LINQ投影关系属性为null

Question

I want to select items from a table, these items have relation properties, can I projecting them if they could be nullable (ie like left join)? And if not how I can workaround this?

class MyProducer
{
  ....
}    

Model model = new Model();
var q = 
    model.Products
    .Select(
      p => 
        new 
        { 
            id = p.Id, 
            producer = p.Producer != null ? new MyProducer { id = p.Producer.Id } : null 
        });

var r = q.ToArray();

When I execute this code I have exception

Unable to create a null constant value of type 'MyProducer'. Only entity types, enumeration types or primitive types are supported in this context.

Answer 1

Why don't you use left join?

using(var model = new Model())
{
    var q = 
    from product in model.Products
    join producer in model.Producers.DefaultIfEmpty()
    on product.ProducerId equals producer.Id
    select new
    {
        Id = product.Id,
        Producer = producer != null ? new MyProducer{ Id = producer.Id} : null
    }
}

Answer 2

Not sure if this is the only way to handle the projection of a possibly null relationship but this is how I have done it in the past. You create your new object and then when setting your properties check for null appropriately. The only problem with this solution is that you now have to check if the 'id' property of 'producer' is '0' to know if the relationship was null. If there is a better way of handling this I would love it if someone would post it here.

var q = 
    model.Products
    .Select(p => new 
        { 
            id = p.Id, 
            producer = new MyProducer 
            { 
               id = p.Producer == null ? 0 : p.Producer.Id
            }
        }
     );

var r = q.ToArray();

Answer 3

You need to cast your null to Struct :

Model model = new Model(); 
var q = model.Products.Select(p => new { id = p.Id, producer =  
    p.Producer != null ? new Struct { id = p.Producer.Id } : (Struct)null });