LINQ左联接-'System.NullReferenceException'错误

Question

I have been trying to create a LINQ statement that will join 3 tables with left outer joins. When I try to run the code, I am receiving a 'System.NullReferenceException' error.

        var model = from con in dbo.Contacts
                    from conSpe in dbo.Contact_Specialties
                        .Where(cs => cs.ContactID == con.ContactID)
                        .DefaultIfEmpty()
                    from lan in dbo.Languages
                        .Where(l => l.ContactID == con.ContactID)
                        .DefaultIfEmpty()
                    orderby con.familyName
                    where (searchFirst == null || con.givenName.StartsWith(searchFirst)) &&
                          (searchLast == null || con.familyName.StartsWith(searchLast))
                    select new PhysicianDirectory
                    {
                        Contact = con,
                        Contact_Specialty = conSpe,
                        Language = lan
                    };

        return View(model);

Below is my code for my Direcory.cs Model in which I want to use the PhysicianDirectory class within my View.

[Table("Contacts")]
public class Contact
{
    [Key]
    public int ContactID { get; set; }
    public string familyName { get; set; }
    public string givenName { get; set; }
    public string additionalName { get; set; }
    public string honorificSuffix { get; set; }
    public string title { get; set; }
    public string School { get; set; }
}

[Table("Languages")]
public class Language
{
    [Key]
    public int LanguageID { get; set; }
    public int ContactID { get; set; }
    public string LanguageSpoken { get; set; }
}

[Table("Contact_Specialties")]
public class Contact_Specialty
{
     [Key]
     public int ContactID { get; set; }
     public int SpecialtyID { get; set; }
     public string Specialty1 { get; set; }
     public string Specialty2 { get; set; }
}

public class PhysicianDirectory
{
    public Contact Contact { get; set; }
    public Language Language { get; set; }
    public Fellowship Fellowship { get; set; }
    public Contact_Specialty Contact_Specialty { get; set; }
}

The 'System.NullReferenceException' error keeps appearing when I try to enter any information from Language class. The Language data will appear if I take out .DefaultIfEmpty() but will only show a small portion that is selected within the Language table. Is there a way for empty space to appear in place of NULL in code so the 'System.NullReferenceException' doesn't appear?

@model IEnumerable<MvcDirectory.Models.PhysicianDirectory>
    <div id="DirectoryList">
        @foreach (var item in Model)
        {
            <a href='@Url.Action("Details", "Profile", new { id = item.Contact.ContactID })'>
                <div class="PhysicianDetails">
                    <img src="~/Images/Capture.png" alt="Physician IMG" />
                    <h4>@item.Contact.givenName @item.Contact.additionalName @item.Contact.familyName, @item.Contact.honorificSuffix</h4>
                    <p>@item.Contact_Specialty.Specialty1</p>
                    <p>@item.Contact_Specialty.Specialty2</p>
                    <p>@item.Contact.title</p>
                    <p>@item.Language.LanguageSpoken</p>
                </div>
            </a>
        }
    </div>

Any suggestions? Thanks!

Answer 1

Just handle item.Language being null in your view:

 <p>@item.Language == null ? "" : @item.Language.LanguageSpoken</p>

In C# 6 this is even simpler:

 <p>@item.Language?.LanguageSpoken</p>

(Assuming that null ends up as an empty string in Razor...)

Answer 2

Since it is a Left join you have to handle null for conSpe and lan

var model = from con in dbo.Contacts
            from conSpe in dbo.Contact_Specialties
                .Where(cs => cs.ContactID == con.ContactID)
                .DefaultIfEmpty()
            from lan in dbo.Languages
                .Where(l => l.ContactID == con.ContactID)
                .DefaultIfEmpty()
            orderby con.familyName
            where (searchFirst == null || con.givenName.StartsWith(searchFirst)) &&
                  (searchLast == null || con.familyName.StartsWith(searchLast))
            select new PhysicianDirectory
            {
                Contact = con,
                Contact_Specialty = (conSpe == null? string.Empty : conSpe),
                Language = (lan == null ? string.Empty : lan)
            };

string.Empty should be handled differently based on what that type is. But you get the idea right.