Value cannot be null. Parameter name: source

Question

Somewhere inside the DbContext is a value that is IEnumerable and is queried with Any() (or Where() or Select() or any other LINQ-method), but this value is null.

Find out if you put a query together (somewhere outside your example code) where you are using a LINQ-method, or that you used an IEnumerable as a parameter which is NULL.

I had this one a while back, and the answer isn't necessarily what you'd expect. This error message often crops up when your connection string is wrong.

At a guess, you'll need something like this:

<connectionStrings>
    <add name="hublisherEntities" connectionString="Data Source=localhost;Initial Catalog=hublisher;Integrated Security=True;" providerName="System.Data.SqlClient" />
</connectionStrings>
<entityFramework>
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.LocalDbConnectionFactory, EntityFramework">
        <parameters>
            <parameter value="Data Source=localhost;Initial Catalog=hublisher;Integrated Security=True" />
        </parameters>
    </defaultConnectionFactory>
</entityFramework>

What's happening is that it's looking for a data source in the wrong place; Entity Framework specifies it slightly differently. If you post your connection string and EF config then we can check.

I just got this exact error in .Net Core 2.2 Entity Framework because I didn't have the set; in my DbContext like so:

public DbSet<Account> Account { get; }

changed to:

public DbSet<Account> Account { get; set;}

However, it didn't show the exception until I tried to use a linq query with Where() and Select() as others had mentioned above.

I was trying to set the DbSet as read only. I'll keep trying...

I was calling Count on an instance of DbSet with a filter of null i.e.

dbSet.Count(null);

I found that passing null here was causing the error so I now call the parameter-less method if the filter is null:

 if (filter == null)
 {
     return dbSet.Count();
 }
 else
 {
     return dbSet.Count(filter);
 }

This sorted the issue for me. This may be an issue for any other methods on DbSet as well.

just as an FYI, somebody may find it useful. I was chasing my tail for this error almost 2 days and was always thinking something big and looking for the classes that might be the issue and finally i found it very stupid issue and it was in my mark up (HTML) code in mypage.ascx. the issue was I have a <asp:EntityDataSource> and this has got a include property and I have some other tables listed here and mistakenly a table was there that has been delete from the database recently and I never noticed and it returning null with other entities. I just removed the stupid table from the include list and I am good to go. hope this can help somebody.