What is the purpose of the UserSecretsId in the .csproj file, and should it be hidden or public?

I created a new Windows Service using the BackgroundService class and .NET 8. When doing so, the .csproj file was automatically generated with a property called UserSecretsId. The value was a hash-like value, along with the name of my project appended to it.

Since I was planning to commit this project to source control on a remote repo, this seemed like a red flag to me. Why would there be a user secret in the .csproj file since that file would normally be added to source control? So my questions are, what is the purpose of the UserSecretsId and should it be hidden or public? Should I remove it before committing my project to source control? If it should be hidden, then why would the .csproj file be automatically generated with this value?

Here's an example of the .csproj file:

<Project Sdk="Microsoft.NET.Sdk.Worker">

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <UserSecretsId>dotnet-MyService-12uaeoj7i-sdfoij-ds398u3ef-3oweu5h</UserSecretsId>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Hosting" Version="8.0.0" />
    <PackageReference Include="Microsoft.Extensions.Hosting.WindowsServices" Version="8.0.0" />
  </ItemGroup>
</Project>

Solution

The UserSecretsId property is used by the Secret Manager tool to store sensitive data during application development.

Note that, despite the name, the tool does not encrypt secrets. From the docs:

The Secret Manager tool doesn't encrypt the stored secrets and shouldn't be treated as a trusted store. It's for development purposes only. The keys and values are stored in a JSON configuration file in the user profile directory.

All it does is providing an abstraction layer so that developers can focus in handling secrets without having to worry where those are actually located.

Therefore, coming back to your question about the need for UserSecretsId in the .csproj file, it is just a way for the tool to identify a path in the local system where to store the actual secrets and it is not a secret value itself, it is literally just a GUID for the app to have its own place to handle secrets.

The path where secrets are stored on Windows OS:
%APPDATA%\Microsoft\UserSecrets\<user_secrets_id>\secrets.json

The path where secrets are stored on Linux OS:
~/.microsoft/usersecrets/<user_secrets_id>/secrets.json

Note that, in Production scenarios, secrets should be secured accordingly, e.g. using Azure Key Vault.

The following page provides a more practical example on how to handle secrets in Development vs Production:
https://learn.microsoft.com/en-us/aspnet/core/security/key-vault-configuration?view=aspnetcore-8.0

Finally, like you I have also noticed that when creating a new Microsoft.NET.Sdk.Worker app with the most recent .NET 8.0 SDK, this property is automatically added and initialized at project creation time.
If you actually do not need to handle secrets in your app, you can simply remove it.