I have a list of entities and I want to check how many of them are common to some of the properties. So I used a dictionary

List<Letter> allPsukim = await _context.Letter.Where(a => a.OrderId == Order.Id)
                                              .ToListAsync();
foreach (var item in allPsukim)
{
    Psukim newPasuk = new Psukim()
    {
        Book = item.Book,
        Pasuk = item.Pasuk,
        Perek = item.Perek
    };

    if (showPsukim.ContainsKey(newPasuk))
    {
        List<int> newList = showPsukim[newPasuk];
        newList.Add(item.Position);
        showPsukim[newPasuk] = newList;
    }
    else
    {
        List<int> newList = new List<int>() { item.Position };
        showPsukim.Add(newPasuk, newList);
    }
}

The problem is that the showPsukim.ContainsKey(newPasuk) always shows that no key exists.

Where can I use entity as a key and check if it exists?

CodePudding user response：

Currently you are comparing Object references and not values. One way to compare Values is to use a string as key that combines the serialized properties that you need for comparison of equality. For example you could key on a concatenation of the Book, the Perek and the Passuk Properties:

string key = item.Book   "-"   item.Perek   "-"   item.Passuk;
dic.Add(key,item);

Then query for the key:

if(dic.ContainsKey(key)){...}

Of course, you could add a Serialize() method to the Letter Class that would return a serialized representation of the Object and use that as a key for your dictionary.

Alternatively you could override the GetHashCode() method.

CodePudding user response：

If it is reference type , it would be a new key when you new one. So try change to a struct, it's key is same when your field is totally same.

public class Psukim 
{
    //...
}

Change to this:

public struct Psukim 
{
    //...
}

or Override GetHashCode() , And you can choose few field to combine a new key.

public class Psukim 
    {
        //...
    
        public override int GetHashCode()
        {
            // set what you want field to combine a key.
            return System.HashCode.Combine(Book, Perek);
        }

        public override bool Equals(object obj)
        {
            Key other = obj as key;
            return other != null && other.Book == this.Book && other.Perek == this.Perek;
        }
    }