.NET Core is open-source and free, with an offline SDK installer. [1] There are plenty of large open C# code bases, like the ASP.NET web framework [2], that you can use to compile and get a good performance score.

And like the other comment said, you can probably get complimentary copies of the IDEs from Jetbrains to run benchmarks with. They're also scriptable which helps.

1 .https://dotnet.microsoft.com/download/visual-studio-sdks

2. https://github.com/aspnet/AspNetCore

3. https://www.jetbrains.com/