To complement the previous answer, there's also the .NET Core SDK, which is has a faster and cleaner implementation of the .NET framework(albeit it's somewhat imcomplete, being a Core CLR. Still really reliable, though). If you don't need C# for WinForms, UWP and WPF development, I really recommend checking it out. By now, most libraries should be supporting .NET Core. Download link: https://dotnet.microsoft.com/download
As for the IDE, there's VS Code, which is the one I'm currently using, and JetBrains Rider. MonoDevelop does have an addon for NET Core support, but I heard it's lacking features(especially on the debug department).