Using LINQ to find item in a List but get Value cannot be null Parameter name source

Question

When using LINQ to get data from a list I encounter this error  How can this be fixed      Value cannot be null  Parameter name  source   var nCounts   from sale in sal               select new                                   SaleID   sale OrderID                    LineItem   from sli in sale LineItems                              group sli by sli Item into ItemGroup                              select new                                                                 Item   ItemGroup Key                                   Weeks   ItemGroup Select s   gt  s Week                                                   foreach  var item in nCounts        foreach  var itmss in item LineItem                   MessageBox Show itmss Item Name           itemsal Add new Roundsman BAL WeeklyStockList itmss Item Name ToString                        itmss Item Code ToString                        itmss Item Description ToString                        Convert ToInt32 itmss Item Quantity   2  2  2  2  2  2  2  2  2                                    Error which i got after execution my LINQ I got that type of result  one line  orginally       System Linq Enumerable WhereSelectListIterator lt Roundsman BAL Sale          lt  f  AnonymousType1 lt int System Collections Generic IEnumerable         lt  lt  f  AnonymousType0 lt Roundsman BAL Items Item System Collections Generic IEnumerable         lt Roundsman BAL WeeklyRecord

User · Answer

Ok - my case was not strictly related to this question, but I ended up on this page when Googling for the same exception; I queried an SQL database with LINQ when I got this exception.

private Table<SubscriptionModel> m_subscriptionsSql;
var query = from SubscriptionModel subscription in m_subscriptionsSql ...

Turned out I forgot to initialize my Table instance variable, that I used in the LINQ query.

m_subscriptionsSql = GetTable<SubscriptionModel>();

User · Answer

The error you receive is from another method than the one you show here  It s a method that takes a parameter with the name  source   In your Visual Studio Options dialog  disable  Just my code   disable  Step over properties and operators  and enable  Enable  NET Framework source stepping   Make sure the  NET symbols can be found  Then the debugger will break inside the  NET method if it isn t your own  then check the stacktrace to find which value is passed that s null  but shouldn t   What you should look for is a value that becomes null and prevent that  From looking at your code  it may be the itemsal Add line that breaks   Edit  Since you seem to have trouble with debugging in general and LINQ especially  let s try to help you out step by step  also note the expanded first section above if you still want to try it the classic way  I wasn t complete the first time around     Narrow down the possible error scenarios by splitting your code  Replace locations that can end up null with something deliberately not null  If all fails  rewrite your LINQ statement as loop and go through it step by step    Step 1  First make the code a bit more readable by splitting it in manageable pieces      in your using-section  add this  using Roundsman BAL      keep this in your normal location var nCounts   from sale in sal               select new                                   SaleID   sale OrderID                    LineItem   GetLineItem sale LineItems                    foreach  var item in nCounts        foreach  var itmss in item LineItem                itemsal Add CreateWeeklyStockList itmss                 add this as method somewhere WeeklyStockList CreateWeeklyStockList LineItem lineItem        string name   itmss Item Name ToString        isn t Name already a string      string code   itmss Item Code ToString        isn t Code already a string      string description   itmss Item Description ToString        isn t Description already a string      int quantity   Convert ToInt32 itmss Item Quantity      wouldn t  int  or  as int  be enough       return new WeeklyStockList                   name                    code                    description                   quantity                    2  2  2  2  2  2  2  2  2                        also add this as a method LineItem GetLineItem IEnumerable lt LineItem gt  lineItems           add a null-check     if lineItems    null          throw new ArgumentNullException  lineItems    Argument cannot be null             your original code     from sli in lineItems     group sli by sli Item into ItemGroup     select new               Item   ItemGroup Key          Weeks   ItemGroup Select s   gt  s Week            The code above is from the top of my head  of course  because I cannot know what type of classes you have and thus cannot test the code before posting  Nevertheless  if you edit it until it is correct  if it isn t so out of the box   then you already stand a large chance the actual error becomes a lot clearer  If not  you should at the very least see a different stacktrace this time  which we still eagerly await     Step 2  The next step is to meticulously replace each part that can result in a null reference exception  By that I mean that you replace this   select new       SaleID   sale OrderID      LineItem   GetLineItem sale LineItems       with something like this   select new       SaleID   123      LineItem   GetLineItem new LineItem   ctor params for empty lineitem here          This will create rubbish output  but will narrow the problem down even further to your potential offending line  Do the same as above for other places in the LINQ statements that can end up null  just about everything    Step 3  This step you ll have to do yourself  But if LINQ fails and gives you such headaches and such unreadable or hard-to-debug code  consider what would happen with the next problem you encounter  And what if it fails on a live environment and you have to solve it under time pressure   The moral  it s always good to learn new techniques  but sometimes it s even better to grab back to something that s clear and understandable  Nothing against LINQ  I love it  but in this particular case  let it rest  fix it with a simple loop and revisit it in half a year or so   Conclusion  Actually  nothing to conclude  I went a bit further then I d normally go with the long-extended answer  I just hope it helps you tackling the problem better and gives you some tools understand how you can narrow down hard-to-debug situations  even without advanced debugging techniques  which we haven t discussed

User · Answer

This error can occur in several places  most commonly running further LINQ queries on top of a null collection  LINQ as Query Syntax can appear more null-safe than it is  Consider the following samples   var filteredCollection   from item in getMyCollection                            orderby item ReportDate                          select item    This code is not NULL SAFE  meaning that if getMyCollection   returns a null  you ll get the Value cannot be null  Parameter name  source error  Very annoying  But it makes perfect sense because LINQ Query syntax is just syntactic sugar for this equivalent code   var filteredCollection   getMyCollection   OrderBy x   gt  x ReportDate     Which obviously will blow up if the starting method returns a null   To prevent this  you can use a null coalescing operator in your LINQ query like so   var filteredCollection   from item in getMyCollection                                     Enumerable Empty lt CollectionItemClass gt                             orderby item ReportDate                          select item    However  you ll have to remember to do this in any related queries  The best approach  if you control the code that generates the collection  is to make it a coding practice to NEVER RETURN A NULL COLLECTION  EVER  In some cases  returning a null object from a method like  getCustomerById string id   is fine  depending on your team coding style  but if you have a method that returns a collection of business objects  like  getAllcustomers    then it should NEVER return a null array enumerable etc  Always always always use an if check  the null coalescing operator  or some other switch to return an empty array list enumerable etc  so that consumers of your method can freely LINQ over the results

User · Answer

This exception could point to the LINQ parameter that is named source    System Linq Enumerable Select TSource TResult  IEnumerable 1 source  Func 2 selector    As the source parameter in your LINQ query  var nCounts   from sale in sal  is  sal   I suppose the list named  sal  might be null

User · Answer

Here are more code examples that will produce the argument null exception   List lt Myobj gt  myList   null    from this point on  any linq statement you perform on myList will throw an argument null exception myList ToList    myList GroupBy m   gt  m Id   myList Count    myList Where m   gt  m Id    0   myList Select m   gt  m Id    0     etc

User · Answer

I think you can get this error if your database model is not correct and the underlying data contains a null which the model is attempting to map to a non-null object   For example  some auto-generated models can attempt to map nvarchar 1  columns to char rather than string and hence if this column contains nulls it will throw an error when you attempt to access the data    Note  LinqPad has a compatibility option if you want it to generate a model like that  but probably doesn t do this by default  which might explain it doesn t give you the error

[c#] Using LINQ to find item in a List but get "Value cannot be null. Parameter name: source"

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