Standard ASP.NET logger LogError to stderr

Question

Is it possible to configure the ASP.NET Core default logger to log error level logs to stderr?

This is my configuration:

builder.Logging.AddJsonConsole(options =>
{
    options.IncludeScopes = false;
    options.TimestampFormat = "HH:mm:ss ";
    options.JsonWriterOptions = new JsonWriterOptions
    {
        Indented = true
    };
});

And usage:

_logger.LogError("Test error log");

Message is nicely formatted, LogLevel is set to Error but it is in std out not err

Answer 1

You can use ConsoleLoggerOptions.LogToStandardErrorThreshold, by configuring it either by explicitly calling AddConsole (which is internally called by AddJsonConsole):

builder.Logging.AddConsole(o => o.LogToStandardErrorThreshold = LogLevel.Error);
builder.Logging.AddJsonConsole(...);

Or by explicit configure call:

builder.Logging.AddJsonConsole(...);
builder.Services.Configure<ConsoleLoggerOptions>(
    o => o.LogToStandardErrorThreshold = LogLevel.Warning);

Also it should be manageable from the config:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information"
    },
    "Console": {
      "LogToStandardErrorThreshold": "Error",
      "FormatterName": "json",
      "FormatterOptions": {
        "JsonWriterOptions": {
          "Indented": true
        },
        "IncludeScopes": false,
        "TimestampFormat": "HH:mm:ss "
      }
    }
  }
}

Also you can look into Serilog which also supports redirecting errors to stderr via standardErrorFromLevel setting - see for example this github issue.

Small demo (for console app):

var services = new ServiceCollection();

services.AddLogging(lb => lb.AddJsonConsole());
services.Configure<ConsoleLoggerOptions>(
    o => o.LogToStandardErrorThreshold = LogLevel.Error);

StringWriter stdErrCapture = new();
Console.SetError(stdErrCapture);

var sp = services.BuildServiceProvider();
var requiredService = sp.GetRequiredService<ILogger<Program>>();
requiredService.LogError("Test");
sp.Dispose(); // dispose to wait for async write from the log queue

var capturedStdErr = stdErrCapture.ToString();
Console.WriteLine(capturedStdErr);