The entry of the function in C++dll cannot be found in C#dll

Just tried, always the DLL in C # cannot find the entrance to the function in C++dll;

For the moment, there are two reasons.

1, no extern "C" _declspec (dllexport), resulting in C # cannot find the portal.

2, the function in C + + is under a namespace, so C # did not find him

In addition, it is said that when the C # function and the parameter memory of C + + functions are not aligned, this problem can also occur, but has not yet been tried.

The entry of the function in C++dll cannot be found in C#dll