c/front dll calling from within manganed c# code

Because the OCX Call Interface is very slow we are trying to connect our application via the cfront.dll to navsion. The casting-problems with ref object-Parameters in DBL_GetFieldData() and DBL_AssignField() are soved by come methods using the DBL_FieldType()-Function to estimate the right type casting. But we have problems with some other functions: The attemp to call DBL_Field_2_Str() always causes a “System.NullReferenceException”. The function is defiend in c# as: Code: [DllImport(“cfront.dll”, SetLastError = true) ] protected static extern void DBL_Field_2_Str( [MarshalAs(UnmanagedType.LPStr)] out string Str, int StrSize, int hTable, int hRec, int FieldNo); Another function, DBL_FieldName() causes this error not always, but on several fields: Code: [DllImport(“cfront.dll”, SetLastError = true) ] [return: MarshalAs(UnmanagedType.LPStr)] protected static extern string DBL_FieldName( int hTable, int FieldNo); Has anyone an idea, why this errors occur? Thanks for your support from Solingen, Germany… greets, Arkadius