Why the anonymous type instance cannot accept null values returned by the entity framework query?

问题

When I try to run the following Entity Framework query:

var l =  (from s in db.Samples
          let action = db.Actions.Where(x => s.SampleID == x.SampleID && x.ActionTypeID == 1).FirstOrDefault()
          where s.SampleID == sampleID
          select new 
          {
             SampleID = s.SampleID,
             SampleDate = action.ActionDate,
          }).ToList();

I get following exception:

The cast to value type 'DateTime' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

The problem probably is that Action.ActionDate is defined as non nullable DateTime in EF model, but the query returns null when there are no relevant actions assigned to Sample.

The workaround is to return a non-anonymous type with nullable properties, but why the anonymous type cannot accept the null result? Can the anonymous type be somehow forced to be created with nullable properties?

回答1:

You are using anonymous types, not generic types. Anonymous types are defined by the compiler at compile time. In the end it's like you created a

class MyEntity
{
    public readonly int SampleID;
    public readonly DateTime SampleDate;
}

The types are "selected" by the compiler based on the types you use at the right of the = in the new. So if SampleID is int and ActionDate is DateTime then those types will be used.

Now, what happens is that when the Entity Framework executes the query and "deserializes" the data in the "MyEntity" class, it tries to convert the null it received by the SQL to a DateTime, and the cast fails. The solution is to define the ActionDate as a DateTime? in the anonymous type:

SampleDate = (DateTime?)action.ActionDate,

or to give SampleDate a value when ActionDate is null:

SampleDate = (DateTime?)action.ActionDate ?? default(DateTime),

(this second solution is untested because I don't have an SQL Server near me, if it works SampleDate will be a DateTime and will contain the date returned by the query or DateTime.MinValue when the date is null)

来源：https://stackoverflow.com/questions/29399001/why-the-anonymous-type-instance-cannot-accept-null-values-returned-by-the-entity

标签

entity-framework

anonymous-types