VB .Net NullReferenceException work-around

Question

This should be simple, but I am having problems. Using the eBay .Net library, sometimes certain fields in a response are Nothing and sometimes contain a value. When they are Nothing they could simple be represented as an empty string, but the developers chose to return them as Nothing so when I try to set a String to the value (when nothing is there) I get a NullReferenceException, see below

Imports System
Imports System.Configuration.ConfigurationManager
Imports System.Globalization
Imports System.Threading
Imports System.Xml

Imports eBay.Service.Core.Soap
Imports eBay.Service.Core.Sdk
Imports eBay.Service.Call
Imports eBay.Service.Util

Public Class eBayOrderImportService
Private Sub ServiceWorkerThread(ByVal state As Object)
    ' Periodically check if the service is stopping.
    Do While Not Me.stopping
        ' Perform main service function here...
        GetLastTime()
        Dim apiContext As ApiContext = GetApiContext()

        Dim apiCall As GetOrdersCall = New GetOrdersCall(apiContext)
        Dim orders As New OrderTypeCollection

        Dim timeFilter As New TimeFilter
        timeFilter.TimeFrom = lastUpdate
        timeFilter.TimeTo = Date.Now


        Dim lastTime As Boolean = SetLastTime()
        apiCall.IncludeFinalValueFee = True
        orders = apiCall.GetOrders(timeFilter, TradingRoleCodeType.Seller, OrderStatusCodeType.Completed)

        Dim order As OrderType

        For Each order In orders
            'do order-wide stuff here
            LogOrder(order)

        Next
        Thread.Sleep(30 * 1000)  ' Simulate some lengthy operations.

    Loop

    ' Signal the stopped event.
    Me.stoppedEvent.Set()
End Sub

Private Sub LogOrder(ByVal Order As OrderType)

    Dim OrderID, AccountID, BillingFirstName, BillingLastName, _
        BillingCompany, BillingEmailAddress, BillingPhone, _
        BillingAddress1, BillingAddress2, BillingCity, _
        BillingStateProvidence, BillingPostalCode, _
        BillingCountry, ShippingFirstName, ShippingLastName, _
        ShippingCompany, ShippingEmailAddress, ShippingPhone, _
        ShippingAddress1, ShippingAddress2, ShippingCity, _
        ShippingStateProvidence, ShippingPostalCode, _
        ShippingCountry, OrderStatus, BillingStatus, _
        OrderDate, ShippingMethod, SalesTax, _
        PreShippingCharge, OrderDiscount, OrderTotalCharged, _
        PaymentMethod, RepeatOrder, GiftCode, CouponCode, RID, _
        OrderNotes, OrderChannel, IsPrinted, IsShipped, PrintDate, _
        ShipDate, ActualShipCharge, DaysInTransit, DeliveryDate, _
        TrackingNumber, ShippedMethod As String


    OrderID = Order.OrderID

    AccountID = ""

    Dim name As String = If(Order.ShippingAddress.Name.ToString(), "None Given")

