Problem with null variable

Question

I’m developing software in C# with MVVM model, and with Visual Studio.

It is a software to manage the members of a university.

We must also use a database, with Code First.

My problem is it gives me the following mistake:

"An Exception of type 'System.Nullreferenceexception' occurred in Universitymembers.exe but was not handled in user code Additional information: Object Reference not set to an instance of an Object."

I’ll point out the error in the code below:

public partial class MainWindow : Window
{
    private static VMUniversityMembers d = new VMUniversityMembers();
    ProjectContext db = d.ProjectContext;
    public MainWindow()
    {
        InitializeComponent();

        using (db)
        {

            var query = (DataContext as VMUniversityMembers).ProjectContext.UniversityMembers.SqlQuery("Select * from UniversityMembers");  **<---- Dá o erro aqui**

            foreach (var item in query)
            {
                listView.Items.Add(item);
            }
        }
    }
}

"VM Universitymembers" is the View Model between the "Universitymembers" Model and the graphical interface:

namespace UniversityMembers.ViewModel
{
    class VMUniversityMembers:BaseModel
    {
        private ProjectContext projectContext;
        private UniversityMember selectedMember = null;

        public List<UniversityMember> Members
        {
            get
            {
                return projectContext.UniversityMembers.Include("Position").Include("Sex").ToList();
            }
        }

        public ProjectContext ProjectContext
        {
            get
            {
                return projectContext;
            }
            set
            {
                projectContext = value;
            }

        }
        public UniversityMember SelectedMember
        {
            get
            {
                return selectedMember;
            }

            set
            {
                selectedMember = value;
            }
        }

        /**
         * Método para adicionar alunos 
         */

        internal void Add()
        {
            var unmbm = new UniversityMember();    

            projectContext.UniversityMembers.Add(unmbm);
            this.selectedMember = unmbm;
            this.OnPropertyChanged("UniversityMembers");    

        }
        internal void Cancel()
        {

        }
        /**
         *Método  para guardar a informacao introduzida  
         *
         */
        internal void SaveContext()
        {
            projectContext.SaveChanges();
            this.OnPropertyChanged("UniversityMembers");

        }
        /**
        * Método para eliminar um utilizador seleccionado na lista 
        * 
        */
        internal void Delete()
        {
            projectContext.UniversityMembers.Remove(SelectedMember);
        }    
    }
}

And the model I use is the Universitymembers:

    namespace UniversityMembers.Models
{
    class UniversityMember : Person
    {           
       public string email;
      // public  Position position;
       public string migration;


        public string Email { get { return email; } set { email = value; } }
      //  public Position Position { get { return position; } set { position = value; } }

        public UniversityMember()
        {

        }
    }
}

The class UniversityMembers extend in the superclass Person, with other attributes.

Answer 1

I will consider that DataContext is the type VMUniversityMembers, if it’s not, that’s the mistake. It seems to be after.

The estate ProjectContext does not seem to be being initialized anywhere, it seems that the problem is there.

UniversityMembers it is certainly not initialized, there is also error.

Some other remarks:

using (db)

That doesn’t work, that’s not how you use it, it won’t release anything there, or worse, it will release what you can’t, but it’s another matter.

This:

public string email;
public string Email { get { return email; } set { email = value; } }

is the same as this:

public string Email { get; set; }

I put in the Github for future reference.

The same goes for all other properties that make something so simple and standardized.