Using DllImport to load an unmanaged dll into a managed application

In my project, I have an unmanaged native C ++ dll and a C # application. I am trying to import a function from an unmanaged dll using DllImport, but I keep getting DllNotFoundException.

Here is my code that calls the DLL.

using System.Runtime.InteropServices;
namespace TestApp
{
  public delegate void UpdateDelegate(string s);
  class Program
  {
    [DllImport("CGPUnmanagedLibrary.dll")]
    internal static extern int parse_raw_gsod_file( 
      [MarshalAs(UnmanagedType.LPStr)]                                               
      string filePath,
      int minTemp, 
      UpdateDelegate callBack);

    static void Main(string[] args)
    {
      UpdateDelegate myCallBack = new UpdateDelegate(Program.Report);
      string path = @"C:\Creative Solutions\Projects\Clyde Garden Planner\Frost Data Database\GSOD Data\GSOD_RAW_DATA\1992\gsod_1992.txt";
      int result = parse_raw_gsod_file(path, 32, myCallBack);
      Console.Write("Parse completed with exit code: " + result.ToString());
      Console.ReadLine();
    } // end main function

    public static void Report(string msg)
    {
      Console.Write("Message is ");
      Console.WriteLine(msg);
    }

  } // End class
} // end namespace

      

        
        
        
      

    

I tried to copy the DLL to the application's output directory, but it still can't find it. I also tried to add the DLL project as a reference, but I get a popup that says it cannot be added. How to properly link an unused DLL to a managed application?

Refresh . Here is the complete error:

Unable to load DLL 'CGPUnmanagedLibrary': The specified module could not be found. (Exception from HRESULT: 0x8007007E)

      

        
        
        
      

    

Update 2 . I know for sure that the DLL is in the same directory as the .exe trying to load it. This makes me think there is a dependency in the DLL that won't load. I only use basic C ++ libraries in DLLs (string, math, iostream, etc.). Any ideas which can't be downloaded and why?

Update 3 - Tested with Walker Dependency Walker Loading my unmanaged C ++ DLL in dependency run showed no errors. I also tried to open my executable in dependent walker and it showed two DLL load errors: GPSVC.DLL and IESHIMS.DLL - doesn't make any sense because I only use standard C ++ libraries in my code. I think it might be because I have a managed C ++ / CLI DLL trying to load an unmanaged DLL (I was trying to implement some C ++ / CLI wrappers). Anyway, since then I started a new VS solution and moved on. See my answer.