Entity Framework EntityType 'UserAccount' has no key defined

Question

I'm getting the following error:

One or more validation errors were detected during model generation:   
\tSystem.Data.Entity.Edm.EdmEntityType: : EntityType 'UserAccount' has no key defined. Define the key for this EntityType.
\tSystem.Data.Entity.Edm.EdmEntitySet: EntityType: EntitySet 'UserAccounts' is based on type 'UserAccount' that has no keys defined.

This error is triggered by the code:

_Db.Database.Initialize(true);

I assume it's not picking up the [Key] attribute on the model for some reason. Originally when I attempted to run the code I hadn't added the key attribute, could this mean something has been created / cached which is preventing this key being applied?

The MVC4 project is almost a blank setup aside from including Entity Framework 5

Model Code

public class AccountContext: DbContext
{
    public AccountContext()
        : base("DefaultConnection")
    {          
    }

    public DbSet<UserAccount> UserAccounts { get; set; }
}

[Table("UserAccount")]
public class UserAccount
{
    [Key]
    [Required]        
    public string Username;

    [Required]
    [DataType(DataType.Password)]
    [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
    public string Password;  

    public string Name;
    public string Surname;

    [Required] 
    public string Email;             
}

Global.asax Initialization

Database.SetInitializer(new DropCreateDatabaseAlways<AccountContext>());
var _Db = new AccountContext();
_Db.Database.Initialize(true);

I've already done some searching and understand the naming conventions e.g. Id / UserId etc. However i'd like to explicitly use [Key] and call the field Username.

Answer 1

I believe EF only allows mapping to properties, not fields as you have used.

Try changing:

[Key]
[Required]        
public string Username;

to

[Key]
[Required]        
public string Username { get; set; }

Answer 2

I'd like to add that if you happen to forget to make the target key property public, you will receive this same error. I came here for the same key error and noticed that I was lacking the "public" attribute. I added it and my error went away.