[LogProperties] Generated Code Incorrectly Calls ToString() on Nullable-Typed Properties (e.g. DateTime?)

[rwoll]

Description

When using the [LogProperties] on an object with nullable-typed properties (e.g. DateTime?), the nullable-typed properties are being forced to string before being added to the state.TagArray while their non-nullable type (DateTime) remains DateTime-typed in the TagArray.

Reproduction Steps

Given the following user-written code:

using Microsoft.Extensions.Logging;

public partial class MyLogger
{
  [LoggerMessage(
    EventId = 1,
    Level = LogLevel.Information,
    Message = "Event triggered!"
  )]
  public static partial void MyLogMessage(ILogger logger, [LogProperties] MyEvent myEvent);

  public class MyEvent
  {
    public DateTime TimeStamp { get; set; }
    public DateTime? TimeStampNullable { get; set; }
  }
}

and csproj:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
    <CompilerGeneratedFilesOutputPath>Generated</CompilerGeneratedFilesOutputPath>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="8.0.1" />
    <PackageReference Include="Microsoft.Extensions.Telemetry.Abstractions" Version="8.9.0" />
  </ItemGroup>
</Project>

Expected behavior

Both myEvent.TimeStamp and myEvent.TimeStampNullable get added to the state.TagArray as DateTimes in the auto-generated logging code.

Actual behavior

In the autogenerated code, myEvent.TimeStamp field gets added to the TagArray as a DateTime (same type as it was in the event), but myEvent.TimeStampNullable is added to the TagArray as a string:

        state.TagArray[2] = new("myEvent.TimeStamp", myEvent?.TimeStamp);
        state.TagArray[1] = new("myEvent.TimeStampNullable", myEvent?.TimeStampNullable?.ToString()); // <--- LINE WITH BUG

Complete generated code:

// <auto-generated/>
#nullable enable
#pragma warning disable CS1591 // Compensate for https://github.com/dotnet/roslyn/issues/54103
partial class MyLogger
{
    /// <summary>
    /// Logs "Event triggered!" at "Information" level.
    /// </summary>
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Gen.Logging", "8.9.0.0")]
    public static partial void MyLogMessage(global::Microsoft.Extensions.Logging.ILogger logger, global::MyLogger.MyEvent myEvent)
    {
        if (!logger.IsEnabled(global::Microsoft.Extensions.Logging.LogLevel.Information))
        {
            return;
        }

        var state = global::Microsoft.Extensions.Logging.LoggerMessageHelper.ThreadLocalState;

        _ = state.ReserveTagSpace(3);
        state.TagArray[2] = new("myEvent.TimeStamp", myEvent?.TimeStamp);
        state.TagArray[1] = new("myEvent.TimeStampNullable", myEvent?.TimeStampNullable?.ToString());
        state.TagArray[0] = new("{OriginalFormat}", "Event triggered!");

        logger.Log(
            global::Microsoft.Extensions.Logging.LogLevel.Information,
            new(1, nameof(MyLogMessage)),
            state,
            null,
            [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Gen.Logging", "8.9.0.0")] static string (s, _) =>
            {
                return "Event triggered!";
            });

        state.Clear();
    }
}

Regression?

Unknown

Known Workarounds

No response

Configuration

It does not look specific to any architecture/.NET version.

.NET SDK:
 Version:           8.0.100
 Commit:            57efcf1350
 Workload version:  8.0.100-manifests.6c33ef20

Runtime Environment:
 OS Name:     Mac OS X
 OS Platform: Darwin
 RID:         osx-arm64
 Base Path:   /usr/local/share/dotnet/sdk/8.0.100/

.NET workloads installed:
 Workload version: 8.0.100-manifests.6c33ef20
There are no installed workloads to display.

Host:
  Version:      8.0.0
  Architecture: arm64
  Commit:       5535e31a71

.NET SDKs installed:
  6.0.405 [/usr/local/share/dotnet/sdk]
  7.0.101 [/usr/local/share/dotnet/sdk]
  7.0.102 [/usr/local/share/dotnet/sdk]
  8.0.100 [/usr/local/share/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.13 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 7.0.1 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 7.0.2 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.13 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 7.0.1 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 7.0.2 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.0 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

Other architectures found:
  x64   [/usr/local/share/dotnet/x64]
    registered at [/etc/dotnet/install_location_x64]

Environment variables:
  Not set

global.json file:
  Not found

Learn more:
  https://aka.ms/dotnet/info

Download .NET:
  https://aka.ms/dotnet/download

Other information

No response

[RussKie]

@dariusclay please triage

[rwoll]

@RussKie and @dariusclay - I've opened a PR with a test case: #5416