Unable to cast object of type 'System.String' to type 'System.Web.Security.MembershipUser'

Question

I'm trying to get a list of usernames and bind them to a DropDownList and I must be missing a trick because I can't seem to cast it to the correct type. The code is below and the title is the error message I'm recieving.

EDIT - QUser inherits from MembershipUser

UserRepository userRepository = new UserRepository();
// retrieve custom user objects 
IEnumerable<QUser> Users = userRepository.GetAllUsers(); 
// just get usernames only
IEnumerable<string> userList = (from u in Users select  u.UserName); 

// set usernames to data source for a DropDownList
Username.DataSource = userList.ToArray(); // Cast error occurs here
Username.DataBind();

I've also tried casting the IEnumerable using the cast method as follows with no luck:

userList.Cast<string>().ToArray();

EDIT:

QUser Class

public class QUser : MembershipUser 
    {
        public QUser(){}

        public QUser(MembershipUser user): 
            base(user.ProviderName, user.UserName, user.ProviderUserKey, user.Email, 
            user.PasswordQuestion, user.Comment, user.IsApproved, user.IsLockedOut,user.CreationDate,
            user.LastLoginDate,user.LastActivityDate,user.LastPasswordChangedDate,user.LastLockoutDate)
        {}

        public string Forename
        {
            get;set;
        }

        public string Surname
        {
            get;set;
        }

        public string Phone
        {
            get;set;
        }

        public string PropertyNameNumber
        {
            get;set;
        }

        public string Street
        {
            get; set;
        }

        public string Town
        {
            get; set;
        }

        public string Area
        {
            get; set;
        }

        public string Postcode
        {
            get; set;
        }

        public DateTime? ExpiredDate
        {
            get; set;
        }
    }

Answer 1

If the u.UserName object is actually a MembershipUser type, that alone won't return the user name.

You'll have to either use u.UserName.UserName (the actual user name property) or explicitly convert it to a string using u.UserName.ToString() in the LINQ query.

If that's the case, I would consider refactoring the QUser class and give the UserName property a more accurate name.

Answer 2

Didn't you 'manually' bind the control to the MemberShipUser type ? Or maybe the expected type for this control datasource isn't "string" but "MemberShipUser", and this is checked at runtime to keep a generic .DataSource interface... ?

Answer 3

Do you think you could humor me and rewrite your code to look like this:

UserRepository userRepository = new UserRepository();
// retrieve custom user objects 
List<QUser> Users = userRepository.GetAllUsers().ToList();
// just get usernames only
List<string> userList = (from u in Users select u.UserName).ToList();
/* snip */
// set usernames to data source for a DropDownList
Username.DataSource = userList.ToArray(); // Cast error occurs here
Username.DataBind();

And see where your exception occurs then. I would be curious to see if the exception occurs sooner with that code. It might provide some insight into the problem.

Answer 4

I just ran your code on my machine and all looks good - even without the cast and removing the ToArray() and all works fine:

Default.aspx

....
<div>
    <asp:DropDownList id="Username" runat="server"></asp:DropDownList>
</div>
....

Default.aspx.cs

public class UserRepository
{
    private List<QUser> AllUsers = new List<QUser>();
    public UserRepository()
    {
        //MyProvider is just a dummy class that inherits 
        //MembershipProvider and has the name "MyProvider"
        //We can use this to spoof the MembershipProviderCollection
        //and create new membership users on the fly.
        MembershipProvider mp = new MyProvider(); 
        MembershipProviderCollection mpc = Membership.Providers;

        //Override the private field _ReadOnly so we can add our
        //spoof provider to the provider collection.  In normal
        //circumstances your provider should be added through web.config
        //this is a dirty hack and should not be used in production code
        Type t = mpc.GetType();
        Type tbt = t.BaseType; //The _ReadOnly field is on the base type
        FieldInfo fi = tbt.GetField("_ReadOnly", BindingFlags.Instance | BindingFlags.NonPublic);
        fi.SetValue(mpc, false);

        //Add our spoof provider... if you don't add the spoof provider
        //then when we create a new user with the provider "MyProvider"
        //everything will fall to pieces...
        mpc.Add(mp);

        //Add a bunch of test users
        AllUsers.AddRange(new[] {
            new QUser(new MembershipUser("MyProvider", "User 1", 1, "[email protected]", "What is your question?", "Some random comment", true, false, DateTime.Now, DateTime.Now, DateTime.Now, DateTime.Now, DateTime.MinValue)),
            new QUser(new MembershipUser("MyProvider", "User 2", 2, "[email protected]", "What is your question?", "Some random comment", true, false, DateTime.Now, DateTime.Now, DateTime.Now, DateTime.Now, DateTime.MinValue)),
            new QUser(new MembershipUser("MyProvider", "User 3", 3, "[email protected]", "What is your question?", "Some random comment", true, false, DateTime.Now, DateTime.Now, DateTime.Now, DateTime.Now, DateTime.MinValue))
        });
    }
    public IEnumerable<QUser> GetAllUsers()
    {
        return AllUsers;
    }
}
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        UserRepository userRepository = new UserRepository();
        IEnumerable<QUser> Users = userRepository.GetAllUsers();
        IEnumerable<string> userList = (from u in Users select u.UserName);

        Username.DataSource = userList;
        Username.DataBind();
    }
}

My only thought therefore is that the DropDownList that you're using is not actually a drop down list? Or there's something up with your ASP.NET development server. Have you tried killing it and restarting it just to be sure it's all working properly?