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Using a DirectX c++ DLL in a C# WPF or Windows Forms Application

I have written a DX11 renderer using c++. I am now looking for a way to implement an Editor/GUI for it.
Since im pretty used to Windows Forms and WPF C# Applications im thinking about putting my renderer inside a dll, load the dll from a c# application and use it to draw into a defined section of the form. Does this even work with a window handle passed by a managed c# appication and how is the performance if it does?
From my understanding, since the actual communication with the DX11 API would still be directly in c++ and the c# part would only tell the dll what to do the performance loss should be almost nothing. Or is this generally a bad idea and implementing the GUI directly in c++ using a library or D2D is the better approach.

In WPF you can use D3DImage which is just like a simple Image in your xaml. The general principle is as follows:
Allocate a Direct3D 9 texture, this will be your rendering target.
Hand it over to the D3DImage.SetBackBuffer.
Put your rendering code inside CompositionTarget.Rendering event handler.
RenderStuffToTexture();
d3dImage.Lock();
d3dImage.AddDirtyRect(new Int32Rect()
{
X = 0,
Y = 0,
Height = d3dImage.PixelHeight,
Width = d3dImage.PixelWidth
});
d3dImage.Unlock();
Since you are using Direct3D 11, you have to use DXGI surface sharing to share the D3D9 textute with D3D11. There are 2 examples I know of that illustrate how to do just that. Microsoft D3D11Image and D3DImageEx (my personal preference, can be found in multiple places online).
Regarding performance, once you use WPF you no longer doing the presentation yourself. You literally write a small render-to-texture logic inside a WPF D3D9-based renderer. So not everything can be controlled including presentation time. From my personal experience, you can definitly render simple scenes at a nice framerate. For a really graphics intensive app and strict FPS requirements I would use a more lightweight (and less intrusive) UI solution.
Note that you can also use WinformHost control to get a HWND and render to it. This has a serious disadvantage if you want to overlay UI controls over the rendered frame - AFAIK you simply can't.

Program architecture for Windows 8 C# & XAML app featuring graphics

If I'm looking to create a game that doesn't necessarily run full screen, but simply needs to feature 2D/3D graphics somewhere in a portion of the screen, what's my best approach?
Some specific questions could be:
What component would the rendered area use?
Are there any game libraries I could leverage for the rendered area?
What would be the most "pure" or "canonical" stack according to Microsoft to use here?

Omega --
Visual Studio 2010 and 2012 are Both WPF apps. WinRT is for Tablets / Mobile. WPF is certainly NOT outdated.
If I were you I wouldn't render everything out the way canvas forces you to, it might be a better approach to have the center item be a UI element named Frame, which is the base element for all UI related content in WPF.
In this way you would be able to leverage all of the possible types of controls in the Frame whether you decided that An ImageSourceType or Canvas is more applicable to a particular features of the game.

Depending on how you want to draw graphics, you could use (but are by no means limited to):
Canvas - which would be totally appropriate for slow moving games. This way you get the benefit of the various WPF layout routines and can define objects inside the scene in XAML/vectors as well.
WPF supports 3D graphics (using Direct3D on the backend) so you could probably set up an orthogonal projection matrix and treat it like a Direct3D context (with the WPF API). I don't have enough experience to know how slow this is compared to D3D, but it's certainly easier (built-in "scene graph" like support from the XAML architecture, for instance).
If you want to go whole-hog with Direct3D you could use SlimDX, which has a WPF shim that I've used in the past, as well as another third party control. There may be other libraries available as well.
Direct blitting to/from a Bitmap using WriteableBitmap (see WriteableBitmapEx for a third-party version with a much friendlier API) or similar.
There are probably a lot of other options too. My preference would be for using Canvas initially if it's a slow-paced game that doesn't need super-fast frame rates (the layout work does incur a fairly substantial overhead, but it's less work and may be easier to get looking exactly the way you want).
If you want absolute control and speed, use D3D through SlimDX, but this is a pretty hefty learning curve if you're new to it.

Is it practical to port code from Flash to C#?

My goal is to gain a better understanding of the characteristics of C#, and become more comfortable creating simple apps. I am fairly competent with Flash (Actionscript 3), and found an old Tic-Tac-Toe game I'd written. I started wondering about porting this code into a C# application. Not knowing much about C#, I'm wondering how difficult the migration would be.
On the one hand, the underlying game logic is syntactically similar, and therefore would be easy to port.
However, as far as the graphics are concerned, I don't even know where to begin. So far, I've only exposed myself to Windows Forms and Console apps in C#.
I'm wondering if these Flash concepts have similar analogs in C#, or if the procedures and syntax are radically different:
Placing graphic elements on a stage
Rendering lines from start/end coordinates
Event listeners on movie clips
Swapping the image inside a graphic element (or, in my Flash version, nextFrame() in a movie clip)

You may want to try developping your little game using Silverlight. Silverlight applications, coded using C# and Xaml, are pretty similar in form with flash applications, and you should find everything you need without trouble.
So I suggest you download the Silverlight SDK (free) and give it a try.

