To access any DLL file from C# windows application or from other application an entry point is required. Otherwise you will get the below exception:
Unable to find an entry point named 'xxx' in DLL 'yyy.dll'
Entry point not found exception / error.
When you create any DLL file in VC++ from a turbo C program then you must have create a definition file where you will define the DLL entry point. You have to create the Definition file within the root folder. This will not resolve your problem. To resolve the problem go to the Project--> Project Properties and define the definition file in the following way:
Hope now you can resolve the problem.