I can now successfully call InitializeSecurityContext and get a status of ContinueNeeded.

Contexts/SecureBufferDataRep.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace NSspi.Contexts
+{
+    public enum SecureBufferDataRep : int
+    {
+        /*
+        #define SECURITY_NATIVE_DREP        0x00000010
+        #define SECURITY_NETWORK_DREP       0x00000000
+        */
+        Nativee = 0x10,
+        Network = 0x00
+    }
+}