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Visual Studio C# Error creating handle but built app runs fine

I have a winforms app (>75k lines of source code), I have just added some new functionality and a new form and I'm getting the 'can't create window handle' error but only when running inside VS. The app builds OK and when i run it as the executable it is using 597 handles, 94 GDI and 83 USER and the new form works just fine apart from my logic errors in it. So as far as I can tell; my app is well below the 10,000 limit where things start to break therefore the problem is in/with VS somewhere but I may have hold of entirely the wrong end of the stick or possibly completely the wrong stick.
I have cleaned, rebuilt, rebooted etc and it stills falls over.
I'm using Process Explorer (thats where the previous numbers came from) but can't seem to find VS in there to check what its total handle count is.
VS2017 on Win 11.
Yes, I have googled it and explored things to look at but I can't find a result that matches 'Studio falling over but the exe running fine', any help / pointers very much appreciated as I obviously can't debug the new form which does make things somewhat difficult.

The underlying issue was to do with a pop-up form being generated by the parent form during its load processes, I found that the parent hadn't been issued a handle before it was trying to show the pop-up. Before you ask... I'm capturing data from a network sensor via the pop-up, which, once the user has accepted the incoming value, feeds the data back to the parent form. I assume that in run-time everything runs faster and it doesn't hit that 'un-coupled' point. Changed the code in the instantiate/load/shown cycle it runs and all is well.
Thanks once again Hans, headache solved.

Why is the ASP.NET/Visual Studio Web Development Server so slow?

(xkcd)
I know that compiling nowadays is much faster than it was before. Yet, for me, it seems that compiling and especially running/debugging ASP.NET projects with the Visual Studio Web Development Server is incredibly slow.
Since the beginning of last summer, I've been working heavily on ASP.NET MVC projects. Of course, the best way to debug them is by using the web server that comes with Visual Studio. When doing that, I get horrendously slow loading times. Chrome dev tools typically report that loading one of my pages had a 3 minute wait time, followed by a short loading time.
I've seen these two questions, but they don't help. While I do most of my debugging work in Chrome, the same happens in IE.
Has anyone else had this problem before? If so, any tips?
Also, I doubt that the problem lies with the speed of my machine. This computer is really fast running Windows 7 and Visual Studio 2010, so I don't see why ASP.NET debugging should be so slow.
UPDATE: In his answer below, Jon Skeet suggested attempting to identify whether the problem is being caused by the environment or by the code itself. I created a brand new MVC project and ran it. The first test appeared to be much faster. However, after testing it a few more times, it's safe to say that the first test was an anomaly - usually, it takes as long as my big project (2 - 3 minutes). Thus, this is a problem with the environment. Thanks in advance for any help!
UPDATE #2: It's been a while since I updated this question. Here are some details I've gathered since my last update:
This delay is occuring on both of my development machines, both running Windows 7 and Visual Studio 2010
This delay is happening for all my MVC2 and MVC3 projects (but I haven't experimented with plain ASP.NET yet)
Plain/vanilla MVC projects experience the same delay as MVC projects with big codebases
Disabling IntelliTrace did not help
Disabling IPv6 did not help
I haven't found a solution for this problem, so I've been stuck with huge wait times. Does anyone know how to solve this?

There's no reason why it should take 3 minutes to start debugging something unless you've got something really strange going on.
I suggest you launch the debugger, try to navigate to a page and then just break into the debugger while it's loading. See where you're losing time. Maybe you're making some call on startup which is failing, but taking minutes to do so.
I've never experienced anything like what you're describing, which suggests it's either in your environment or in your code - and if it's a generally fast computer, that suggests it's somewhere in the code.
If you create a brand new MVC project and debug into that, does that take a long time?
You might also want to run Wireshark when you start debugging - see whether something in your app is trying to fetch a network resource of some description without you realising it.

This is due to the IPv6 DNS problem within Firefox and can be fixed by setting the network.dns.disableIPv6 setting to true within the about:config
Slow DNS response - Firefox on localhost and Visual Studio or Cassini

This is actually a known performance issue with MVC, caused by throwing a ton of exceptions internally on startup. If you have IntelliTrace turned on, this will wreck havoc with it. Try disabling IntelliTrace and see if that improves it at all.
Source: http://connect.microsoft.com/VisualStudio/feedback/details/535799/debugging-mvc-is-very-very-slow

Check out if this helps:
http://blogs.msdn.com/b/mahuja/archive/2008/07/08/resolving-very-slow-symbol-loading-with-vs-2008-during-debugging.aspx

I met the same problem these days, working under Win7, VS2010, and developing a Silverlight solution with 10+ projects. It takes about 3 minutes to launch IE, and waits 3 minutes to get back VS after closing IE. I overcame the problem by trying different ways. But the only valuable method:
Unload all projects not for debug
Start debug the solution (it works normal )
Reload other projects
VS works normal here. (VS works normal)
It seems the problem is just related with the VS environment on my side. It has nothing to do with ASP.NET Development Server or IIS.

