（obj！= null）不起作用

Question

It seams simple but I really don't get it.I am using linq and Entity Framework to retrieve an object from Database by a simple query like this

loggedinUser = (from user in context.Users
                 where user == _guid
                 select user).ToList()[0];

I know that I can use .FirstOrDefault() , but I don't think that my problem has something to do with the way I get my user.

After getting user If I check it with this condition

if (loggedinUser != null)
   {
     ToLocation = String.Format("{0} {1} {2} {3}", loggedinUser.StreetAddress,loggedinUser.City, loggedinUser.Province, loggedinUser.PostalCode);
   }

it doesn't work, as if it is null , but it's not.when I use this condition it works.

if (loggedinUser == null)
{ }
else
{
  ToLocation = String.Format("{0} {1} {2} {3}", loggedinUser.StreetAddress,loggedinUser.City, loggedinUser.Province, loggedinUser.PostalCode);
}

I always use this type of condition (Obj != null) and it works,the answer should be simple, but I don't really get why it doesn't this time. Am I missing something?

The only point is that the class for this entity is located in another project.Can it be the problem?

The project which this class is located in is in VB.Net :

<Table("Users")>
Public Class User
<Key()>
Public Property UserID As Integer

Public Property Username As String
Public Property PasswordEncrypted As String
Public Property LastLogin As DateTime 
Public Property CreatedByUserID As Integer
Public Property DateCreated As DateTime
Public Property Deleted As Boolean '?
Public Property Email As String
Public Property StreetAddress As String
Public Property City As String
Public Property Province As String
Public Property PostalCode As String

<NotMapped()>
Public Property Lat As Double

<NotMapped()>
Public Property Lon As Double

Public Property GUID As String

<NotMapped()>
Public Property EULAAgreed As DateTime

Public Overrides Function ToString() As String
    Return Username
End Function

Public Overrides Function Equals(ByVal obj As Object) As Boolean
    If (obj Is System.DBNull.Value) Then
        Return MyBase.Equals(obj)
    ElseIf (TypeOf obj Is String) Then
        Return MyBase.Equals(obj)
    Else
        Try
            Return UserID = CType(obj, Entities.User).UserID
        Catch ex As Exception
            Return UserID
        End Try
    End If
End Function

End Class

Answer 1

It's because LINQ-SQL/EF don't give you null. They give you DBNull.

Change your comparison to this:

if (loggedinUser != DBNull.Value)

Answer 2

It's possible that you have code like this:

if (loggedinUser != null)
    doSomething();
    doSomethingElse();

The problem is that the if 's scope ends with the first statement. Include braces if you have multiple statements.

if (loggedinUser != null)
{
    doSomething();
    doSomethingElse();
}

Answer 3

Although you got the answer for your question. but still these answers also worth a read.

Well, null is not an instance of any type. Rather, it is an invalid reference.

However, System.DbNull.Value, is a valid reference to an instance of System.DbNull (System.DbNull is a singleton and System.DbNull.Value gives you a reference to the single instance of that class) that represents nonexistent* values in the database.

*We would normally say null, but I don't want to confound the issue.

So, there's a big conceptual difference between the two. The keyword null represents an invalid reference. The class System.DbNull represents a nonexistent value in a database field. In general, we should try avoid using the same thing (in this case null) to represent two very different concepts (in this case an invalid reference versus a nonexistent value in a database field).

Keep in mind, this is why a lot of people advocate using the null object pattern in general, which is exactly what System.DbNull is an example of.

Original Source

and a very good explanation by Marc in What is the point of DBNull?