Hopefully you all already know about the most excellent LinqPad app. I've used it for the longest time, mainly to easily open up SQLCE database. Yes, I know, SQLCE is very much end of life, but I've done a LOT of testing with it over the years since it's been available in Umbraco, as an easy way to do quick prototyping while testing reports on the issue tracker.
LinqPad
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Screenshot of the configuration screen for attaching a SQLCE database, the important fields being: "SQLCE 4.0" selected and a path filled in at "Attach database file"
But there's a lot more that LinqPad can do! It's a mini IDE as well, super interesting for rapid prototyping. I know we can do a some of this stuff online at Try Dot Net as well but I often want to try things that need a NuGet package to be installed and then there's the magic Dump extension method that is advertized as: "LINQPad’s Dump method is famous for its capacity to eat almost anything!"
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Screenshot of a piece of code that splits a string on commas and with the Dump() method lists each item from the split string in the results area
LinqPad is free, but personally I've found it worth paying $59 for the Pro edition, mostly to get autocomplete and tooltips in my code, making my rapid prototyping even more rapid!