LINQ to Objects查询忽略层次结构中的空对象

Question

I want to use LINQ-to-objects to query a collection of objects several levels deep, to extract a value. For example, given an Entity object I want to get the emailAddress string if a flag is set to 1, otherwise null:

 Entity.CommunicationCollection.Communication.CommunicationDetail.EmailAddress

Where CommunicationDetail looks like this:

 public class CommunicationDetail
 {
     public int Flag;
     public string EmailAddress;
 }

The query I came up with looks like this:

 string email = Entity.CommunicationCollection.FirstOrDefault(x => x.Communication.CommunicationDetail.Flag == 1).EmailAddress;

The problem with this is that any null objects in the hierarchy will cause a null ref exception. Is there a way to structure the query to somehow ignore the nulls, without first checking every object? (The above is a simplified example, the nesting in the project I'm working on is much deeper.)

Answer 1

Well, you either have to check everything or use ?? for defaulting, which would still be awkward. I suspect you want something like:

var email = Entity.CommunicationCollection
                  .Where(x => x.Communication != null &&
                              x.Communication.CommunicationDetail != null &&
                              x.Communication.CommunicationDetail.Flag == 1)
                  .Select(x => x.Communication.CommunicationDetail.EmailAddress)
                  .FirstOrDefault();

Note that email can still be null here...

Obviously if x.Communication can never be null, or x.Communication.CommunicationDetail can never be null, you can remove those checks - it's not clear what can be null in your model.

Answer 2

I highly suggest creating a class with a generic method called something like IsNull, or IsNotNull and allowing it to take any any object type and checking the object to see if it is null. That would allow you to not have to have separate validation for each object your are querying

public class NullChecker where T : class
{
    public static bool IsNotNull<T>(default(T) type)
    {
        return type != null;
    }
}