    BillingFirstName = name.Substring(0, name.IndexOf(" "))
    BillingLastName = name.Substring(name.IndexOf(" ") + 1)
    BillingCompany = If(Order.ShippingAddress.CompanyName.ToString(), "")
    BillingEmailAddress = If(Order.TransactionArray(0).Buyer.Email.ToString(), "")
    BillingPhone = If(Order.ShippingAddress.Phone.ToString(), "")
    BillingAddress1 = If(Order.ShippingAddress.Street1.ToString(), "")
    BillingAddress2 = If(Order.ShippingAddress.Street2.ToString(), "")
    BillingCity = If(Order.ShippingAddress.CityName.ToString(), "")
    BillingStateProvidence = If(Order.ShippingAddress.StateOrProvince.ToString(), "")
    BillingPostalCode = If(Order.ShippingAddress.PostalCode.ToString(), "")
    BillingCountry = If(Order.ShippingAddress.CountryName.ToString(), "")
    ShippingFirstName = If(BillingFirstName, "")
    ShippingLastName = If(BillingLastName, "")
    ShippingCompany = If(Order.ShippingAddress.CompanyName.ToString(), "")
    ShippingEmailAddress = If(Order.TransactionArray(0).Buyer.Email.ToString(), "")
    ShippingPhone = If(Order.ShippingAddress.Phone.ToString(), "")
    ShippingAddress1 = If(Order.ShippingAddress.Street1.ToString(), "")
    ShippingAddress2 = If(Order.ShippingAddress.Street2.ToString(), "")
    ShippingCity = If(Order.ShippingAddress.CityName.ToString(), "")
    ShippingStateProvidence = If(Order.ShippingAddress.StateOrProvince.ToString(), "")
    ShippingPostalCode = If(Order.ShippingAddress.PostalCode.ToString(), "")
    ShippingCountry = If(Order.ShippingAddress.CountryName.ToString(), "")
    OrderStatus = If(Order.OrderStatus.ToString(), "")
    BillingStatus = If(Order.OrderStatus.ToString(), "")
    OrderDate = If(Order.CreatedTime.ToString("MM/DD/yyyy"), "")
    If Order.TransactionArray(0).Taxes IsNot Nothing Then
        Dim tmpTax As Double = 0.0
        Dim tmpTrans As TransactionType
        For Each tmpTrans In Order.TransactionArray
            tmpTax = tmpTax + tmpTrans.Taxes.TotalTaxAmount.Value
        Next
        SalesTax = tmpTax.ToString()

    Else
        SalesTax = "0.0"
    End If


    ShippingMethod = If(Order.ShippingServiceSelected.ShippingService.ToString(), "")
    ShippingMethod = ShippingMethod & ":" & If(Order.ShippingServiceSelected.ShippingServicePriority.ToString(), "")

    OrderTotalCharged = If(Order.Total.Value.ToString(), "")
    OrderChannel = "eBay"
    comm = New OdbcCommand
    comm.CommandText = _
           "INSERT INTO Orders (OrderID, AccountID, BillingFirstName, BillingLastName, " & _
           "BillingCompany, BillingEmailAddress, BillingPhone, BillingAddress1, " & _
           "BillingAddress2, BillingCity, BillingStateProvidence, BillingPostalCode, " & _
           "BillingCountry, ShippingFirstName, ShippingLastName, ShippingCompany, " & _
           "ShippingEmailAddress, ShippingPhone, ShippingAddress1, ShippingAddress2, " & _
           "ShippingCity, ShippingStateProvidence, ShippingPostalCode, ShippingCountry, " & _
           "OrderStatus, BillingStatus, OrderDate, SalesTax, ShippingMethod, OrderTotalCharged, OrderChannel) " & _
           "VALUES('" & OrderID & "', '" & AccountID & "', '" & BillingFirstName & "', '" & _
           BillingLastName & "', '" & BillingCompany & "', '" & BillingEmailAddress & "', '" & _
           BillingPhone & "', '" & BillingAddress1 & "', '" & BillingAddress2 & "', '" & BillingCity & "', '" & _
           BillingStateProvidence & "', '" & BillingPostalCode & "', '" & BillingCountry & "', '" & _
           ShippingFirstName & "', '" & ShippingLastName & "', '" & ShippingCompany & "', '" & _
           ShippingEmailAddress & "', '" & ShippingPhone & "', '" & ShippingAddress1 & "', '" & _
           ShippingAddress2 & "', '" & ShippingCity & "', '" & ShippingStateProvidence & "', '" & _
           ShippingPostalCode & "', '" & ShippingCountry & "', '" & OrderStatus & "', '" & _
           BillingStatus & "', '" & OrderDate & "', '" & SalesTax & "', '" & ShippingMethod & "', '" & _
           OrderTotalCharged & "', '" & OrderChannel & "')"
    ' Dim orderResult As Integer = comm.ExecuteNonQuery()
    sysLog.WriteEntry(comm.CommandText)
End Sub
End Class

There is updated code please note no exception is thrown until: BillingCompany = If(Order.ShippingAddress.CompanyName.ToString(), "") is executed. The Name property has a value which successfully stored into its variable, while Order.ShippingAddress.CompanyName is set to Nothing (this property does exist, and can sometimes have a value). I updated the code to include Anthony Pegram's answer which did not help.

