Logging in ASP.NET Core (Part 1 - Configuration)

• Intro
• Default Config
  • Categories
• Customize the basic config
  • Log Level

Intro

There are multiple ways to log informations in ASP.NET and theres a wide variety of frameworks to make it fit your usecase.

But this article will be about the Default logging of ASP.NET, because it's important to understand the fundamentals before you advance to more complex toolings.

Default Config

When you start with one of the Visual Studio Application Templates logging is enabled by default.

But its only configured to a degree and only to show C# and ASP.NET logs of a certain level.

Lets checkout the default appsettings.Development.json:

{
  "DetailedErrors": true,
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  }
}

There are only two root keys. While DetailedErrors might sound 'interesting', its not what we're looking for, since it only deals with Exceptions.

What we're looking for is the Logging Section. It's a Dictionary of Category and their logging level. The Default key is special, since it provides the default level for all unconfigured categories.

Categories

Everytime a Logger is created, a category is specified. That category is included with each log message created by that instance of the Logger. The category string is arbitrary, but the convention is to use the class name.

For example in our current Config the Category Microsoft.AspNetCore affects the logging of the Asp.NET Framwork itself.

With the current configuration we have a Log level of Warning from the AspNetCore Framework and Information for everything else.

Let's try it, start your Application using F5 and have a Look at the output in the CMD Window that opens.

The output in your CMD window should look similiar to this:

info: Microsoft.Hosting.Lifetime[14]
      Now listening on: https://localhost:7252
info: Microsoft.Hosting.Lifetime[14]
      Now listening on: http://localhost:5252
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: C:\Users\magoerlich\source\repos\BlazorSandbox\BlazorSandbox\

Since Microsoft.Hosting has not a specified log level, it inherits the Default one and so we see these logs.

But we don't see any Microsoft.AspNetCore entries, since the log level is set to warning and there currently should'nt be any warnings in your application.

Customize the basic config

Lets change the Logging level of Microsoft.AspNetCore to Information as well.

To do this, either delete the Microsoft.AspNetCore line from your config, to make it inherit the default as well, or you can set it explicitly like this:

{
    ...
    "Logging": {
        ...
        "Microsoft.AspNetCore": "Information"
        ...
    }
}

Now lets restart the application and watch a flood of loggs flow through our CMD Window:

info: Microsoft.AspNetCore.DataProtection.KeyManagement.XmlKeyManager[63]
      User profile is available. Using 'C:\Users\magoerlich\AppData\Local\ASP.NET\DataProtection-Keys' as key repository and Windows DPAPI to encrypt keys at rest.
info: Microsoft.Hosting.Lifetime[14]
      Now listening on: https://localhost:7252
info: Microsoft.Hosting.Lifetime[14]
      Now listening on: http://localhost:5252
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: C:\Users\magoerlich\source\repos\BlazorSandbox\BlazorSandbox\
info: Microsoft.AspNetCore.Hosting.Diagnostics[1]
      Request starting HTTP/2 POST https://localhost:7252/_blazor/negotiate?negotiateVersion=1 text/plain;charset=UTF-8 0
info: Microsoft.AspNetCore.Authentication.Cookies.CookieAuthenticationHandler[7]
      Identity.Application was not authenticated. Failure message: Unprotect ticket failed
info: Microsoft.AspNetCore.Routing.EndpointMiddleware[0]
      Executing endpoint '/_blazor/negotiate'
info: Microsoft.AspNetCore.Routing.EndpointMiddleware[1]
      Executed endpoint '/_blazor/negotiate'
info: Microsoft.AspNetCore.Hosting.Diagnostics[2]
      Request finished HTTP/2 POST https://localhost:7252/_blazor/negotiate?negotiateVersion=1 text/plain;charset=UTF-8 0 - 200 316 application/json 60.4700ms
info: Microsoft.AspNetCore.Hosting.Diagnostics[1]
      Request starting HTTP/1.1 GET https://localhost:7252/_blazor?id=U9EOBELNcAdSNsKl2ac1UA - -
...
...

Your output might vary slightly, but the important change you should be able to observe is that there are more logs than before and all of them Start with info: Microsoft.AspNetCore ....

These messages were supresed through the config before.

Log Level

There are more log levels than just Information and Warning. Each with its own semantics and verbosity.

Here's a list, try a few of these with the Microsoft.AspNet Category to experiment:

• None
• Critical
• Error
• Warning
• Information
• Debug
• Trace

You can find further information about the semantics of these levels in the .NET Core Documentation

Thats it for today. In the next part, we'll take a look at how to use all of this when writing logs from your own code.