c# project migrated from .net framework 4.0 to 4.5 return error System.AccessViolationException

Question

I have a Windows Forms project built in C#, using .NET framework 4.0. The project was built in VS2010.

This project uses functions written in C, which are used by the program through dlls. Everything was working fine in 4.0, I have a 32 bit and also a 64 bit version for this.

But, I recently had to migrate it to VS2015 with.Net Framework 4.7.2.

The 32 bit application ran without any issues. No new prompts from Visual Studio while opening it in 2015 either.

But, the 64 bit application has some issues. The compilation and build was good. But when I try and use one of the C functions, it fails with the following error:

An unhandled exception of type 'System.AccessViolationException' occurred in ODT Admin.exe

Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

I tried to change the target framework to an older version and this problem starts right after.Net framework 4, so.Net Framework 5 also has this.

Any help would be really appreciated.

Thanks.