Linq然后可能是空的

Question

I'm trying to sort a view model binding on multiple properties. The problem is that the second property might be null, and I get a null reference exception.

return this.People
  .OrderBy(x => x.Car.Name)
  .ThenBy(x => x.Pet.Name);

What if Pet is null? How do I still do a ThenBy sort by Pet.Name?

Answer 1

This should return null Pets before non-null Pets.

return this.People
  .OrderBy(x => x.Car.Name)
  .ThenBy(x => x.Pet != null ? x.Pet.Name : "");

Answer 2

If you want people with no pets to be sorted above those with pets, you can use this:

return this.People
  .OrderBy(x => x.Car.Name)
  .ThenBy(x => x.Pet == null ? string.Empty : x.Pet.Name);

If you're going to be doing many sort operations involving pets, you could make your own PetComparer class that inherits from Comparer<Pet> , like this:

public class Pet
{
    public string Name { get; set; }
    // other properties
}

public class PetComparer : Comparer<Pet> // 
{
    public override int Compare(Pet x, Pet y)
    {
        if (x == null) return -1; // y is considered greater than x
        if (y == null) return 1; // x is considered greater than y
        return x.Name.CompareTo(y.Name);
    }
}

Now, your query would look like this:

return this.People
  .OrderBy(x => x.Car.Name)
  .ThenBy(x => x.Pet, new PetComparer());

Note: this will do the opposite of the query at the top of this answer--it will sort people without pets to the bottom (within a car name).

Answer 3

您可以对宠物和汽车使用Null对象模式 ，以避免在这种情况下对null进行任何额外检查，并将可能的NullReferenceException风险降至最低。

Answer 4

Using the null conditional ( ?. ) and null coalescing ( ?? ) operators together you can do this -

return this.People
  .OrderBy(x => x.Car.Name)
  .ThenBy(x => x.Pet?.Name ?? string.Empty);