Simple test:

static (int, int) Foo(int i)

{

  return (i+2, 1*2);

}

Current "result" :
Predefined type 'System.ValueTuple`2' is not defined or imported

Expected:
To compile (and work of course :-)

This includes updating "4.7.2" "Compiler" to accept C# 7.x and at least linking the proper dll with ValueTuple-a for "Roslyn 2.0"

This is especially valuable with MVC fiddle type as one cannot add additional views or define their own routing.

Allow the usage of debugging such as stepping into a function, out of a function, run, memory mapping, local variables and the whole nine yards. It is vital if you don't want users to write constantly Console.WriteLine() statements.

Kindly, add debugging!

allow us to

1) View the compiled razor
2) allow code folding of codetags and collapse/expand code for readability

Support for System.Diagnostics.Debug.Assert could be added. Given that it's an internal verification system that fails when the assertion is incorrect, it could be helpful to at least throw a message in the output window when an assertion wasn't working

ScriptCS has a syntax for loading assemblies and works better as a single file. I don't know if you plan on allowing .NET Fiddles (?...is that the plural of the thing you make in .NET Fiddle?) to load other assemblies, but I think it would be really cool to be able to do that.

Currently, dotNetFiddle throws when trying to use Akka.NET.
There is an error message saying that some methods in JSON.NET are missing.
We would love to have the ability to run live Akka samples from our docs site.

We need the same quick keyboard shortcuts as visual studio...
CTRL+D - Duplicate line
CTRL+K+C - Comment out line
CTRL+K+U - Uncomment line
ALT+ENTER - Automatically add or suggest missing namespace reference and add usings to the class (like resharper)

Collaboration doesn't work at all.
I clicked 'collaborate' and sent the link to other machine. I opened it in broswer and clicked 'yes, join' But I don't see collaborator in the host and in the remote machine. I don't see the changes was made.Nothing

Please enable vim key bindings. VsVim is one of the most downloaded Visual Studio extension, which is an indication of Vim key binding's popularity among .net devs.

When you add a nuget reference save it in the fiddle so when you open the fiddle again, either on the same machine or a different one, the packages are still there and you don't have to search for them again

In CodePen, you can show the HTML, CSS, and JS on one half of the iframe and the result on the other half, as well as open and close them by clicking on tabs located at the top of the iframe. It would be awesome if we could have the code be on one tab, and the console output on the other tab

Allow you to work on whole projects instead of a single at a time. Having the tree listed on the side allowing you to jump from page to page and looking more like a whole project. Also being able to download your project when it's completed

When trying to install System.Xml.XPath from NuGet (any version listed in the menu), installation is not allowed and this message is included at the end of the description of the package: "When using NuGet 3.x this package requires at least version 3.4". I think that updating NuGet is an important and urgent step for the functionality of this site. Gracias!

For example: I define a class Person in a fiddle named PersonClass.
Then I base a new fiddle PersonUseExample on existing fiddle PersonClass, so I don't need to declare the class again, just démonstrate how I can use it.

The idea is probably easier to implementis but similar to the exiting request: https://dotnetfiddle.uservoice.com/forums/228764--net-fiddle-ideas/suggestions/6056568-allow-multiple-files-for-console-projects

This would really help to make samples of bugs / issues in a repository. One could import all the files of a repository, then write a "Test" that fails and the owner of the repository could then check out why it fails easily.

See for example me manually uploading the files in a competitors product:
https://repl.it/@devedse/ErrorWhenLoadingJpgWithExif

I'm creating some samples for a project, and when using Console.WriteLine inside Tasks, the messages usually won't appear in the log window.

I'm currently required to add a "Thread.Sleep(1000)" or "Task.Delay(1000)" to allow the console buffer to appear.

It would be much better for everyone if this was handled by .NET Fiddle by default.

Now that Blazor server-side has been released and client-side in preview (due to be released with .Net 5 in November), you should think about supporting the project type on .Net Fiddle