Could be any language, you can't tell the details from the final result. For example, any code written in C# can be translated to VB automatically with a pretty good success rate by automated translators - so you can't tell if an app was compiled from teh C# or VB version of the code. Similarly, C++, Fortran, Cobol, F# and a large number of other languages can also code for .NET and produce the same results:
Category:.NET programming languages - Wikipedia[
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But it doesn't stop there! Oh no, there are a huge number of languages which can be used to create native Windows applications and you can't tell by looking which it was.
The only way to find out is ask the author - he may tell you!
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