Using LINQ to search for an item in a list, but get "The value cannot be null. Parameter name: source"

Question

When using LINQ to retrieve data from a list, I encounter this error. How can this be fixed?

The 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 => 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)); } } 

The error I received after executing my LINQ I got this result (one line, original):

System.Linq.Enumerable.WhereSelectListIterator <Roundsman.BAL.Sale, <> f__AnonymousType1 <INT, System.Collections.Generic.IEnumerable <<> f__AnonymousType0 <Roundsman.BAL.Items.Item, System.Collections.Generic.IEnume. BAL.WeeklyRecord → →>

Answer 1

The error you get is different from the one you show here. This is a method that accepts a parameter named "source". In the Visual Studio Options dialog box, turn off My Only Code, turn off the Step over Properties and Operators, and turn on the Enable Source .NET Framework Step Step. Make sure .NET characters can be found. Then the debugger will break inside the .NET method if it is not your own. then check stacktrace to see which value is passed with a null value, but shouldn't.

What you need to look for is a value that becomes null and prevents this. If you look at your code, it could be line break itemsal.Add .

Edit

Since you seem to have problems with debugging in general and LINQ, especially, try to help you step by step (also pay attention to the extended first section above, if you still want to try the classic way, I was not completely the first time):

• Reduce possible error scenarios by splitting the code;
• Replace places where something null may be intentionally not null ;
• If all else fails, rewrite the LINQ statement as a loop and go through it step by step.

Step 1

First make the code more readable by dividing it into manageable parts:

 // 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<LineItem> 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 => s.Week) } } 

The above code, of course, is because of my head, because I don’t know what types of classes you have, and therefore I can’t check the code before publishing. However, if you edit it until it is correct (if it is not so out of the box), then you already have a big chance that the actual error becomes much clearer. If not, this time you should at least see another glass (which we are still expecting!).

Step 2

The next step is to carefully replace each part, which can lead to the exclusion of a null reference. By this I mean that you are replacing 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 a garbage outlet, but narrows the problem even further to your potential line of violation. Do the same as above for other places in LINQ statements that can end with null (almost everything).

Step 3

You will need to do this step yourself. But if LINQ fails and gives you such headaches and such unreadable or hard to debug code, think about what will happen to the next problem you are facing? And what if he fails in a living environment, and you have to solve it under the pressure of time =

Moral: it’s always good to learn new techniques, but sometimes it’s even better to return to something clear and understandable. Nothing against LINQ, I like it, but in this particular case, give it a break, fix it with a simple cycle and rename it after six months or so.

Conclusion

Actually, there is nothing to conclude. I went a little further than usual with a long extended answer. I just hope this helps you better solve the problem and gives you some tools to understand how you can narrow down hard-debugging situations, even without advanced debugging methods (which we did not discuss).

Answer 2

This exception may point to a LINQ parameter named by the source:

  System.Linq.Enumerable.Select[TSource,TResult](IEnumerable`1 source, Func`2 selector) 

Since the source parameter in your LINQ query ( var nCounts = from sale in sal ) is ' sal ', I assume that a list named "sal" can be null.

Answer 3

I think you can get this error if your database model is incorrect and the underlying data contains zero that the model is trying to map to a non-zero object.

For example, some auto-generated models may try to map nvarchar (1) columns to char rather than a row, and therefore, if this column contains null values, it will cause an error when trying to access data.

Note. LinqPad has a compatibility option if you want it to create such a model, but it probably doesn’t do this by default, which may explain that it does not give you an error.

Answer 4

Well - my case was not strictly related to this issue, but I got to this page when Googling for the same exception; I requested a SQL database with LINQ when I received this exception.

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

It turned out that I forgot to initialize my table instance variable, which I used in the LINQ query.

 m_subscriptionsSql = GetTable<SubscriptionModel>(); 

Answer 5

Here are some code examples that will result in the null argument being thrown:

 List<Myobj> myList = null; //from this point on, any linq statement you perform on myList will throw an argument null exception myList.ToList(); myList.GroupBy(m => m.Id); myList.Count(); myList.Where(m => m.Id == 0); myList.Select(m => m.Id == 0); //etc... 

Answer 6

This error can occur in several places, most often further LINQ queries are executed on top of an empty collection . LINQ as a query syntax may seem more null than it is. Consider the following examples:

 var filteredCollection = from item in getMyCollection() orderby item.ReportDate select item; 

This code is not NULL SAFE, which means that if getMyCollection() returns null, you will get a Value cannot be null. Parameter name: source error Value cannot be null. Parameter name: source Value cannot be null. Parameter name: source . Very annoying! But that makes sense, because LINQ Query syntax is the syntactic sugar for this equivalent code:

 var filteredCollection = getMyCollection().OrderBy(x => x.ReportDate); 

Which will obviously explode if the original method returns zero.

To prevent this, you can use the zero coalescence operator in your LINQ query as follows:

 var filteredCollection = from item in getMyCollection() ?? Enumerable.Empty<CollectionItemClass>() orderby item.ReportDate select item; 

However, you will need to keep this in mind in any related queries. A better approach (if you are managing the code that generates the collection) is to make it a coding practice, NEVER RETURN THE NEXT COLLECTION, EVER. In some cases, returning a null object from a method like " getCustomerById(string id) " is fine, depending on the command line style, but if you have a method that returns a collection of business objects, for example, " getAllcustomers() ", then it should NEVER return a null array / enumerable / etc. Always always use an if check, a null coalescence 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.