I was having the same problem. It ended up being a problem with ColdFusion being installed on my local machine (as painful as it is for me to admit). ColdFusion puts Handler Mappings named AboMapperCustom-* that were causing the latency. I uninstalled ColdFusion because I no longer need it (thank God). Another solution could be to remove each AboMapperCustom entry through the web.config. Example:
<handlers>
<remove name="AboMapperCustom-15397" />
<remove name="AboMapperCustom-20358" />
<remove name="AboMapperCustom-47286" />
</handlers>
Make sure the 5-digits following AboMapperCustom- match those in your local IIS Handler Mappings.
HTH

On Visual Studio 2012 + Win10 x64, i've noticed that if you
go to the taskbar and find the asp.net dev server there,
right click it and do a "show details"
close the box (NOT 'stop', just close via 'x')
then the server loads almost instantly

VS 2010 - Error when opening User Control / Form with Designer

C#, VS2010, WinForm application:
Sometimes I do have the problem that I get an error message when opening some of my controls / forms. All code compiles and the application runs properly. Opening the control in the designer gives me:
The designer loader did not provide a root component but has not indicated why.
From my experience I can tell, it is usually something in my code which does not get properly initialized, e.g. a property is not set which is somehow available at runtime, but not when opened with the designer. The only issue is, that the root cause is hard to find.
Q: Is there a chance to somehow use the debugger when the designer opens my component in Visual Studio 2010? That would help a lot and the problem tackling is most likely a matter of minutes then.
Remark: Just to make this clear, I know how to use the debugger ;-), I only have no idea how I could tell VS2010's designer to open my control in Debug mode.
As of 2nd SEP 2010 added:
Thanks for your help. Basically it is the MSDN Library article describing how to do it.
I have managed to set it up and run the second instance
(there was not much to understand how to do it).
It only fails in my case, because the 2nd VS2010 instance (debugging Design Time) fails to find the
symbols for my custom control. I have added the symbols manually under Debugging / symbols. No
result, still "Breakpoint won't be hit because of missing symbols"
BTW, using this approach it is helpful to exclude some stuff from the symbol loading (via modules window), because this will safe a lot of time.
Now if have to figure out how to get the symbols resolved and then I can tackle the issue.....

The vast majority of design time problems with custom controls are caused by code in the event handlers or method overrides in your control running at design time as well as run time. That's normally desirable, you get instant feedback when you change a property in the Properties window for example.
But not desirable when the code depends on something that's available at runtime but not design time. Like a dbase connection or a file that's stored in the build folder. That can generate exceptions and Visual Studio isn't very robust against handling exceptions at design time. Worst case, you can crash VS to the desktop without any diagnostic. But anything is possible.
Review the code in your control and make sure that the bits of code that should only run at runtime are wrapped like this:
if (!DesignMode) {
// etc..
}
Hard cases can be diagnosed with the debugging tips in this MSDN Library article.

To debug your control in design mode, you need two instances of Visual Studio. In the first instance, open the project which contains this control source code. In the Project Properties, Debugger, set command line which calls another Visual Studio instance (msdev? don't know exactly for VS2010 - take it from the shortcut), Then execute "Start Debugging" command. Another Visual Studio instance starts. In this instance open client project which uses your control on the form.

Debugging C# applications

I´m coming from a PHP background where my debugging "tools" are basically echo, var_dump and exit. While I know the importance of debugging, I never tried to use/learn a debugging tool.
Now I´m learning C# and I think I can´t really program without an extensive knowledge of this area.
So my question is: where can I learn what is and how to do debugging? I know what is a breakpoint (conceptually), but how to use it? How to use Step into and Step over? Basic things like that.
As a related question, there is anything like var_dump in C# (Visual Studio), where I can inspect any object.
I find very difficult and painful to do a foreach for every array/list to see the contents, specially now that I´m still learning the language.

Microsoft has an extensive guide on C# debugging in Visual Studio that might be helpful. VS has a lot of powerful debugging functionality; for example, rather than doing a foreach to see the contents of an array as you were describing, you could set a breakpoint (pausing the execution of the program) and select the variable you wish to see the contents of (array or list or whatever) and see what it contains, without having to write any extra code. Step Into and Step Over can be used to continue execution of the program but only incrementally so that you can continue to see how variables change, where the flow of execution currently is, etc.

This has been covered earlier on StackOverflow:
Best Visual Studio 2008 Debugging Tutorial?

