My personal thoughts
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1) What Visual Studio version to choose and how to transfer VB6?
Latest.
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2) Whether is it possible to transfer VB6? what better language for interface to choose: VB.net or C#?
Rewrite the same (or better)
GUI
using
C#
.
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3) Can you advice please how can I do that for make less changes and to keep high performance ?
Probably you haven't to worry about
GUI
code performance. You might keep existing
C++
functional classes and maintain performace. You could try to improve a bit your classes using modern
C++
features.
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4) Is it possible to write interface in C# and leave functionality on C++
Yes.
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5) What language C# or VB.net will be better for performance
It doesn't matter.
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6) How than to integrate GUI project & C++?
It is called
Interop[
^].
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7) From what to start?
Start writing a minimal
C# GUI
able to use your
C++ COM
objects.
In another approach you could rewrite everything using
C#
or, if you are masochist,
VB.NET
.