Do you know if there's a .NET client for The Content Delivery API? I know there's one for Heartcore.

From what I can see,

Umbraco.Headless.Client.Net

is actually called the "Umbraco headless client". It doesn't look like it's actually Heartcore-specific. Maybe except for some minor stuff like

ProjectAlias

There's no .NET client at the moment for the Content Delivery API.