EF6中的两个列表之间的区别，使用except不起作用

Question

I am trying to calculate the Difference between two list my list type has this structure :

public partial class Assistance
{

    public int Id { get; set; }
    public string AssistanceName { get; set; }
    public string AssistanceTell { get; set; }
}

and here is my query :

           List<Assistance> assistanceWithoutExpert;
            AssistanceJurorRepository assistanceJurorRepository = new AssistanceJurorRepository();

            List<Assistance> assistancesWithExpert = assistanceJurorRepository.FindBy(i => i.User.Permission == "Assistance").Select(i => i.Assistance).ToList();
            List<Assistance> AllAssitance = GetAll().ToList();
            assistanceWithoutExpert = AllAssitance.Except(assistancesWithExpert).ToList();
            return assistanceWithoutExpert;

As you can see i have a list that holds all assitances called AllAssitance and a list that hold assitances that has expert called assistancesWithExpert i need to calculate assistanceWithoutExpert .So but after executing the result is all records in AllAssitance why ?

Best regards

Answer 1

You need to use the overload which takes a comparer . It doesn't work out of the box with complex types.

public class AssistanceComparer: IEqualityComparer<Assistance>
{
    public bool Equals(Assistance x, Assistance y)
    {
        return x.ID == y.ID;
    }

    public int GetHashCode(Assistance assistance)
    {
        return assistance.ID.GetHashCode();
    }
}

Usage:

assistanceWithoutExpert = AllAssitance.Except(assistancesWithExpert, new AssistanceComparer()).ToList();

Answer 2

The comparison is done using the default equality comparer. From MSDN :

Produces the set difference of two sequences by using the default equality comparer to compare values.

You need to define a custom comparison by overriding GetHashCode and Equals .

The default equality comparer, Default, is used to compare values of the types that implement the IEqualityComparer generic interface. To compare a custom data type, you need to implement this interface and provide your own GetHashCode and Equals methods for the type.

In short, you need to add the following code to your class Assistence if you want to compare the instances through the Id property.

public override int GetHashCode() {
    return Id.GetHashCode();
}

public override bool Equals(obj otherInstance) {
    return (otherInstance is Assistence) && ((Assistence)otherInstance).Id == Id;
}

A note of warning though if you want to compare on some field that isn't unique. See this link .