How to assign a nullable int property in an anonymous type in LINQ with a Union?

Question

I have two select statements in LINQ with a Union.

A RoleID needs to have a null value in one of the selects. I am getting the error below.

If the RoleID has a value, it works fine. Reports is an EF entity with properties.

It can be anything in this example. Example is simple for illustration purposes.

Code in LinqPad:

var list = Reports.Select(r => new
    {
        RoleID = 3
    })
    .Union(Reports.Select(r => new
    {
        RoleID = new Nullable<int>()    <= error
        //RoleID = (int?) null          <= error
        //RoleID = 4                    <= works
    }));
list.Dump();

How do I make it work with a null value and make RoleID of type int?

Error Message:

'System.Linq.IQueryable' does not contain a definition for 'Union' and the best extension method overload 'System.Linq.ParallelEnumerable.Union(System.Linq.ParallelQuery, System.Collections.Generic.IEnumerable)' has some invalid arguments Instance argument: cannot convert from 'System.Linq.IQueryable' to 'System.Linq.ParallelQuery'

Answer 1

Your first Select query returns sequence of anonymous objects where RoleID have type int. To union both sequences they should have same type of anonymous objects. So you need to change first query:

var list = Reports.Select(r => new
                {
                    RoleID = (int?)3
                })

Keep in mind, in second query you also should have nullable RoleID to match type of anonymous object:

 .Union(Reports.Select(r => new
    {
        RoleID = new Nullable<int>() 
        //RoleID = (int?)null
        //RoleID = (int?)4
    }));

BTW Why have union on two selects from same source? Looks like you over simplified your sample queries.

Answer 2

You need RoleID to be nullable in the first anonymous class as well:

Reports.Select(r => new
               {
                   RoleID = (int?)3
               }

See also these questions:

• Two anonymous classes will have be share the same type if they use the same properties, with the same types, at the same order: is order of field important in anonymous types automatic initialization?
• You can also create a list of an anonymous type and add to it: Add an anonymous class of object to an anonymous list
• Also, you usually want to use Concat, not Union: Union Vs Concat in Linq