How to add AES GCM with bcrypt for .NET framework?

[pavinan]

I installed this CsWin32 package in netstandard2.0 class library. But it is giving error to use c#9.0 to support nint keyword. Is it it not possible to use aesgcm with this in .net framework?

I have used BCRYPT* in that NativeMethods.txt

[AArnott]

Down-level C# language version support is spotty. You should be able to fix this by adding this to your project file:

<LangVersion>9</LangVersion>

Of course, I like to go all the way and use 12.

C# 12 has the appropriate guards to keep you on the right path when targeting older runtimes.

[pavinan]

But .netstandard supports only c# 7.3 right? But I will check if it works with 9 or 12.

[AArnott]

.NET Standard doesn't support any language at all. It's a set of ref assemblies. Runtimes also don't support any particular language. The language is only a means to generate ecma-335 assemblies, which C# 7.3 and C# 12 both do.
So yes, you absolutely can use C# 12 when targeting .NET Standard 2.0.