A Function Declared Dllimport May Not Be Defined -

// myheader.h __declspec(dllimport) void myFunction(); // Client sees this But then the DLL's own .cpp file also includes this header and tries to define myFunction , causing the same error. 1. Use a macro to switch between dllimport and dllexport Common pattern (in a shared header):

The error message occurs in Microsoft C++ when you declare a function with __declspec(dllimport) but then attempt to provide a definition (implementation) for that function in the same module (usually a .cpp file being compiled into an executable or DLL). Why does this happen? __declspec(dllimport) tells the compiler: "This function exists in a different DLL — do not generate code for it here; instead, generate a call via the import library." a function declared dllimport may not be defined

If you then define the same function (write its body) in the same source file or a linked object file, you are contradicting that directive. The compiler rightly complains because it doesn't know whether to treat the function as imported from a DLL or as a local function. Problematic code (in an executable project): // myheader