[WP7][MVVM Light toolkit] Button Command raised too early, before binding when using mvvm-light toolkit

Question

I'm using the mvvm-light toolkit in my windows phone 7 application.

I have in my view :

<TextBox Height="78" HorizontalAlignment="Left" Margin="108,33,0,0" VerticalAlignment="Top" Width="313" Text="{Binding MyValue, Mode=TwoWay}" />
<Button Content="Go" Height="78" HorizontalAlignment="Left" Margin="127,252,0,0" Name="button1" VerticalAlignment="Top" Width="213" cmd:ButtonBaseExtensions.Command="{Binding DoCommand}"  />

My view model is :

    public class MainPageViewModel : ViewModelBase
    {
        public ICommand DoCommand { get; internal set; }
    public MainPageViewModel()
    {
        DoCommand = new RelayCommand(() =>
            {
                DoSomethingWith(MyValue);
            }, () => true);

    }

    private const string MyValuePropertyName = "MyValue";
    private string _myValue;
    public string MyValue
    {
        get { return _myValue; }
        set
        {
            if (_myValue == value)
                return;
            _myValue = value;
            RaisePropertyChanged(MyValuePropertyName);
        }
    }
}

In the emulator, when I type value in the textbox, and I click the button, I can see that I'm going first in the relaycommand lambda expression and with a breakpoint I see that MyValue is null. Then, the breakpoint in the setter of MyValue is reached, and the correct value goes in MyValue.

What am I doing wrong ? Of course, I would like that the setter can be reached before the RelayCommand...

Thanks in advance for any help.

indyfromoz indyfromoz · Accepted Answer · 2010-10-25T09:15:43

It is possible that you have hit the TextBox DataBinding issue with the TextChanged event. This is a recognized problem in Silverlight 3, see this thread discussing this issue and the workaround. A neat solution is perhaps to use behaviors as discussed in this article.

HTH, indyfromoz

[WP7][MVVM Light toolkit] Button Command raised too early, before binding when using mvvm-light toolkit

1 Answers