COM(C++) programming tutorials?
Since you're asking for websites, you can try this introduction to COM on The Code Project, and how to handle COM in plain C and in C++ on the same site. And of course, you have MSDN.
Great C tutorial?
The C Programming Language (often referred to as "K & R") is almost universally considered to be the best resource for learning C.
C++ tutorial for experienced C programmer
This might be of some use: C++ tutorial for C users.
If you're looking for a book, check out "C++ for C Programmers" by Ira Pohl (Amazon).
C programming in Visual Studio
Short answer: Yes, you need to rename .cpp files to c, so you can write C:
https://msdn.microsoft.com/en-us/library/bb384838.aspx?f=255&MSPPError=-2147217396
From the link above:
By default, the Visual C++ compiler treats all files that end in .c as C source code, and all files that end in .cpp as C++ source code. To force the compiler to treat all files as C regardless of file name extension, use the /Tc compiler option.
That being said, I do not recommend learning C language in Visual Studio, why VS? It does have lots of features you are not going to use while learning C
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