In .net 6.0, the application needs secret.json to run. in any case, if you are migrating from .net core 3.1 or .net 5.0 in order to run your application in .net 6.0:

1. run dotnet user-secrets init in your project folder using command-line tools.
2. make sure you have a secret.json file in your app domain ...\bin\Debug\net6.0. It doesn't matter if your secret.json file is empty for now.

Hit F5 and run your application!

https://github.com/dotnet/runtime/issues/48485

Basically, it's a new feature in .NET6 that 'secrets.json' is not optional by default!

AddUserSecrets(this IConfigurationBuilder configuration, Assembly assembly, bool optional);

That 'optional' parameter should be set to 'true' in your code.

 var configuration = new ConfigurationBuilder()
    .AddEnvironmentVariables()
    .AddCommandLine(args)
    .AddJsonFile("appsettings.json")
    .AddUserSecrets<Program>(true)
    .Build();

Unlike environment variables, user secrets are placed in a settings file similar to appsettings.json. Having similar structured off-project settings is great when you need to copy keys and values between files and there is support for adding, removing, and listing values as I will show you later in this post.

To understand user secrets, let's resume the example from the previous post. In there I had an appsettings.json file looking like this:

{
  "AppSettings": {
    "ConnectionString": "http://localhost:9000"
  },
  ...
}

In order to override the AppSettings:ConnectionString setting on individual machines, each user needs to add a user secret with the same name. The easiest approach is to right-click the project and select Manage User Secrets:

This creates and opens a new empty JSON file named secrets.json. The file is placed beneath C:\Users\<username>\AppData\Roaming\Microsoft\UserSecrets\<id> where <username> matches your Windows user and is a randomly generated GUID. The important thing to notice here is that the file is located outside your project directory. In order to "bind" the secrets.json file location to your project, Visual Studio added a bit of markup to the csproj file:

<PropertyGroup>
  <UserSecretsId>dda25df4-9a88-4a7e-8502-2134b74e4729</UserSecretsId>
</PropertyGroup>

In case you are not using Visual Studio, you can generate a random GUID and add the <UserSecretsId> manually.

In case you want to override the AppSettings:ConnectionString setting, add a similar structure to the secrets.json file:

{
  "AppSettings": {
    "ConnectionString": "http://localhost:9000?user=mehdidaustany&password=1234"
  }
}

You can also collapse settings like this:

{
  "AppSettings:ConnectionString": "http://localhost:9000?user=mehdidaustany&password=1234"
}

Finally add created secrets.json in the root of your project.