Firstly, Flash is to WPF (close enough) as ActionScript is to C#.
The WPF/Silverlight model is much more similar to Flex that it is to Flash. Everything is added to the UI tree as a object, even lines.
Likewise, adding event handlers to controls (like a button click) can be done in the "code behind" (the code that lives with the view), but the recommendation is to use the MVVM pattern. If you are new to the concepts of separation of concerns and unit testing, feel free to start with the simpler "code behind" method.
While WPF and Silverlight are very similar, I'd recommend starting with Silverlight as the SDK and available samples are richer. You can easily move onto WPF later on (though porting an application from Silverlight to WPF is not automatic).
Swapping images, as you mentioned, would be done via "Visual States" in Silverlight (or possibly changing the image reference, which is more "hacky").
Have a look at the following links to get started:
Learn # Silverlight.net
Silverlight on MSDN
Shapes and Drawing (Silverlight)

If you're looking specifically to do games and the like, you may wish to look into the free XNA framework. However, there will be differences as Flash gives you far more ability to "set up" things beforehand and modify them.
Placing graphic elements on a stage
If you go the XNA route, you will be drawing sprites using the spritebatch, you tell them where and how to draw and that's where they will go
Rendering lines from start/end coordinates
In windows forms you can do this via a simple System.Drawing call, however if you wish to do this in XNA, you will either have to make a 1 pixel square and stretch/rotate it to what you want, or use 3d primitives (Though this will limit you to a 1 pixel line)
Event listeners on movie clips
Look into delegates, but there isn't really an equivalent for movie clips to my knowledge
Swapping the image inside a graphic element (or, in my Flash version, nextFrame() in a movie clip)
This is fairly simple, depending on what you mean. If you want to, say, animate a sprite. You can do this by moving the source rectangle or changing the texture of the spritesheet. If you mean the screen as a whole, this is mostly handled for you provided you use the spritebatch. In windows forms you'll have to do more of it yourself, but the base concepts are the same.
Overall it's not that bad, but if that doesn't sound appealing check out Silverlight. It's basically C# styled flash so you may find the transition easier.
Good luck and hope this helps.

Which one has a faster runtime performance: WPF or Winforms?

I know WPF is more complex an flexible so could be thought to do more calculations. But since the rendering is done on the GPU, wouldn't it be faster than Winforms for the same application (functionally and visually)?
I mean when you are not running any games or heavy 3d rendering, the GPU isn't doing heavy work, right? Whereas the CPU is always busy.
Is this a valid assumption or is the GPU utilization of WPF a very minor operation in its pipeline?
EDIT: The application that I am interested is a 3d modeling and animation software, where you have 3d viewports to navigate and edit the scene, and objects inside the scene. But I want to use WPF because of its modern architecture, and it's from scratch.
EDIT2: Also for my purposes I will use DirectX hands down for the app itself because of the high end requirements of the software. As for people using lower end or computers without a dedicated GPU, that's OK since they aren't not in my primary customer area. Just like other high end 3d software for film and games, it will be understandable to require a powerful computer to fully benefit from the application.

Provided the machine has a GPU, you'll get better rendering performance in WPF.
We have a large desktop application that we wrote in WinForms, and are now porting to WPF. We've witnessed much better rendering performance, particularly when resizing windows or redrawing controls.
We've also found that WPF "controls" are more lightweight than WinForm controls. If I recall right, WPF controls do not necessarily require an operating system handle, and don't register for Windows window messages via WndProc, at least not independently.
For your case, since you're building a 3d modeling app, which kind of assumes some 3d hardware on the machine, you should absolutely use WPF over WinForms.

For the app scenario you describe, I would expect WPF to outperform WinForms for 3D work on a full featured GPU by a wide margin.
The difference between the application types is more than just the rendering the 3D vector pipeline. WPF's internal architecture is radically different than WinForms, specifically designed to overcome the caveats learned from years of prior experience with the Windows GDI and WinForms apps.
(WinForms is a relatively thin wrapper around Windows GDI and User model that was originally created in the late 1980's. The Windows User control model has evolved over the past 25 years, but the core architectural patterns are largely unchanged.)
For example, WPF always separates UI rendering from application logic. When the WPF window goes to draw something, the actual rendering happens on a background thread. The refreshed visuals are flipped to the display during the video retrace interval, so you don't get partial blits or "tearing" artifacts on screen.
WinForms does none of this. If you render to DirectX or OpenGL surfaces in a WinForms app, you have to do the work of flipping the video page and making sure it happens at the right time to avoid screen tearing artifacts.
Wpf's default controls are GPU aware and can be custom styled with glows and transparency and whatnot all GPU accelerated. WinForms controls do not benefit significantly from GPU features, since about the only things WinForms (Windows User controls) uses for rendering are 2D bitblit and rectangle fill. Glows, transparency, animations are all possible with WinForms, but you have to do all the work to implement them.
In WPF, UI slickness is mostly a matter of designing and styling to get WPF to do it for you. In WinForms, you have to push the pixels yourself.

