How to implement OAuth2 for a single tool, without using it as my application's authorization solution - oauth-2.0

Question

I currently have a MVC site, in .NET Core, backed by a public API. My users must log in (there are no [Anonymous] controllers), and authentication is already successfully being done using the DotNetCore.Authentication provider. All that is well and good.
What I'm now trying to do (by user request) is implement functionality for a user to read and view their Outlook 365 calendar on a page within my site. It doesn't seem too hard on the surface... all I have to do is have them authenticate through microsoftonline with my registered app, and then -- once they have given approval -- redirect back to my app to view their calendar events that I am now able to pull (probably using Graph).
In principle that seems really easy and straightforward. My confusion comes from not being able to implement authentication for a single controller, and not for the entire site. All of the OAuth2 (or OpenID, or OWIN, or whatever your flavor) examples I can find online -- of which there are countless dozens -- all want to use the authorization to control the User.Identity for the whole site. I don't want to change my sitewide authentication protocol; I don't want to add anything to Startup.cs; I don't want anything to scope outside of the one single controller.
tldr; Is there a way to just call https://login.microsoftonline.com/common/oauth2/v2.0/authorize (or facebook, or google, or whatever), and get back a code or token that I can use for that user on that area of the site, and not have it take over the authentication that is already in place for the rest of the site?

Answer 1

For anybody else who is looking for this answer, I've figured out (after much trial and error) how to authenticate for a single user just for a short time, without using middleware that authenticates for the entire application.
public async Task<IActionResult> OfficeRedirectMethod()
{
Uri loginRedirectUri = new Uri(Url.Action(nameof(OfficeAuthorize), "MyApp", null, Request.Scheme));
var azureADAuthority = #"https://login.microsoftonline.com/common";
// Generate the parameterized URL for Azure login.
var authContext = GetProviderContext();
Uri authUri = await authContext.GetAuthorizationRequestUrlAsync(_scopes, loginRedirectUri.ToString(), null, null, null, azureADAuthority);
// Redirect the browser to the login page, then come back to the Authorize method below.
return Redirect(authUri.ToString());
}
public async Task<IActionResult> OfficeAuthorize()
{
var code = Request.Query["code"].ToString();
try
{
// Trade the code for a token.
var authContext = GetProviderContext();
var authResult = await authContext.AcquireTokenByAuthorizationCodeAsync(code, _scopes);
// do whatever with the authResult, here
}
catch (Exception ex)
{
System.Diagnostics.Trace.WriteLine(ex.ToString());
}
return View();
}
public ConfidentialClientApplication GetContext()
{
var clientId = "OfficeClientId;
var clientSecret = "OfficeClientSecret";
var loginRedirectUri = new Uri(#"MyRedirectUri");
TokenCache tokenCache = new MSALSessionCache().GetMsalCacheInstance();
return new ConfidentialClientApplication(
clientId,
loginRedirectUri.ToString(),
new ClientCredential(clientSecret),
tokenCache,
null);
}
I don't know if that will ever be helpful to anybody but me; I just know that it's a problem that doesn't seem to be easily solved by a quick search.