The value ServicePointManager’s of SecurityProtocol property is

ServicePointManager.SecurityProtocol == Ssl3 | Tls /* == Default == 0xF0 */

I don’t want to use explicitly typed protocols for the SecurityProtocol property. Instead, I want the default to be initialized to SystemDefault (0x00).

Additionally, if I explicitly change the value:

ServicePointManager.SecurityProtocol = SecurityProtocolType.SystemDefault;

an exception occurs when I try to communicate:

WebException Status: ReceiveFailure (The underlying connection was
closed: An unexpected error occured on a receive)

-> ArgumentException (The specified value is no valid in the ‘SslProtocolType’ enumeration. Parameter name> sslProtocolType)

The solution configuration is as follows:

• (A) Executable EXE project: C++ native application without CLR
  support (Common Language Runtime Support: No Common Language Runtime
  Support)
• (B) DLL project: C++/CLI library with support for managed
  code (Common Language Runtime Support: .NET Framework Runtime Support
  /clr), which targets .NET Target Framework Version: v4.8.

The initialization of the ServicePointManager.SecurityProtocol property value probably has something to do with the AppDomain.CurrentDomain.SetupInformation.TargetFrameworkName value.
The value of TargetFrameworkName is null even though the assembly (B) has a TargetFrameworkAttribute specified with a value of v4.8. The application domain is internally initialized according to the result of Assembly.GetEntryAssembly, which in this case returns null for image (A) (image A is not a managed assembly).