Blazor Web App: Adding Custom Claims to Current User

I recently had a requirement where I want to add Custom Claims to the current user in a Blazor Web Application. I used IClaimsTransformation, but it doesn't work with Blazor. Instead, the correct option for Blazor is to implement a custom AuthenticationStateProvider.

public class ApplicationAuthenticationStateProvider : AuthenticationStateProvider
{
    private readonly AuthenticationStateProvider _authenticationStateProvider;
    private readonly IUserService _userService;
 
    public ApplicationAuthenticationStateProvider(AuthenticationStateProvider AuthenticationStateProvider, IUserService userService)
    {
        _authenticationStateProvider = AuthenticationStateProvider;
        _userService = userService;
    }
 
    public override async Task<AuthenticationState> GetAuthenticationStateAsync()
    {
        AuthenticationState authenticationState = 
            await _authenticationStateProvider.GetAuthenticationStateAsync();

        if (authenticationState.User.Identity?.IsAuthenticated != true)
        {
            return authenticationState;
        }
 
        // Check if user already has the UserId claim
        if (authenticationState.User.HasClaim(x => x.Type == ApplicationClaimTypes.UserId))
        {
            return authenticationState;
        }
 
        // TODO: Get the user from the database using current claims and IUserService
        string applicationUserId = "someUserId";
 
        ClaimsIdentity claimsIdentity = new ClaimsIdentity();
        claimsIdentity.AddClaim(new Claim(ApplicationClaimTypes.UserId, applicationUserId));
 
        authenticationState.User.AddIdentity(claimsIdentity);
 
        return authenticationState;
    }
}

Register ApplicationAuthenticationStateProvider,

builder.Services.AddScoped<ApplicationAuthenticationStateProvider>();

Use,

// This is some service where you want to use authenticationState
public class BlazorIdentityService : IIdentityService
{
    private readonly ApplicationAuthenticationStateProvider _applicationAuthenticationStateProvider;
 
    public BlazorIdentityService(ApplicationAuthenticationStateProvider applicationAuthenticationStateProvider)
    {
        _applicationAuthenticationStateProvider = applicationAuthenticationStateProvider;
    }
 
    public async Task<string> GetCurrentUserId()
    {
        AuthenticationState authenticationState = 
            await _applicationAuthenticationStateProvider?.GetAuthenticationStateAsync();
 
        return authenticationState.User?.Claims?.SingleOrDefault(x => x.Type == ApplicationClaimTypes.UserId)?.Value;
    }
}

Hope this helps.

Happy Coding.

Regards,

Jaliya