This is a really tough question to answer.
A huge portion of WPF performance is your GPU. A good GPU works wonders for making WPF perform well. WPF can be very performant. If your requirements are to have a decent GPU, since it's a 3D modeling program, you'll probably find WPF performance to be as good or better than Windows Forms - though this really depends on what you're using.
That being said, it's usually difficult to compare - mostly because WPF allows you to add a lot of visual effects that people tend to never even attempt in Windows Forms. Many WPF applications "seem" faster even though they're actually slower in some instances because of extra visual clues.
That being said, if you have a LOT of controls, WPF can actually outperform Windows Forms by a fair amount. In Windows Forms, each control requires a separate window handle, and receives its own message sets. With a lot of controls, this can actually slow things down pretty dramatically.
The real question here should be is WPF perf. "good enough" for your application. If you're doing a 3D modeling application, chances are WPF will not be your bottleneck- it should be fine.

For 3D modeling, WPF is clearly a lot better choice than WinForms -- but it's still a long ways from ideal. Winforms has no (direct) support for 3D rendering at all, and WPF has some. For a program that's aimed primarily toward 3D rendering, however, you might be better off with something dedicated more specifically to 3D rendering tasks, such as OpenGL or Direct3D.
On their own, neither of those provides a huge advantage over WPF, but unless what you're doing is fairly specialized, I wouldn't use those directly though. I'd use something like OpenSceneGraph or Ogre3D, that can use either of those for its rendering, but provides a much higher-level interface and does more to manage rendering your scene.
Either or both of these probably could use WPF for the actual drawing -- but I'm pretty sure neither one does, at least currently. I don't know whether they ever will either -- they might, but I doubt it's a real priority, since they already support OpenGL and Direct3D. For their purposes WPF provides little advantage.

Seeing as WPF actually uses DirectX under the hood and the fact that you use XAML to specify DirectX objects, WPF is by far the easiest solution.
Example coding a camera in xaml:
<PerspectiveCamera x:Key="Camera"
Position="0, 0, 4"
LookDirection="0, 0, -4"
UpDirection="0, 1, 0"
FieldOfView="30"/>

Is it smart building an OpenGL C# application to replace GDI

I have developed a quite large application using MFC. Naturaly, I used GDI for drawing, CCmdTarget for event routing, and the document-view architecture.
It was a convenient development path.
Now, the client is interested in converting this application to .Net.
I would prefer (and they too) writing the new product in C#.
The application displays and interacts with thousands of graphic objects, so
I figured going with GDI+, although seems natuaral, can cause performance issues,
So I am thinking of using OpenGL, specifically - OpenTK - as the graphics library (it's 2D).
I know that OpenGL works differently that these Windows APIs, which rely on Invalidation of portion of the screen. OpenGL has a rendering loop which constantly draws to the screen.
My question is:
Is this an acceptable way to go, thinking of:
performance - will the users need special graphics cards (hardware?). It is graphics intensive, but it's not a high-end game
printing and print preview - are these things complex to achienve?
multiple selection and context menus
Is this library goes well inside windows forms?

I don't think so. Use WPF if you can or DirectX if you can't.
I know it might not be fair but if I'm programming on .NET (microsoft) on windows (microsoft) I'd rather use DirectX ... which is also from microsoft.
As a side note: don't reinvent the wheel. Recoding user controls in open-gl can be very time consuming, if you do make sure you have a good reason.

In my experience developing CAD-like software, the benefits of OpenGL and DirectX are fast depth testing, smooth rotation and panning, lighting and powerful texture capabilities. Obviously there are other benefits but, despite what most tutorials would lead you to believe, implementing a rendering system using either of these APIs is a significant undertaking and should not be taken lightly.
Specifically:
If it is a 2D app and you already have it implemented in GDI then switching to GDI+ will be much easier. Additionally, on modern hardware, 2D GDI or GDI+ can be about as fast as 2D OpenGL or DirectX. And ultimately, the end-user probably won't notice the difference, especially with double buffered support in GDI+.
You do not need (and probably don't want) a continuous rendering loop for your app. In OpenGL and DirectX you can explicitly invalidate the window when your scene changes.
If you go with OpenGL or DirectX you will need to consider putting your objects into display lists or vertex arrays (buffers) for fast drawing. This is not difficult but managing objects in this way adds complexity to the system and will most likely significantly change the architecture of your rendering system.
Printing in either OpenGL or DirectX can also be tedious. On the one hand you can render to a bitmap and print that out. However, for high quality images you may want vectorized images instead, which are difficult to produce with either of these rendering frameworks.
I would also stay away from writing GUIs in OpenGL or DirectX...unless you're really looking for a challenge ;~)
Finally, and this is just an annoyance from an install perspective, the Managed DirectX run-time library that must be installed on the user's machine is around 100 MB.

I have no experience with C#, but I have once built a layer system for a drawing program that used openGL for rendering.
To make this layer I asked openGL for the current framebuffer and converted it to an image to use as a texture under the current canvas. So I guess from there its pretty easy to go to printing and print preview.

Direct X and Open GL much faster than GDI+.

You can also use an TAO framework as an alternative to OpenTK.