Develop Reference

Is there a way to generate C# Code from Class Diagrams?

I am using Visual Studio 2019. I cannot seem to find any option to make UML Class Diagram and generate code from it. The language which I'm working on is C#. If there is a way to do it, kindly tell me which libraries and frameworks are required in Visual Studio. You can also recommend another IDE if it does the job with a little briefing. Thank you.

First, you need to install Class Designer component. https://learn.microsoft.com/en-us/visualstudio/ide/class-designer/how-to-add-class-diagrams-to-projects?view=vs-2019
After your design you can generate de code: https://learn.microsoft.com/en-us/visualstudio/modeling/generate-code-from-uml-class-diagrams?view=vs-2015

You can generate c# code from diagram with Astah UML, it is totaly free for student or it's a free trial and you can reinstall it as you want.
https://astah.net/products/free-student-license/

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).

Visual Studio Class Diagram Viewer stand alone

Visual Studio Makes Class diagrams and if we open it in text editor program it is a simple XML. Surely we can generate that on runtime. But the problem is although you have visual studio to see the Class Diagram Generated but what solution is there if we want to deploy application where there is no Visual Studio Installed?
Is there any possibility or available plugin or library that can generate Class diagram or UML diagram from XML?
Can we somehow use Visual Studio Class Diagram to view it on runtime?
Not Sure but what if we generate xml on Back end and then if possible create Image (png or gif) using some builtin plugin and display that? (I am really not sure about this idea :()
I have seen NClass It is very good but a bit complex and it is not developed through XML.

I suggest to use PowerToys for Class Designer (http://modeling.codeplex.com). It improves the Class Designer and allows you to export diagrams to HTML. This way is easy to distribute to any machine.
As an alternative, you can take a different path with reflection, in this case you don't need the visual studio class diagram; the diagrams are generated by inspecting the requested types. I suggest to read the articles by Sacha Barber (http://www.codeproject.com/Articles/17823/100-Reflective-Class-Diagram-Creation-Tool and http://www.codeproject.com/Articles/207280/200-Reflective-Class-Diagram-Creation-Tool). Maybe you can customize his application.

In Visual Studio Enterprise 2017:
Quit Visual Studio if it is running
Start Visual Studio Installer
Click dropdown list "More" and select "Modify"
In section "Code tools", check the option for "Class Designer"
Click button: Modify
Result: now the Class Diagram should load from Solution Explorer as UML.

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.