[BUG] MissingMethodException: set_ServerCertificateCustomValidationCallback

[stevejgordon]

When using the agent with .NET Framework 4.6.2, the user may experience the following exception.

System.MissingMethodException: Method not found: 'Void System.Net.Http.HttpClientHandler.set_ServerCertificateCustomValidationCallback(System.Func`5<System.Net.Http.HttpRequestMessage,System.Security.Cryptography.X509Certificates.X509Certificate2,System.Security.Cryptography.X509Certificates.X509Chain,System.Net.Security.SslPolicyErrors,Boolean>)'.

This API is technically missing (see also) from .NET Framework 4.6.2, despite being in netstandard2.0.

This doesn't appear at compile time due to the build picking up a specific system.net.http package (see #2112). It can sometimes be worked around with binding redirects but this is not always possible in no-code instrumentation scenarios.

Options:

• We could review switching to System.Net.Http.WinHttpHandler (see docs) on `net462 which may avoid the issue while still supporting the server callback API usage.
• We could drop support for certificate calls back on net462 (or fallback to ServicePointManager) to avoid this exception at runtime.
• We could review manually depending on a specific NuGet package version (see Update System.Threading.Tasks.Dataflow to resolve transistive resolution issues on net462 #2112).

The easiest option, for now, would be removing the certificate callback code on net462 as this shouldn't impact many/any consumers and should avoid the exception.

References:

• Issue with .NET Standard 2.0 and .NET Framework 4.6.1 for System.Net.Http assembly dotnet/runtime#29314
• Reference to System.Net.Http breaks after changing TargetFramework to net472 microsoft/dotnet#737
• https://stackoverflow.com/questions/49598467/how-to-choose-target-framework-from-nuget-package