for the kind of feature you are describing out of the box. The full Visual Studio on Windows does have support for that. When you type the name of a class/type you want to use, eg. JsonConvert, it will detect that the missing type is available on NuGet and offer you the ability to download the correct package and add the using statement. (In the screenshot I already have the NuGet package installed, so it only needs to add the using statement)
This feature is available in the Community edition of Visual Studio, which you can download for free from the visualstudio.com website.
This does require you to run Windows, so I'm not sure if you consider this an option.
Now on the wishful thinking side: VS Code does support extensions these days and it has the power of the Roslyn engine, so theoretically someone could write an extension that will offer this functionality in the future.
You could also try getting it added to the core editor, by opening an issue on GitHub: https://github.com/Microsoft/vscode/issues
I'm afraid neither of these will really help you in the short term though.

Aspect Oriented Programming with Roslyn

Does roslyn or visual studio 2015 provide API to rewrite IL or "something like that"? Let me explain...
I've read in msdn magazine's article Use Roslyn to Write a Live Code Analyzer, that it is possible to write custom code analyzers distributed via nuget packages, without need for visual studio plugins.
I'm really curious, if roslyn compiler and visual studio 2015 will allow to implement e.g. Code Cotracts such way, that no plugin is required to build it correctly.
Also, I would like to gain productivity and readability of my code with aspects like [NotifyPropertyChanged] data annotation on properties, but again, if it won't build correctly on every machine with visual studio 2015, it's not a good idea.

It is possible to use DispatchProxy from System.Reflection.DispatchProxy package. Unfortunately I haven't found any examples how to use it, but it looks like a replacement for RealProxy class with some differences (examples how to use RealProxy: hear and here).

How to implement code folding in C#

I'm beginning to work on a COBOL/BASIC IDE at work (to replace the one that we have currently that's a slight step up from Notepad). It'll be made in C#. The management is really interested in implementing some Visual Studio type features, and a big one is code folding. I've looked on MSDN, but I didn't see any good way to collapse lines (or to add the expand/collapse buttons).
I'm assuming that the text area should be a RichTextBox. Am I off track here? I suppose it could be done with some sort of modified TreeView, but that seems a little wrong to me. Is there some way of doing this that I'm just missing?

Why not use an existing IDE and extend it? Writing one from scratch is a huge undertaking (you need a parser, lexer, syntax highlighter and more), and is even more complicated if you need to support multiple languages (you mention COBOL and Basic).
Notepad++ has syntax coloring and one can add languages to it - COBOL is one of the ones installed by default. It supports code folding and has many plugins (you can write your own, that will suit your needs).
Edit:
Eclipse is another excellent IDE that has similar support, and as mentioned in the comments has a COBOL plug-in.

I suggest you take a look at SharpDevelop. It's a pretty good IDE with a bunch of Visual Studio like features already built in. It's written in C# and fully supports code folding with syntax highlighting in several languages. Plus, it's Open Source under the LGPL license. So, if you don't want to base your app on SharpDevelop then you can still reuse some of their controls like the code editor or windowing toolkit.
You should consider adding the COBOL language to SharpDevelop instead of starting from scratch. If you can't do this, then you can still use the SharpDevelop code as a decent reference on how to make a good IDE work.

Sometimes embedding Eclipse or a full-fledged editor is not appropriate. It's overkill or overweight or wrong for some other reason. I appreciate the first inclination suggested in other posts to not re-invent here, but in some cases a small invention is what is necessary. For example, the textbox used to make Stack Overflow posts .... is neither Eclipse nor an embedded Visual Studio. I wonder why?
It's important to ask the question - build it or buy it? - but sometimes, the correct answer is BUILD IT.
XPathVisualizer provides a simple example of a code-folding text editor implemented in C#, and based on a RichTextBox. It's not VB, though - it's an XML editor. But the general principles apply.
Here's a look at it.
To implement XML syntax colorization dynamically, while the user types, it uses a separate background thread. The reasons why and some of the details are described in a separate answer on Stack Overflow.
You could do something similar for your COBOL/VB thing. XPathVisualizer is open source, licensed with MS-PL, so you can browse and borrow.

