In making a C# custom DSC resource, you follow the same conventions as an MOF-based custom DSC resource, with the exception of the PowerShell .psm1 code file being replaced by the C# DLL.

You start with creating the MOF schema file:

[ClassVersion("1.0.0"), FriendlyName("xExampleThing")]
 class xExampleThing : OMI_BaseResource
 {
 [Key, Description("path")] String Path;
 [Write, Description("Should the file be present"), ValueMap{"Present","Absent"}, Values{"Present","Absent"}] String Ensure;
 };