So, your compiled applications can be run in a "debug" mode from which visual studio can monitor the internal workings of the application and even control it.
A break point can be placed just about anywhere in your code by clicking to the far left of the line (kinda in the margin of the visual studio text editor). When that line of code has been reached, the visual studio debugger will actually pause the execution of your program and bring you back to the editor where you can literally hover over a variable or object or whatever and see everything about it.
There is also a "Locals" window available that will give you the break down of all of your locally scoped items - this should pop up by default at the bottom of your screen when debugging.
In debug mode you can navigate the execution of your code line-by-line
F10 will continue with the next line of code.
F11 will attempt to drill down into what ever functions are on the current line of code
Ctrl-D will bring up a "Quick Watch" window giving you all information about the currently selected variable/object.
Once you are in debug mode there are tons of things you can do - in some cases you can even edit the code as you go.
The easiest way to get into debug mode is to use the little "play" button up at the top of visual studio - and when a break point is reached it will enter debug mode and highlight the currently executing line of code.
You can also hit F10 from the editor and your application will be started and paused on the very first line of code.
By comparison, in PHP, you had to actually write "debugging code" into your application - using Visual Studio you can actually monitor the execution of your code without adding a thing to your existing code.
I hope that gets you started.
You might want to also read up on your IDE a bit to. There is a metric ton of stuff in visual studio that will help you navigate your code in ways you never imagined in most PHP editors.

If you've already downloaded Microsoft Visual Studio, you'd might want to check out the Visual C# Express Library available for free over at: http://msdn.com/express/
It's located down the bottom of the page and is very useful. It contains pretty much every answer you might be looking for as a beginner to the C# Language. ...Welcome to C#, my friend :-D

Is there a way to *completely* disable Edit and Continue?

I was wondering if there was a way to completely lock my code while debugging it within Visual Studio 2008. The code documents lock automatically when running as 64 bit applications, which I greatly prefer; however, I do most of my coding making add-ins for Excel, which is 32 bit. The result is that even though I target 'AnyCPU', the VS host knows that it is running within a 32 bit process and, therefore, the source code is not locked while the code is running hosted in Visual Studio.
I can turn off Edit and Continue by going to Tools > Options > Debugging > Edit and Continue, and then unchecking the 'Enabled Edit and Continue' check box. This does not completely lock the code, however. This does prevent any edits in the code from being executed in the current run, but it does not prevent mouse clicks or keystrokes from actually changing the code.
Again, when working with 64 bit applications this does not occur -- the code is completely locked. I greatly prefer the code to be completely locked for at least a couple of reasons:
I can accidentally hit a key or the like while debugging, which I definitely do not want to do. It's rare, but it is an issue.
Many of my automated tests drive the user interface via SendKeys. When stepping through such a test using the debugger, however, I can sometimes forget that some of the aspects involve SendKeys, which means that keystrokes wind up getting sent to the Visual Studio IDE instead of Excel.
In issue #2, above, the unit test fails, which is fine -- my bad -- but having all the keystrokes sent to the code module and destroying my code is completely unacceptable.
Does anyone have any ideas here? Can one completely lock the code when running hosted in Visual Studio while compiled against a 32 bit CPU?
Some related posts on this issue, but none of which directly address this:
How to: Enable and Disable Edit and Continue
“Changes to 64-bit applications are not allowed” when debugging in Visual Studio 2008
How do I enable file editing in Visual Studio’s debug mode?
How does “Edit and continue” work in Visual Studio?
Can we edit our code while running the application
Editing C# while debugging
Thanks in advance for any help or ideas...
Mike

Here is a trick I use under Visual Studio 2005 (don't have a chance to test under Visual Studio 2008, but it should work):
Open the executable assembly's properties
Go to the Debug tab
Check the Enable unmanaged code debugging checkbox
The code documents should stay locked, even when a breakpoint is hit, and any attempt to change it should trigger a popup saying "Changes are not allowed when unmanaged debugging is enabled".

Hey there - sorry I can't help you with completely locking your code - I have the opposite desire: to completely UNLOCK it during debug, but I can help you with your second issue.
I suggest that you consider checking the active window before sending any keys and if the active window is other than your target site, pause the execution of your test until focus is returned that that window.
I know it's not the solution you want, but it probably wouldn't hurt to prevent other similar issues.
Best of luck!
Adam

Here is the best I could come up with. It works, but there are some steps you may not want to take.
Essentially, the technique is to set the files of your project to Read-Only when you run the application, and then set them back to writable once your application ends.
However, in VS2k8, by default, setting a file to Read-Only still allows you to edit the file. You need to first turn off the setting "Allow editing of read-only files..." in Tools > Options > Environment > Documents.
Second, you need to add the following key to the registry as a DWORD and set its value to 1:
HKCU\Sofware\Microsoft\Visual Studio\9.0\Source Control\UncontrolledInMemoryEditDialogSuppressed
This still won't work completely. What you then have to do is set your Source Control for that project to Visual Source Safe. (<-- this is the step I'm assuming you won't like.)
Then restart VS2k8.
At this point if you set one of your files to read-only, you will see that Visual Studio will not let you edit this file at all. When you try, it plays your computer's exception music.
Now, to make your files read-only when you run the app, set a post-build process to do this. That's easy.
Harder, is to set them back to writable once your app finishes running. The simplest solution is probably a batch file shortcut.