How to fix C++ file linking error in VS 2017（LNK2005)?

Question

C++ / C#

Hey!
There was a problem in VS 2017 with a file linking error (LNK2005), I will attach screenshots for clarity.
There are 3 files in the project:

• MyForm.h is the file of the C++\CLI form itself, here I do #include "source.cpp", which most likely fails, I also use methods for interacting with the form like openFileDialog, etc.
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• Source.cpp is the main script for 1.3k lines, I need to call the function void solve() {...} in MyForm.h, which works only within the standard libraries and Source.cpp itself, without calling anything "from within", for simplicity, I commented everything out and left it in the whole file:

  void solve() {
  //
  }

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• MyForm.cpp - needed only to define the entry point to the application, nothing happens there.
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Perhaps I messed up with the namespace or the connection, but if you create the H1.h header, where to write:

#include "source.cpp"
void solve();

- also does not work (
In Google I found information about redefining the same function in several files, but I have only 1 file with a definition and I include it only 1 time;
If the thought is that the error is due to the fact that I somehow connect like this: Source.cpp -> MyForm.h -> MyForm.cpp... Tell me
how to do it right!

Answer 1

1. *.h files should contain only function headers and declarations of global variables.
2. *.cpp files are not written in #includes (only *.h is written in them), *.cpp are compiled separately and then the resulting object files are linked together into one executable.
3. To avoid including the same file multiple times, use #ifndef:

// файл MyForm.h
#ifndef __MYFORM__
#define __MYFORM__
// здесь содержимое файла
#endif