Here are some notes on how to override the appsettings.json using launchSettings.json in the Properties folder to control ASP.NET Core applications on your local workstation.

There are several good articles to provide an overview of the launchSettings.json:

To quote the .NET Core documents:

The environment for local machine development can be set in the Properties\launchSettings.json file of the project. Environment values set in launchSettings.json override values set in the system environment.

The ASP.NET Core framework contains a built-in mechanism for configuring the application. I suspect it was born out of the need to make it easier to configure web application hosted in Azure. Azure uses environment variables to inject settings into the websites it hosts. This makes sense since environment variables are pretty universal, and work well with Windows and Linux hosted applications. The built-in mechanism relies on the notion that all application settings can be reduced down to a set of key value pairs.

Microsoft incorporates a few configuration providers as part of the ASP.NET Core web application framework. By default, the WebHost.CreateDefaultBuilder routine (called in the Program class), includes settings from the following providers:

  • Microsoft.Extensions.Configuration.Json
  • Microsoft.Extensions.Configuration.UserSecrets
  • Microsoft.Extensions.Configuration.EnvironmentVariables
  • Microsoft.Extensions.Configuration.CommandLine

Note that order matters. After loading the settings from the JSON files, then it loads the user secrets, followed by the environment variables and lastly the command line arguments. So if you have a settings, say "Security:SessionTimeout" defined in your JSON File:

  "Security": {
    "SessionTimeout": 20

And you have an environment variable with the same name:

"Security:SessionTimeout" = 25

The configuration value loaded into the application will be 25, not 20. The same override can be made via the command line.

This is described in more detail here: Configuration in ASP.NET Core

Using the Launch Settings

So in the launchSettings.json file, find the profile you want to override. In my case, I usually use the Project profile, names after the Project being run (usually the last one in the file). Add an environmentVariables node:

  "profiles": {
    "RefArch01": {
      "commandName": "Project",
      "launchBrowser": true,
      "launchUrl": "api/values",
      "environmentVariables": {
      "applicationUrl": "http://localhost:51715/"

In this example, I am overriding the ASPNETCORE_ENVIRONMENT variable to use a Local environment configuration, rather than the ones that Microsoft configures automatically. The WebHost.CreateDefaultBuilder automatically includes the settings from the appsettings.{Environment}.json file, which is not always desirable. Setting it to Local means only the appsettings.json file is processed, ignoring the Development settings, for example.

To override the SessionTimeout value of the Security node, add this variable:

"environmentVariables": {
  "Security:SessionTimeout": 25

How to handle arrays

When configuring Serilog, the appsettings.json file contains an array of Sinks to log messages. Overriding that array is possible as long as you get the syntax correct.

Let’s say you have the following:

  "PossibleValues": [{ "Name": "Value1" }, { "Name": "Value2" }, { "Name": "Value3" }]

This results in the following values being added to the configuration:


This is also possible to override via environment variables:

PossibleValues:1:Name = "Value42"

You can even add elements to the array:

PossibleValues:3:Name = "Value42"

However, if you want to reduce the size of the array, then you are out of luck. Environment variables cannot be set to an empty string (or anything equivalent). So if you need to adjust the array, you either have to do it using a provider other than the environment variables, or you have to adjust the array in code (i.e programmatically).