Why doesn't .NET find the OpenSSL.NET dll?

Try the latest version of OpenSSL.NET (0.4.1) which should now include prebuilt libeay32.dll and ssleay32.dll binaries that link to the CRT statically. Alternatively, you can build these libraries yourself or use an 'official' build from openssl.org.

Without looking at your code exactly, I get that error when I:

• do not have the dlls in the path of the executable (not where your sln resides, but where the .exe is made, typically in bin/debug or bin/x86/debug or whatever).
• do not have the proper signature of the calling function (ie, I left out an integer parameter, the return types don't match, etc).
• am not marshalling the types properly (ie, BOOL is marshalled as a bool, while bool is marshalled as a unsigned single byte integer, etc)-- while this last one may not cause the exception, it can cause decidedly funky behavior.
• am on a 64 bit platform and am calling a 32 bit dll. The pointer sizes will be all different, and the dll will probably just crash and cause that exception.

EDIT: When all else fails, try dependency walker, because it sounds like your dlls are calling other dlls that aren't in your path or in the directory of the executable.

For anyone else out there still experiencing this issue (and have verified that the necessary prerequisites exist in their correct locations:

Check the OpenSSL.NET installation documentation and ensure its prerequisites are installed. In my case, a user was missing the Microsoft Visual C++ 2010 Redistributable Package (x86) dependency which is called out in the OpenSSL.NET documentation.