ASP.NET is a framework, written on top of .NET framework. .NET framework applications or services (libraries such as ASP.NET) can be written in C# or VB.NET, which are then converted to IL code as to remove the ambiguity between C#, VB.NET or Visual C++ code.
ASP.NET code that you write in C# is similar to VB.NET in every way. The performance cap is also similar, because both languages are compiled down to IL code before they are actually executed. Thus the question, "ASP.NET is written in" doesn't make any sense as a few modules may be written in either language.
.NET has gone open source, ASP.NET code would also be available on GitHub if they would publish it (
or perhaps they already have had). If you have a look at the
.NET Core[
^] repo at GitHub, you will find the following languages being used,
1. C# 98%
2. Visual Basic 0.8%
3. C++ 0.3%
4. C 0.1%
5. Misc languages.
So from this, it is clear that Microsoft prefers to use C# as their core language for all of their frameworks. C# compiles down to IL to remove the difference between VB.NET, C# or Visual C++.
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