Azure B2C: how to Include domain hint in Sign Up/In links from asp.net core control - asp.net-core

Question

I'm using Azure B2C, I would like to use Google to sign in users. I would like to do something like this in my asp.net core page to achieve it:
<a asp-area="MicrosoftIdentity" asp-controller="Account" asp-action="SignIn" asp-route-domainHint="google.com">Google Sign Up/In</a>
So that the Google challenge page is used to sign in the user.
However, this code results in the non-social ("organic") sign in page being used, instead, because the domain_hint parameter is not included in the auth redirect.
Looking at the account controller code, here: https://github.com/AzureAD/microsoft-identity-web/blob/master/src/Microsoft.Identity.Web.UI/Areas/MicrosoftIdentity/Controllers/AccountController.cs
The only way to pass a domain hint to the Account controller, is by using the Challenge action.
<a asp-area="MicrosoftIdentity" asp-controller="Account" asp-action="Challenge" asp-route-domainHint="google.com">Google Sign Up/In</a>
This almost works.
But it loops back again to Google challenge page, after you choose your google account, repeatedly.
Is there a way to pass a domain hint and use the SignIn action of the Account controller? Or some other approach for generating Sign In calls from an asp.net core page and include a domain hint?
I have seen this answer, but would prefer to not build up the entire href address from scratch: Azure B2C with domain hint instead of IdP buttons
Thanks very much

Answer 1

To redirect users to an external identity provider, do the following:
Check the domain name of your external identity provider. For more information, see Redirect sign-in to a social provider.
Complete the Support advanced scenarios procedure.
In the OnRedirectToIdentityProviderFunc function, add the following line of code to the OnRedirectToIdentityProvider function:
private async Task OnRedirectToIdentityProviderFunc(RedirectContext context)
{
context.ProtocolMessage.DomainHint = "facebook.com";
// More code
await Task.CompletedTask.ConfigureAwait(false);
}
https://learn.microsoft.com/en-us/azure/active-directory-b2c/enable-authentication-web-application-options#preselect-an-identity-provider

Answer 2

Jas Suri's answer helped quite a bit, but there are a few code-related steps that I did to complete the solution:
To get that domain_hint parameter into the authentication link, you add some code to the ConfigureServices() method of the Startup class of your ASP.NET Core project:
services.AddAuthentication(OpenIdConnectDefaults.AuthenticationScheme)
.AddMicrosoftIdentityWebApp(options =>
{
Configuration.Bind("AzureAdB2C", options); // Assuming an appsettings.json section named AzureAdB2C with config data
options.Events ??= new OpenIdConnectEvents();
options.Events.OnRedirectToIdentityProvider += OnRedirectToIdentityProviderFunc;
});
The key part was subscribing to the OnRedirectToIdentityProvider event. Add an event handler for it in Startup.cs, which could look like this:
private async Task OnRedirectToIdentityProviderFunc(RedirectContext context)
{
var domainHint = context.HttpContext.Request.Query["domainHint"];
context.ProtocolMessage.DomainHint = domainHint.Count > 0 ? domainHint[0] : null;
await Task.CompletedTask.ConfigureAwait(false);
}
The middle line above gets the domainHint from the query string. So, you must set its value in your razor view as follows. Take note of the asp-route-domainHint attribute, which ASP.NET Core adds to the rendered Href as a query string value.
<a asp-area="MicrosoftIdentity" asp-controller="Account" asp-action="SignIn" asp-route-domainHint="google.com">Google Sign Up/In</a>
In the final rendered HTML, this link becomes (note the domainHint querystring parameter):
Google Sign Up/In
When a user clicks on that link, the Account controller in Microsoft.Identity.Web will redirect the user to the authentication link with Microsoft (note the domain_hint parameter near the end) which results in the social auth provider's screen being full-screen:
https://your-ADB2C-intance/your-ADB2C-domain/b2c_1a_signup_signin/oauth2/v2.0/authorize?client_id=..etc..etc....&domain_hint=google.com&state=..etc..etc..