Try explicitly to tell the compiler how to import the dll. Ex:

using System;
using System.Runtime.InteropServices;

namespace BitmapProcessingCs
{
    public static class NativeMethods
    {
        [DllImport("BitmapProcessingCpp.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern void GenerateBitmap(IntPtr src, int dimension);
    }
}

I solved the same problem in these steps:

step 1) If you program your custom DLL in C++ using Visual studio,then at the property page of your project set the Common Language Runtime Support (/clr)parameter to Common Language Runtime Support (/clr).

step 2) To function deceleration in .h file use __declspec(dllexport) keyword like below:

__declspec(dllexport) double Sub(int a,int b);

step 3) Build and export DLL file, then use the Dependency Walker software to get your function EntryPoint.

step4) Import DLL file In the C# project and set EntryPoint and CallingConvention variable like below:

[DllImport("custom.dll", EntryPoint = "?Sub@@Y234NN@Z", CallingConvention = CallingConvention.Cdecl)]

    public static extern double Sub(int a,int b);

Have a look at MSDN's library : https://msdn.microsoft.com/en-us/library/ms164891.aspx

According to them, you'll get an exception if you try to evaluate an exception object if no exception has occurred. But, since you're in the catch block, an excption has occurred... Without seeing your code it's hard to guess, but is it maybe possible that you have multiple threads running, and the exception was consumed by one thread before being handled by another? Doesn't really make sense but it's worth looking at. A safe(r) option would be to ensure you check if the exception is null in your logger, before trying to get detail from it. Also worth looking at: The HttpRequestException contains an inner exception, which might be the source of your null reference? Just speculating.