.NET target framework compatibility and the compiler on .NET 4.5 or higher

I'm fairly experienced in .NET development but today I was forced to wrap my head around something I'd never thought about before:

How do the installed .NET Framework, the .NET Framework target in Visual Studio and the C# compiler work together?

Concrete example: The System.dll contains the enum System.Net.SecurityProtocolType. On .NET 4.5 this enum contains the members SSl3 , Tls, Tls11, and Tls12. With .NET 4.7, the member SystemDefault was added.

So, targeting .NET 4.7.x, this code compiles fine:

var p = SecurityProtocolType.SystemDefault;

However, when I target .NET 4.5.x, this code does not compile (as one would expect).

What puzzles me here is why this works considering that .NET 4.7 is an in-place update to .NET 4.5 (i.e. when installing .NET 4.7, the System.dll of .NET 4.5 is replaced with the one of .NET 4.7).

How does the compiler know that I can't use SystemDefault on .NET 4.5 but can use it on 4.7? Is this done via some kind of API file known to the compiler?

Side fact: When I target .NET 4.5 and have .NET 4.7 installed, a call to Enum.GetValues(typeof(SecurityProtocolType) will give me SecurityProtocolType.SystemDefault. So I'm fairly certain that my .NET 4.5 application uses the .NET 4.7 System.dll.

Jon Skeet
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How does the compiler know that I can't use SystemDefault on .NET 4.5 but can use it on 4.7? Is this done via some kind of API file known to the compiler?

Yes, I'd expect it to be done via the reference assemblies. A reference assembly is an assembly which includes just the accessible API definitions, effectively.

On my Windows machine, these are in C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework

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