Get claims and subscription in Web Api Controller (.Net Core 2.1)

No need to cast, at least if you are using .Net Core 3.1. Simply access the values like this from a Controller:

var nameIdentifier = User.FindFirst(ClaimTypes.NameIdentifier);
var name = User.FindFirst(ClaimTypes.Name);
var givenName = User.FindFirst(ClaimTypes.GivenName);
var surname = User.FindFirst(ClaimTypes.Surname);
var email = User.FindFirst(ClaimTypes.Email);
var mobilePhone = User.FindFirst(ClaimTypes.MobilePhone);
var authenticationMethod = User.FindFirst(ClaimTypes.AuthenticationMethod);
var emails = User.FindFirst("emails");

From an access_token you can read values like this:

var handler = new JwtSecurityTokenHandler();
var jwtSecurityToken = handler.ReadJwtToken(adb2cTokenResponse.access_token);

var givenName = jwtSecurityToken.Claims.First(claim => claim.Type == JwtRegisteredClaimNames.GivenName).Value;
var familyName = jwtSecurityToken.Claims.First(claim => claim.Type == JwtRegisteredClaimNames.FamilyName).Value;
//Unless Alternate email have been added in Azure AD there will only be one email here. 
//TODO Handle multiple emails
var emails = jwtSecurityToken.Claims.First(claim => claim.Type == ADB2CJwtRegisteredClaimNames.Emails).Value;

public struct ADB2CJwtRegisteredClaimNames
{
    public const string Emails = "emails";

    public const string Name = "name";
}

Try casting HttpContext.User.Identity as ClaimsIdentity.

claimsIdentity = User.Identity as ClaimsIdentity;

// alternatively
// claimsIdentity = HttpContext.User.Identity as ClaimsIdentity;

// get some claim by type
var someClaim = claimsIdentity.FindFirst("some-claim");

// iterate all claims
foreach (var claim in claimsIdentity.Claims)
{
    System.Console.WriteLine(claim.Type + ":" + claim.Value);
}

Here are the .NET Core specific docs that support the HttpContext.User.Identity property.

• HttpContext
• HttpContext.User
• ClaimsPrincipal
• ClaimsPrincipal.Identity