Cast a projected list using LINQ returns a list of null values?

Question

When I try to Cast the projected list in the BuildTypes method, I get a list of null values. I have also tried using the .Cast(), but I get an error that some of the properties cannot be cast. I can post the error if it is helpful. Here is my code:

public class AuditActionType: EntityValueType
{
}

private List<T> BuildTypes<T>(XDocument xDocument) where T: EntityValueType
{
    var types = 
        (from ty in xDocument.Descendants("RECORD")
         select new
            {
                Id = GenerateGuid(),
                Name = ty.Element("Name").Value,
                EntityStatus = _activeEntityStatus,
                DateCreated = DateTime.Now,
                DateModified = DateTime.Now
            } as T).ToList();

    return types;
} 

So I would call it like this:

var auditActorTypes = BuildTypes<AuditActorType>(auditActorTypesXml)

I have a ton of types I need to pull from an XML file and didn't want to duplicate code for each type.

Answer 1

You are trying to cast an anonymous object as type T, which cannot be done. The anonymous type is it's own unique type and in no way related to the T passed in.

Instead, you could supply the new() constraint on type T to mean it needs a default constructor, and then perform a new T() instead of creating a new anonymous type:

private List<T> BuildTypes<T>(XDocument xDocument) where T: EntityValueType, new()
{
    var types = 
        (from ty in xDocument.Descendants("RECORD")
         select new T()
            {
                Id = GenerateGuid(),
                Name = ty.Element("Name").Value,
                EntityStatus = _activeEntityStatus,
                DateCreated = DateTime.Now,
                DateModified = DateTime.Now
            }).ToList();

    return types;
} 

This is assuming, of course, that Id, Name, EntityStatus, DateCreated, and DateModified are all properties of the base EntityValueType.

Answer 2

You cannot do this with your current code as new { } creates an anonymous type which has no relationship with T (it is neither a child, nor is it of type T). What you could do instead is implement Id, Name, EntityStatus, DateCreated and DateModified as properties on your EntityValueType class and change:

private List<T> BuildTypes<T>(XDocument xDocument) where T: EntityValueType

To:

 private List<T> BuildTypes<T>(XDocument xDocument) where T: EntityValueType, new()

Which specifies that any type argument passed to our method must have a parameterless constructor which allows use to actually construct an object of type T generically by changing:

select new { ... } as T

To:

select new T { ... }

End result:

public class EntityValueType
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    // Change this to the correct type, I was unable to determine the type from your code. 
    public string EntityStatus { get; set; }
    public DateTime DateCreated { get; set; }
    public DateTime DateModified { get; set; }
}

public class AuditActionType: EntityValueType
{
}

private List<T> BuildTypes<T>(XDocument xDocument) where T: EntityValueType, new()
{
    return (from ty in xDocument.Descendants("RECORD")
        select new T
            {
                Id = GenerateGuid(),
                Name = ty.Element("Name").Value,
                EntityStatus = _activeEntityStatus,
                DateCreated = DateTime.Now,
                DateModified = DateTime.Now
            }).ToList();
}