c＃列表包含项目

Question

I have a class named accessoire :

   class accessoire 
    {
       public int value1 { get; set; }
        public string Value2 { get; set; }    
    }

then i have a List of that product

List<accessoire> accessoires

And i have an UI where the user pick the product he wants from a DataGridview and when he selected it launch an event that add this item to the list :

       private void ProductBrowser_OnItemAdded(Accessoire item)
        {

        if (Cart.Contains(item))
        {
            MessageBox.Show("Produit deja ajoutée au panier ! ");
        }
        else
        {
            Cart.Add(item);
            ProductView.Rows.Add(item.Ref, item.Name, Function.CatName(item.Cat), item.SellPrice, "1", Convert.ToDouble(item.SellPrice) * Convert.ToDouble(item.QtetoSell));
            TotalPriceSet();
            MessageBox.Show("Produit Ajouté !");
        }

    }

this doesnt work , but when i change it to :

    private void ProductBrowser_OnItemAdded(Accessoire item)
    {
        var InList = Cart.Find(product => product.Ref == item.Ref);
        if (Cart.Contains(InList))
        {
            MessageBox.Show("Product already in list ! ");
        }
        else
        {
            Cart.Add(item);
            ProductView.Rows.Add(item.Ref, item.Name, Function.CatName(item.Cat), item.SellPrice, "1", Convert.ToDouble(item.SellPrice) * Convert.ToDouble(item.QtetoSell));
            TotalPriceSet();
            MessageBox.Show("product added !");
        }

    }

it works , but i'am still wondering why the first code doesnt work it keep adding that item to the list ? in other way how does the method .Contains() works ? what does it check to know if the item is or the list on not ?

Answer 1

The reason it doesn't find the object in the list is because it is a reference comparison, comparing the instances of the object, not the values. You can have two instances of your class with the same values in their properties, but if you compare them, they are not equal:

accessoire item1 = new accessoire();
item1.value1 = 1;
item1.value2 = "one";

accessoire item2 = new accessoire();
item2.value1 = 1;
item2.value2 = "one";

if(item1 == item2) MessageBox.Show("Same");
else MessageBox.Show("Different");

When you select the item from the list to compare with, you are pulling the specific instance, which does exist in the list.

Answer 2

You need to implement the Equals method of accessoire to properly compare all properties/fields in it. The default implementation of Equals uses ReferenceEquals, which only works if the two instances are in fact the same.

Answer 3

if (Cart.Contains(item)) 

is matching by equality.
If object.Equals(T) is not satisified, it will fail. That means the smallest change, even whitespace in a string, will return false. You'll also get a false result if you have two instances of the same class. Contains must refer to exactly item .

var InList = Cart.Find(product => product.Ref == item.Ref) is a match by property. This means that other properties of the product/item can all be different, as long as .Ref matches. I presume Ref is a primary key, which is why you're not getting problems in your result where Find() returns the wrong item.

You can get around the difference by overriding Equals for Cart , but I don't recommend it. It can make debugging hell later.

Answer 4

Just implement the equals method

  // override object.Equals
    public override bool Equals(object obj)
    {
        //       
        // See the full list of guidelines at
        //   http://go.microsoft.com/fwlink/?LinkID=85237  
        // and also the guidance for operator== at
        //   http://go.microsoft.com/fwlink/?LinkId=85238
        //

        if (obj == null || GetType() != obj.GetType())
        {
            return false;
        }


        var data = (accessoire)obj;

        return this.Ref.Equals(data.Ref);
    }

    // override object.GetHashCode
    public override int GetHashCode()
    {

        return this.Ref.GetHashCode()
    }

Answer 5

You were doing reference compare and the references don't match in your first example, but do in your second. You probably want to do equality comparison. Are the values of both objects the same.

Below is your class implemented with the various methods used for equality comparing. You would just need to modify them to suit your purposes.

// IEquatable<T> provides typed equality comparing
class accessoire : IEquatable<accessoire>
{
   public int Value1 { get; set; }
   public string Value2 { get; set; }

   // you can override Equals.
   public override bool Equals(object obj)
   {
      return this.Equals(obj as accessoire);
   }

   // this comes from IEquatable<T>
   public bool Equals(accessoire other)
   {
      if (ReferenceEquals(null, other))
      {
         return false;
      }

      // return the comparison that makes them equal.
      return
         this.Value1.Equals(this.Value1) &&
         this.Value2.Equals(this.Value2);
   }

    public override int GetHashCode()
    {
       unchecked
       {
          int hash = 37;
          hash *= 23 + this.Value1.GetHashCode();
          hash *= 23 + this.Value2.GetHashCode();
          return hash;
       }
    }

    // allows you to check equality with the == operator
    public static bool operator ==(accessoire left, accessoire right)
    {
       if (ReferenceEquals(left, right))
       {
          return true;
       }

       if ((oject)left == null || (object)right == null)
       {
          return false;
       }

       return left.Equals(right);
    }

    public static bool operator !=(accessoire left, accessoire right)
    {
       return !left.Equals(right);
    }
}