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 => 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<Roundsman.BAL.Sale, <>f__AnonymousType1<int,System.Collections.Generic.IEnumerable <<>f__AnonymousType0<Roundsman.BAL.Items.Item,System.Collections.Generic.IEnumerable <Roundsman.BAL.WeeklyRecord>>>>>

Answer 1

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<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 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).

Answer 2

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.

Answer 3

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.

Answer 4

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 => 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<CollectionItemClass>()
                         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.

Answer 5

Here are more code examples that will produce the argument null exception:

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

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>();