Everything is declared properly, to shorten the code I just showed a relevant example. Consider this inside of a loop for each order returned from a GetOrders() call, sometimes the Order.ShippingAddress.CompanyName will be nothing, at these times is it possible to handle it as an empty string? I tried the ToString() method. In other languages I could $CompanyName = this || that;

Anything similar in VB .Net?

Solution 1

You can use If(a, b) to coalesce a null to another value. Example:

Dim obj as String = Nothing
Dim foo as String = If(obj, "foo")

The output of foo in this example will be the string "foo". If you had a non-null string assigned to obj, then foo would also reference that string.

However, I have a strong feeling that your null reference exception could be happening not on the property, but on the object. Order or ShippingAddress could be null. Accessing properties or methods on a null reference is an error. Simply accessing a null value by storing it to a variable is not an error in and of itself.

If you are getting your exception on one of these lines

CompanyName = Order.ShippingAddress.CompanyName 
State = Order.ShippingAddress.StateOrProvince

It is because either Order or ShippingAddress is null.

Check and see if these objects could actually return as nothing. If so, you will need to apply null-checking around them prior to accessing their properties.

---

Your update:

There is updated code please note no exception is thrown until:

BillingCompany = If(Order.ShippingAddress.CompanyName.ToString(), "")

is executed.

This can throw because Order can be null, ShippingAddress can be null, or CompanyName can be null. It is an exception to access a property or a method on a null reference. If Order is null, you will get an error accessing ShippingAddress. Similarlu, if ShippingAddress is null, you cannot access CompanyName. If CompanyName is null, you cannot call ToString().

You have to at some point verify which object is null. I don't have confidence that any of them will not be null. Do you? Check. Step through your program and observe the object states.

If it does come down to properties like CompanyName being null, and if CompanyName is a string, omit the ToString() call.

BillingCompany = If(Order.ShippingAddress.CompanyName, "")

If CompanyName is not a string, you will simply have to compare it to Nothing before calling ToString() or accessing a property.

Solution 2

Take a look at this line:

CompanyName = Order.ShippingAddress.CompanyName

If the CompanyName property in Order.ShippingAddress.CompanyName is Nothing, that code is still perfectly legal and will not trigger a NullReferenceException. The exception is triggered because either Order.ShippingAddress or Order itself is Nothing. You get the exception because you're trying to look for a specific property on an object that doesn't exist. Based on your code sample it's easy to see why this might be the case:

Dim Order As OrderType

That line declares a variable, but it never actually gives that variable a value. Order is an object reference variable... that is not yet set to an instance of an object. Now it sounds like you may be omitting the code where an instance is set, but it's worth mentioning because if so, that's a very basic part of how this code is supposed to work and would make sense to include at least a stub for that in the sample.

Comments

[Mike L.]

I updated the code, please see the For/Next loop which ?should? be setting the value of Order as it iterates, and this Order is passed to another sub routine to actually process it. Properties that are not set to nothing have no problem.

[Anthony Pegram]

@MikeL, you have completely changed the code in the question. That generally makes the answers you have already received not useful. Generally speaking, don't do that. Start a new question if you have to.

[Anthony Pegram]

That being said, it still looks like you need to heed the advice of the answers. Either Order or Order.ShippingAddress could be null. Other objects could be null. Or it could indeed be the string properties that are null, and you are calling ToString() on them, getting the exception. This completely nullifies the safety of If(a, b). If a is null, If(a.ToString(), b) will throw. You haven't highlighted where the error is occuring, so we're left to guess. Have you stepped through the code?

[Anthony Pegram]

Please, put a breakpoint where the NRE is happening. Examing all of the object references on that line. See which one is null, and add appropriate null checks. Continue stepping through the code until you have verified that anything that could possibly be null is accounted for. And don't call methods or properties on null references.

[Andrew Morton]

Please make sure that VB.NET code you post on Stack Overflow compiles with Option Strict On. Also, you should explain the code and, in this case, explain what you're adding that the previous answer did not.

[cursorrux]

Please add further details to expand on your answer, such as working code or documentation citations.

[Suhaib Janjua]

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker.