c# distinct 不起作用。 我做错了什么？

Question

I'm developing an asp.net core application.
I have a code for getting property values.

var properties = _context.Properties.Select(p => new {
    p.Name,
    Values = p.Values.Distinct()
}).Distinct();

But Distinct() doesn't work. What I'm doing wrong?

Answer 1

The problem is that the second Distinct does not know how to compare the items. Probably you mean distinct by name, instead it does distinct by all the properties.

Instead of creating an anonymous type, create a named one (ie Property ). Then declare a IEqualityComparer<T> for this type:

class PropertyNameComparer : IEqualityComparer<Property>
{
    public bool Equals(Property x, Property y) => x.Name.Equals(y.Name);
    public int GetHashCode(Property p) => p.Name.GetHashCode();
}

(for sake of simplicity, I'm not handling nulls here.)

var properties = _context.Properties.Select(p => new Property {
    Name = p.Name,
    Values = p.Values.Distinct()
})
.AsEnumerable()
.Distinct(new PropertyNameComparer());

Note the .AsEnumerable() to separate the query from the second Distinct to make it LINQ-to-Objects, because EF cannot convert the IEqualityComparer<T> to SQL.

But the real question is, why are you getting duplicate properties in the first place? And if you do, what do you want to happen to the values of the duplicates? Will they contain the same values or different values? My implementation just takes the first property with its values and ignores the values of the duplicates. Instead, you might want to group by name and to union the values. But that's not clear from your question.