Does MailKit continue to work with UWP (.NET Native)?

[al-kau]

After updating the MailKit package version from 4.17.0 to 4.18.0, my UWP (old-style) project fails to build in the Release configuration.

error : Error: NUTC300D:Internal Compiler Error: Type 'System.ReadOnlySpan`1<System.Byte>' is not valid for use outside of the managed environment, but is passed to native code while computing call to 'static Interop+BCrypt+NTSTATUS __Interop.BCrypt_dll_PInvokes.BCryptSetProperty__1(System.IntPtr, System.UInt16*, System.ReadOnlySpan`1<System.Byte>, System.Int32, System.Int32)'. while compiling method 'static Interop+BCrypt+NTSTATUS __Interop.BCrypt_dll.BCryptSetProperty__1(Microsoft.Win32.SafeHandles.SafeBCryptHandle, System.String, System.ReadOnlySpan`1<System.Byte>, System.Int32, System.Int32)'.
error : ILT0005: returned exit code 1

[CaptainAni187]

The error doesn't come from MailKit's own code but from a dependency that changed in 4.18.0. Comparing the netstandard2.0 dependencies on NuGet:

• MimeKit 4.17.0 uses System.Security.Cryptography.Pkcs 8.0.1.
• MimeKit 4.18.0 uses System.Security.Cryptography.Pkcs 10.0.0 (and MailKit moved System.Formats.Asn1 from 8.0.1 to 10.0.0).

System.Security.Cryptography.Pkcs 10.0.0 adds a new dependency for netstandard2.0, Microsoft.Bcl.Cryptography 10.0.0, and that package includes the BCryptSetProperty P/Invoke from your error, which passes a ReadOnlySpan<byte> to native code. The .NET Native compiler can't marshal spans across P/Invokes, so it fails. It only shows up in Release because that's the configuration where UWP actually runs the .NET Native toolchain.

For an old-style UWP project, the practical option is to stay on MailKit and MimeKit 4.17.0. You can't pin System.Security.Cryptography.Pkcs back to 8.0.1 while using 4.18.0, because MimeKit 4.18.0 requires 10.0.0 or newer and NuGet will report that as a downgrade (NU1605).

Longer term, .NET Native only gets maintenance fixes and newer libraries will keep running into this. UWP apps can now target modern .NET (since .NET 9) with Native AOT instead, which doesn't have this restriction, so that's the way to keep getting MailKit updates.

[jstedfast]

Thanks for answering this question!