The Direct3D11Helpers class
is a custom class. I suggest you could refer to the Github: https://github.com/MicrosoftDocs/SimpleRecorder/blob/e6ec15684e4c51abca518b4cadd2d89d2b359509/CaptureEncoder/Direct3D11Helpers.cs
SharpDX is an open-source managed .NET wrapper of the DirectX API. SharpDX stop being maintained in 2019 so it's unclear if it's still safe to be used as a dependency.