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Learn how to correctly retrieve user access tokens in a Blazor server application using AzureAdB2C and the Microsoft.Identity.Web library.
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Accessing User Tokens with AzureAdB2C in Blazor Applications

In the realm of web applications, especially those utilizing Azure Active Directory B2C (AzureAdB2C), accessing user tokens can sometimes present challenges. If you're working on a Blazor server-side application that employs the Microsoft.Identity.Web package for authentication, you may have encountered issues when trying to retrieve access tokens for users. This guide will guide you through the problem and provide a smooth solution to ensure your application interfaces effectively with Microsoft Graph API using user tokens.

The Problem

You are employing the Microsoft.Identity.Web package to allow user authentication via AzureAdB2C. When attempting to retrieve the user's token to interact with Microsoft Graph, you receive a NullReferenceException. This occurs specifically when you call the method GetAccessTokenForAppAsync(scopes) within your Blazor application’s service layer.

Common Scenario

Here's a breakdown of the components involved in your scenario:

Application Type: Blazor Server Side

Authentication Method: AzureAdB2C for user sign-ins using Microsoft Personal accounts.

Current Implementation: You are using the method intended for application-only tokens, not user delegated tokens.

The Solution

When you want to access tokens for signed-in users, it’s essential to use the appropriate method designed for acquiring user tokens. Instead of calling GetAccessTokenForAppAsync(scopes), which is intended for app-specific tokens, you should use GetAccessTokenForUserAsync(scopes). This method directly addresses the needs of obtaining an access token specifically for user delegation. Let’s see how to implement this:

Steps to Access User Tokens

Adjust Your Service Method:
Update your service method to replace the old token acquisition method. Below is an illustrative snippet on how to implement this change effectively:

[[See Video to Reveal this Text or Code Snippet]]

Check Dependencies:
Make sure you have correctly configured your services in the Startup.cs file, as per the initial setup provided in your application.

Error Handling:
Implement proper error handling to manage scenarios where the user token cannot be retrieved. This allows for a better user experience and aids in troubleshooting.

Testing:
Test the implementation by invoking the method within your application after a user is logged in. Ensure that it retrieves the access token without any exceptions.

Conclusion

By switching from GetAccessTokenForAppAsync(scopes) to GetAccessTokenForUserAsync(scopes), you will effectively address the exception issue and successfully retrieve the user token necessary for making delegated calls to Microsoft Graph. This seamless access is essential for integrating AzureAdB2C into your application framework, paving the way for a versatile and user-friendly experience.

With this guide, you should have a clear and effective path to accessing user tokens in your Blazor server application. If you encounter any issues or have further questions regarding AzureAdB2C or Microsoft identity integration, feel free to reach out in the comments below!