If your team is used to "Visual Studio features," then I'll assume you use Visual Studio there at the office. Here are my suggestions:
Base your IDE on Visual Studio, for the following reasons:
Use Visual Studio 2010 if possible. The SDK is greatly improved from 2008/earlier.
Use Visual Studio 2008/2005 otherwise. At the moment, all of my commercial IDE products only support 2005/2008.
If your team uses Visual Studio, they will hate Eclipse. Not even an option to consider in this case unless you choose to use an existing Eclipse plug-in, saving you the time of creating a new IDE.
If your team isn't using Visual Studio 2010, you can use the Visual Studio 2010 Shell in Integrated Mode for free (Isolated Mode is not what you want). This lets you use Visual Studio 2010 for your IDE for now, and should the team upgrade later to one of the full versions of Visual Studio 2010, the IDE for your language will cleanly integrate into the full version. Edit: Visual Studio Shell is basically the core of Visual Studio without any specific languages (C#, C++, VB, etc.) included. Microsoft provides this core for free, and it's a great option specifically for people interested in creating their own language support.
Read my answers in the following two questions:
Here's a longer post on my reasons for using 2010 over 2008: How do I implement intellisense support for a custom DLR language in VS2008?
Here's a very long answer on implementing various features (most of which are based on the 2005/2008 SDK): How do i implement intellisense for my language in visual studio?

Writing a complete IDE is a HUGE task. I would recommend trying to find an existing one that has what you want, or make adaptions to an existing open source IDE.
To answer your question: I guess that the Visual Studio IDE uses a custom control, written from scratch, rather than a RichText control.

C# Class/Object visualisation software

In Visual Studio 2005 and prior you could export your code to Visio and view the relationships between the objects and what methods, properties and fields it had. This was great as it allowed you to tweak the appearance to improve the look.
In VS 2008 that option is gone, replaced (supposedly) with the class diagram system. It is functional but not pretty.
What I am looking for is some software that allows me to visualise in a prettier way or at least provide me with enough options to filter, arrange and so on so that I could touch it up in something else.

Do you require the software to be free? If not, you might consider a UML tool like Sparx Enterprise Architect. It can import your code, and has two add-ons that go further in terms of Visual Studio integration.

Not quite what you're asking for but it's worth checking out some of the add-ins for .Net Reflector
E.g.
Graph
Dependency Structure Matrix PlugIn for .NET Reflector
100% Reflective Class Diagram Creation Tool

Visual Studio 2010 Ultimate supports UML class, sequence, component, use case, and activity diagrams. It also supports creating sequence, dependency graphs, and layer diagrams from code. Other tools include Architecture Explorer, which lets you browse and explore your solution.
For more info, see the following links:
To download the RC release, visit: Microsoft Visual Studio 2010 Ultimate RC
To see the RC documentation, see Modeling the Application.
To discuss these tools, visit the Visual Studio 2010 Architectural Discovery & Modeling Tools forum.

Have you tried the PowerToys?

NDepend is certainly the .NET tool you are looking for. It comes with a dependency graph coupled with a dependency matrix. You can try NDepend straight on your code by downloading the free Trial Edition.
More on NDepend Dependency Graph
More on NDepend Dependency Matrix:
Disclaimer: I am part of the tool team

There is a decent, free version of a UML diagramming tool called Argos here http://argouml.tigris.org/features.html. Out of the box it can both forward and reverse engineer Java, I'm fairly certain there are add-ins for Visual Studio that help support forward and reverse engineering of C# code. I would look into that.

Not your answer, but related...
Codeplex's VisioAutoExt library:
http://www.codeplex.com/visioautoext
Visio ActiveX component:
http://www.c-sharpcorner.com/UploadFile/mgold/VisioInDotNet12032006222024PM/VisioInDotNet.aspx

When i search for code in codeplex i found this http://vsdm.codeplex.com/ maybe help you.

With VS2005 & VS2008 you have the option of designing your own modelling language (DSL) here you can choose how your model is rendered, it would be a big investment if its just to make the diagrams look a little better, but it does provide you with option to customise how the code is generated from your model.

You may have tried PowerDesigner, it runs under Eclipse through a plugin and it also supports Visual Studio. The only downside is that it only works on Windows.
Another application for modeling is System Architect from IBM (Popkin). It has been used by some major corporations and is still maintained.