Sharepoint Custom SPFieldLookup save just one value

I have write a custom field that extends SPFieldLookup. I set AllowMultipleValues = true; Here is Field Control Value:

public override object Value
    {
        get
        {
            EnsureChildControls();
            SPFieldLookupValueCollection vals = new SPFieldLookupValueCollection();
            ICollection s = TinBaiLienQuanPicker.SelectedIds;
            if (s != null && s.Count > 0)
            {
                foreach (var i in s)
                {
                    ListItem z = availableItems.Find(x => (x.Value == i.ToString()));
                    if (z != null)
                    {
                        vals.Add(new SPFieldLookupValue(int.Parse(z.Value), z.Text));
                    }
                }
            }
            return vals;
        }
        set
        {
            EnsureChildControls();
            base.Value = value as SPFieldLookupValueCollection;
        }
    }

When control save field data, I see it return a collection which hav开发者_运维百科e multiple value. But when I retrieve data again, I receive only the first value. I get value from Control Field ' ListItemFieldValue property.

Please give me a tip. Thank you very much.

Data storage logic is different in the database for lookup field with one value compared to multiple values. Check that in the field type definition xml ParentType is set to LookupMulti instead of Lookup.

You have to inherit the field control class from MultipleLookupField not from LookupField. Are you sure you are doing this?