Use a standard C++ dll in c# application

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C++

C#

DLL

Hi I have a dll written in C++ that I want to use in C# (a common problem).
I am trying to make a test to understand how to use the dll.
So I created a new standard dll in C++ with these simple lines:

extern "C" __declspec (dllexport) short __stdcall TestFunction(short i){
    return i+123;
}

Then I am trying to use this test function in an C# application using the code:

[DllImport("Dll.dll")]
static extern short TestFunction(short i);

and then

short x=3;
x=TestFunction(x);

Compiled!
But I got this error runtime:

An unhandled exception of type 'System.BadImageFormatException' occurred

I think the problem is that the dll is not managed, but I have to use it as standard C++ and unmanaged. Right?
Anyone knows hot to reach my target?
Thank you for any help