There are a lot of different “fiddles”. There’s JSFiddle, SQLFiddle, HTMLFiddle, RegExFiddle, PythonFiddle, R-Fiddle, GoFiddle. There is even a page that curates a list of fiddles.
For .NET there’s one too. It supports C#, F# and VB.NET. For trying something quickly or sharing it online this is a nice way to do it: