How do you get the UserID of a User object in ASP.Net MVC?

It seems you cannot get it from the User object but you can get it this way:

Guid userGuid = (Guid)Membership.GetUser().ProviderUserKey;

Firstly, this answer is not strictly an MVC answer, but an ASP.NET answer. The fact that your site is MVC is irrelevant to solving the problem, in this case.

Hmm. I'm not very sure how you are handling your users in your system but it sounds like you using the (very evil) asp.net membership provider that comes out of the box with .net. This is hinted by the fact that you said

• aspnet_Users.UserID
• UserID is a uniqueidentifier (read: GUID).

With the default forms authentication system, which uses the default FormsIdentity, it only has a single property called Name (as you correctly noted). This means it has only one value where to place some unique user information. In your case, you are putting Name/UserName/DisplayName, in the Name property. I'm assuming this name is their Display Name and it is unique. Whatever value you are putting in here, it HAS TO BE UNIQUE.

From this, you can grab the user's guid.

Check this out.

using System.Web.Security;

....

// NOTE: This is a static method .. which makes things easier to use.
MembershipUser user = Membership.GetUser(User.Identity.Name);
if (user == null)
{
    throw new InvalidOperationException("User [" + 
        User.Identity.Name + " ] not found.");
}

// Do whatever u want with the unique identifier.
Guid guid = (Guid)user.ProviderUserKey;

So, every time you wish to grab the user information, you need to grab it from the database using the static method above.

Read all about the Membership class and MembershipUser class on MSDN.

Bonus Answer / Suggestion

As such, i would CACHE that result so you don't need to keep hitting the database.

... cont from above....
Guid guid = (Guid)user.ProviderUserKey;

Cache.Add(User.Identity.Name, user.UserID); // Key: Username; Value: Guid.

Otherwise, you can create your own Identity class (which inherits from IIdentity) and add your own custom properties, like UserID. Then, whenever you authenticate (and also on every request) you can set this value. Anyway, this is a hard core solution, so go with the caching, right now.

HTH

If you are using the ASP.NET Membership (which in turn uses the IPrincipal object):

using System.Web.Security;
{
  MembershipUser user = Membership.GetUser(HttpContext.User.Identity.Name);
  Guid guid = (Guid)user.ProviderUserKey;
}

User.Identity always returns the state of the current user, logged in or not.

Anonymous or not, etc. So a check for is logged in:

if (User.Identity.IsAuthenticated)
{
  ...
}

So, putting it all together:

using System.Web.Security;
{
  if (User.Identity.IsAuthenticated)
  {
    MembershipUser user = Membership.GetUser(HttpContext.User.Identity.Name);
    Guid guid = (Guid)user.ProviderUserKey;
  }
}

If you are using your own IPrincipal object for authorization, you just need to cast it to access the Id.

For example:

public class MyCustomUser : IPrincipal
{
    public int UserId {get;set;}

    //...Other IPrincipal stuff
}

Here is a great tutorial on creating your own Form based authentication.

http://www.codeproject.com/KB/web-security/AspNetCustomAuth.aspx

That should get you on the right path to creating an authentication cookie for your user and accessing your custom user data.

Its the ProviderUserKey property.

System.Web.Security.MembershipUser u;
u.ProviderUserKey