What's the most annoying api you've ever used?
 
Mine pretty much has to be the winsock NSP as it is probably the hardest api in Windows to use
behind the relatively harmless looking
 
INT WSALookupServiceBegin(
  LPWSAQUERYSET pQuerySet,
  DWORD dwFlags,
  LPHANDLE lphLookup
);
 
lay 10 pointers of PAIN! (in the first level alone!)
 
typedef struct _WSAQuerySet {
  DWORD dwSize;
  LPTSTR lpszServiceInstanceName;
  LPGUID lpServiceClassId;
  LPWSAVERSION lpVersion;
  LPTSTR lpszComment;
  DWORD dwNameSpace;
  LPGUID lpNSProviderId;
  LPTSTR lpszContext;
  DWORD dwNumberOfProtocols;
  LPAFPROTOCOLS lpafpProtocols;
  LPTSTR lpszQueryString;
  DWORD dwNumberOfCsAddrs;
  LPCSADDR_INFO lpcsaBuffer;
  DWORD dwOutputFlags;
  LPBLOB lpBlob;
} WSAQUERYSET, 
 
Needless to say its pretty difficult to use (and even harder to get to work with C#) but it does try to cover a lot of different types of name resolution and other network events (Bluetooth,PNRP,NLA,DNS,SAP,WINS,X500,NETBT). But that doesn’t make it anymore fun to use:)
 
What's your nightmare?