How to check if an item is already in a ListBox

问题:

Say I have a view model defined this way

public class DataVM
{
    public int number { get; set; }
    public string name { get; set; }
}

Then somewhere in my code I want to do this to populate DataListbox:

List<DataVM> data = new List<DataVM>();

for (int i = 0; i < data.Count; i++)
{
    if (DataListbox.Items.Contains(data[i]))
    {
        //do nothing
    }
    else
    {
        DataListbox.Add(data[i]);
    }
}

However, this line if (DataListbox.Items.Contains(data[i])) always evaluate to false even when that item is already in DataListbox and it should evaluate to true. I don't get why it doesn't work.

What am I doing wrong here and how do I fix it?

回答1:

The reason why your code always evaluates false is because the .NET framework compares the pointers to the memory and not the variables content by default when using checking for equality of two objects. So instead of using the built in Contains function you should iterate through all elements of the listbox and check by comparing an unique property if the item was already added to the listbox:

You would have to do something like this (using LINQ; Replace data[i].name and item.Value with the unique property):

bool listContainsItem = DataListbox.Items.Any(item => item.Value == data[i].name);

Or by using "old" coding style:

for (int i = 0; i < data.Count; i++)
{
   bool itemAlreadyAdded = false;
   foreach (var item in DataListbox.Items)
   {
       if (item.Value == data[i].name)
       {
            itemAlreadyAdded = true;
            break;
       }
   }
   if (itemAlreadyAdded)
   {
        //do nothing
    }
    else
    {
        DataListbox.Add(data[i]);
    }
}

回答2:

The Contains method uses the Equals method of the class being checked.

In this case the DataVM class needs to override the Equals method

public class DataVM
{
    public int number { get; set; }
    public stringname { get; set; }

    public override bool Equals(object obj)
    {
        bool areEqual ;

        areEqual = false ;

        if((obj != null) && (obj instanceOf DataVM))
        {
            //compare fields to determine if they are equal
            areEqual = (DataVM(obj)).number == this.number ;
        }

        return areEqual ;
    }
    public override int GetHashCode()
    {
         //calculate a hash code base on desired properties
         return number ; 
    }
}

When you override the Equals method is necessary for you to override also the GetHashCode method

回答3:

You cannt match your own classes. Even if the Properties are the same, it´s baisicaly not the same Object (In your case its not the same DataVM). It´s like trying to match your Blue car with an ather one, baisicaly it´s the same but the location for examle is different. Try matching properties of the object or write an own function. (I made a function)

List<DataVM> data = new List<DataVM>();

for (int i = 0; i < data.Count; i++)
{
    if (ListContainsDataVM(DataListbox.Items, data[i]))
    {
        //do nothing
    }
    else
    {
        DataListbox.Add(data[i]);
    }
}

public bool ListContainsDataVM(List<DataVM> DataVMList, DataVM myDataVM)
{
  foreach (var dataVm in DataVMList)
  {
    if (dataVm.number == myDataVM.number && dataVm.stringname == DataVM.stringname)
    {
      return true;
    }
  }
  return false;
}

回答4:

I cannot reply to previous answer, but this is the version as it should be I believe

bool listContainsItem = DataListbox.Items.Cast<DataVM>().Any(item => item.Value == data[i].name);

it was missing the cast...