searching a Combo box for a Generic item

14 Sep 2006 10:14 PM

Chuck P

I have a DisplayValueItem object that is used to fill a ComboListBox
with items. It used to be the Value and Display members were string.
I changed it to a generic (T, string) and now the below code fails.

x = new DisplayValueItem < int >(1, "stuff");
clbOutcomes.Items.Add(x);
bFound = clbOutcomes.Items.Contains(x);
bFound = clbOutcomes.Items.Contains(new DisplayValueItem < int >(1,
"stuff"));

bFound for x comes out true
bFound for new comes out false

I don't understand why it worked before I turned the DisplayValueItem
from a non-Generic (string, string) into a Generic (T,string).

   public class DisplayValueItem
        {
            private string mValue;
            private string mDisplay;
            public DisplayValueItem(string ValueField, string
DisplayField)
            {
                mValue = ValueField;
                mDisplay = DisplayField;
            }
            public string Value
            {
                get { return mValue; }
            }

            public string Display
            {
                get { return mDisplay; }
            }

        }

   public class DisplayValueItem<T>
        {
            private T mValue;
            private string mDisplay;
            public DisplayValueItem(T ValueField, string DisplayField)
            {
                mValue = ValueField;
                mDisplay = DisplayField;
            }
            public T Value
            {
                get { return mValue; }
            }

            public string Display
            {
                get { return mDisplay; }
            }

        }

15 Sep 2006 3:48 AM

Walter Wang [MSFT]

Hi Chuck,

I'm not sure what do you mean by "why it worked before I turned the
DisplayValueItem from a non-Generic (string, string) into a Generic (T,
string)" because using your non-Generic version of DisplayValueItem,
following code also turns out the same true and false result:

    DisplayValueItem y = new DisplayValueItem("1", "stuff");
    comboBox1.Items.Add(y);
    bFound = comboBox1.Items.Contains(y);
    bFound = comboBox1.Items.Contains(new DisplayValueItem("1", "stuff"));

Which I think is expected because you didn't override Equals in
DisplayValueItem to compare by the fields in it.

ComboBox internally uses a non-Generic ArrayList to store the items.
ArrayList uses Object.Equals to find an item. If your intention is to find
item by both mValue and mDisplay member in your class DisplayValueItem, you
need to override Equals like following:

    public class DisplayValueItem
    {
        public override bool Equals(object obj)
        {
            if (obj == null) return false;
            if (obj.GetType() != this.GetType()) return false;
            DisplayValueItem other = obj as DisplayValueItem;
            return mValue.Equals(other.mValue) &&
mDisplay.Equals(other.mDisplay);
        }
    }

    public class DisplayValueItem<T>
    {
        public override bool Equals(object obj)
        {
            if (obj == null) return false;
            if (obj.GetType() != this.GetType()) return false;
            DisplayValueItem<T> other = obj as DisplayValueItem<T>;
            return mValue.Equals(other.mValue) &&
mDisplay.Equals(other.mDisplay);
        }
    }

I hope this helps. Please reply to let us know whether or not you need
further information. Thank you.

Sincerely,
Walter Wang (waw***@online.microsoft.com, remove 'online.')
Microsoft Online Community Support

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15 Sep 2006 8:13 PM

Chuck

thanks,
maybe it was one of those cases where it was comparing strings on the
stack.

I got a warning about the hashcode so I used this too:

public override int GetHashCode()
            {
                return mDisplay.GetHashCode() ^ mValue.GetHashCode();
            }

On Fri, 15 Sep 2006 03:48:12 GMT, waw***@online.microsoft.com (Walter
Wang [MSFT]) wrote:

Show quote

>        public override bool Equals(object obj)
>        {
>            if (obj == null) return false;
>            if (obj.GetType() != this.GetType()) return false;
>            DisplayValueItem other = obj as DisplayValueItem;
>            return mValue.Equals(other.mValue) &&
>mDisplay.Equals(other.mDisplay);
>        }