I took [Praetorian's answer][answer] and made an extension class that inherits `TextBox` so you don't have to muddle up your view's code behind with this behavior.

**C-Sharp**:



    public class TextBoxUpdate : TextBox
    {
        public TextBoxUpdate()
        {
            TextChanged += OnTextBoxTextChanged;
        }
        private void OnTextBoxTextChanged(object sender, TextChangedEventArgs e)
        {
            TextBox senderText = (TextBox)sender;
            BindingExpression bindingExp = senderText.GetBindingExpression(TextBox.TextProperty);
            bindingExp.UpdateSource();
        }
    }

**VisualBasic**:



    Public Class TextBoxUpdate : Inherits TextBox

        Private Sub OnTextBoxTextChanged(sender As Object, e As TextChangedEventArgs) Handles Me.TextChanged
            Dim senderText As TextBox = DirectCast(sender, TextBox)
            Dim bindingExp As BindingExpression = senderText.GetBindingExpression(TextBox.TextProperty)
            bindingExp.UpdateSource()
        End Sub

    End Class

Then call like this in **XAML**:



    <local:TextBoxUpdate Text="{Binding PersonName, Mode=TwoWay}"/>


[answer]: https://stackoverflow.com/a/4833196/1366033

Is there a way to get a TextBox in Windows Phone 7 to update the Binding as the user types each letter rather than after losing focus? 

Like the following WPF TextBox would do:



    <TextBox Text="{Binding Path=TextProperty, UpdateSourceTrigger=PropertyChanged}"/>

"UpdateSourceTrigger=PropertyChanged" equivalent for a Windows Phone 7 TextBox

Here are [Tim S's examples](https://stackoverflow.com/a/6781292/1366033) as extension methods on top of the Collection Class:

### CS with `FirstOrDefault`



    public static void ReplaceItem<T>(this Collection<T> col, Func<T, bool> match, T newItem)
    {
        var oldItem = col.FirstOrDefault(i => match(i));
        var oldIndex = col.IndexOf(oldItem);
        col[oldIndex] = newItem;
    }

### CS with Indexed Loop



    public static void ReplaceItem<T>(this Collection<T> col, Func<T, bool> match, T newItem)
    {
        for (int i = 0; i <= col.Count - 1; i++)
        {
            if (match(col[i]))
            {
                col[i] = newItem;
                break;
            }
        }
    }

### Usage

Imagine you have this class setup



    public class Person
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

You can call either of the following functions/implementations like this where the `match` parameter is used to identify the item you'd like to replace:



    var people = new Collection<Person>
    {
        new Person() { Id = 1, Name = "Kyle"},
        new Person() { Id = 2, Name = "Mit"}
    };

    people.ReplaceItem(x => x.Id == 2, new Person() { Id = 3, Name = "New Person" });


----

### VB with Indexed Loop
        


    <Extension()>
    Public Sub ReplaceItem(Of T)(col As Collection(Of T), match As Func(Of T, Boolean), newItem As T)
        For i = 0 To col.Count - 1
            If match(col(i)) Then
                col(i) = newItem
                Exit For
            End If
        Next
    End Sub  

### VB with `FirstOrDefault`



    <Extension()>
    Public Sub ReplaceItem(Of T)(col As Collection(Of T), match As Func(Of T, Boolean), newItem As T)
        Dim oldItem = col.FirstOrDefault(Function(i) match(i))
        Dim oldIndex = col.IndexOf(oldItem)
        col(oldIndex) = newItem      
    End Sub

How do I update a single item in an ObservableCollection class?

I know how to do an Add.  And I know how to search the ObservableCollection one item at a time in a "for" loop (using Count as a representation of the amout of items) but how do I chage an existing item.  If I do a "foreach" and find which item needs updating, how to I put that back into the ObservableCollection>

