GetHashCode返回不同​​的值

Question

I'm using Linq-to-Sql to query a SQL Server database. This query returns a List of an entity in my database. My underlying data is NOT changing.

Once I have received the List, I call GetHashCode on it in order to test for equality. Oddly, the hash value is always different. Why would it always be different?

Thank you,

Answer 1

Are different beacuse they are different object references.

You need to override Equals() and GetHashCode() for your object, based on the object data, if you want to behave in that way.

Here you have an example about how to do it, and here a blog post about the guidelines overriding the GetHashCode() method. Hope it helps.

class TwoDPoint : System.Object
{
    public readonly int x, y;

    public TwoDPoint(int x, int y)
    {
        this.x = x;
        this.y = y;
    }

    public override bool Equals(System.Object obj)
    {
        if (obj == null) return false;

        TwoDPoint p = obj as TwoDPoint;
        if (p == null) return false;

        // Return true if the fields match
        return (x == p.x) && (y == p.y);
    }

    public override int GetHashCode()
    {
        return x ^ y;
    }
}

As Servy said in his comment, keep in mind that even overriding GetHashCode() method, you won't be able to have a collision-free hash with that type of data (ever), you can only reduce the collision rate. You need to use Equals() to ensure objects with the same hash are really the same

Answer 2

Did you override GetHashCode() ? If not the default implementation is to give you a hash code based on the reference of the list. It will not have anything to do with the contents of the list.

So two different instances means two different hash codes.

To check for list equality override Equals (and GetHashCode() ) on your entity class and use SequenceEqual on the list.