You're using DateTime.Now, aren't you? Be careful, because it may bring some troubles to your application. Here I'll explain why and I'll talk about time zones and formatting in C# DateTime.
I don't have to tell you why you need to write clean code. Here you'll see some tips about how to name things and how to structure functions
Sometimes on your tests you need to access test files, for example for mocking external data. With manifest resources you can easily reference files for your tests.
BenchmarkDotNet allows you to test the performance on .NET methods. So let's answer a question: is the Enum.HasFlag method really that slow?
Validating inputs is crucial for every application. If you want an easy and versatile way, you can try FluentValidation.