Why default generic host adds EventLogLogger (Windows-only) when targeting net8.0 (not net8.0-windows)?

[bairog]

Both Host.CreateDefaultBuilder() and modern Host.CreateApplicationBuilder() add EventLog logger provider to host DI. This logger throwns System.PlatformNotSupportedException on non-Windows OS.
Same happens (even on Windows) when host is used in a netstandard2.0 library that is dynamically loaded by a net8.0 console application (sample repo). So I have to add the following code to a IHostBuilder setup:

builder
    .ConfigureLogging(builder =>
    {
        builder.ClearProviders();
        builder.AddConsole();
        builder.AddDebug();
        builder.AddEventSourceLogger();                    
    })

What is the use for default generic host to add logger provider that throws exceptions? Why not to add it only for net8.0-windows?

[huoyaoyuan]

OS-specific target framework is not commonly used. Most service applications are built with OS-neutral framework. That's why SupportedOSPlatformAttribute exists.

Same happens (even on Windows) when host is used in a netstandard2.0 library that is dynamically loaded by a net8.0 console application (sample repo).

Is the correct version of Microsoft.Extensions.Hosting.dll copied to the folder at all?

[huoyaoyuan]

This is incorrect approach. The version of dependency library should be picked by entry application, as NuGet does. You need to use the net8.0 version of Microsoft.Extensions.Hosting.

[bairog]

Could you elaborate a bit more: my netstandard2.0 plugin library uses Generic Host (and depends on Microsoft.Extensions.Hosting.dll). net8.0 console application (plugin loader) doesn't use Generic Host (and therefore doesn't depend on Microsoft.Extensions.Hosting.dll). What do you advice to do in this situation?

[huoyaoyuan]

The best approach is to target the same framework (net8.0) and do dotnet publish with RID to resolve the correct dependencies. Otherwise, you will need to replicate the NuGet framework resolving logic somewhere for dependencies of plugins.

[bairog]

My plugin library is intended to be used from both .NET and .Net Framework 4.7.2. Thats why it targets netstandard2.0.

[huoyaoyuan]

Then you need to resolve publish its dependencies separately for each framework. For many packages there isn't a single binary that works on both runtimes (#113006 (comment)).

[davidfowl]

It only adds the event logger on windows

runtime/src/libraries/Microsoft.Extensions.Hosting/src/HostingHostBuilderExtensions.cs

Lines 313 to 317 in d3dffb9

	if (isWindows)
	{
	// Add the EventLogLoggerProvider on windows machines
	logging.AddEventLog();
	}

[huoyaoyuan]

It's unrelated to the Microsoft.Extensions.Hosting. You are loading the incorrect version of Microsoft.Extensions.Logging.EventLog->System.Diagnostics.EventLog, which contains throwing-only logic. It's similar to #110604.

[bairog]

You are loading the incorrect version of Microsoft.Extensions.Logging.EventLog->System.Diagnostics.EventLog, which contains throwing-only logic.

Could you please add more clarification. Unlike #110604 I don't load anything into AssemblyLoadContext. I simply call Host.CreateDefaultBuilder().Build() in my netstandard2.0 plugin library (that cannot target any other TFM). And if I dive into Microsoft.Extensions.Logging.EventLog sources I see the following (looks like line 26 throws that PlatformNotSupportedException):

[huoyaoyuan]

Unlike #110604 I don't load anything into AssemblyLoadContext.

It's unrelated to ALC, it's just the side effect of loading incorrect binary. Take #113006 as another example. It's also unrelated to what you are doing in the plugin library.

And if I dive into Microsoft.Extensions.Logging.EventLog sources I see the following (looks like line 26 throws that PlatformNotSupportedException):

You need a step closer. The throw happens in System.Diagnostics.EventLog, which is another level of dependency.

[huoyaoyuan]

In other words, it's totally unrelated to whatever you are doing in high level callers, including your plugin, or the Microsoft.Extensions.* libraries.

The fundamentally wrong thing is that you are loading lib\netstandard2.0 version of System.Diagnostics.EventLog into a Windows process. It's a throw-only version. You should use runtimes\win\lib\net8.0 version for a net8.0 process, and lib\net462 version for a .NET Framework process. The two versions are not compatible.