How do I pass the information from View to ViewModel with DelegateCommand?

Question

In my View, I have a button.

When the user clicks this button, I want to have the ViewModel save the context of the TextBlock in the database.

<StackPanel HorizontalAlignment="Left" VerticalAlignment="Top">
    <TextBlock Text="{Binding FirstName}"/>
    <TextBox Text="Save this text to the database."/>
    <Button Content="Save" Command="{Binding SaveCommand}"/>
</StackPanel>

However, in my DelegateCommand in my ViewModel, the "Save()" method doesn't pass any arguments, so how do I get data from the view at that point?

#region DelegateCommand: Save
private DelegateCommand saveCommand;

public ICommand SaveCommand
{
    get
    {
        if (saveCommand == null)
        {
            saveCommand = new DelegateCommand(Save, CanSave);
        }
        return saveCommand;
    }
}

private void Save()
{
    TextBox textBox = ......how do I get the value of the view's textbox from here?....
}

private bool CanSave()
{
    return true;
}
#endregion

Answer 1

Check out this MSDN article by Josh Smith. In it, he shows a variation of DelegateCommand that he calls RelayCommand, and the Execute and CanExecute delegates on RelayCommand accept a single parameter of type object.

Using RelayCommand you can pass information to the delegates via a CommandParameter:

<Button Command="{Binding SaveCommand}" 
        CommandParameter="{Binding SelectedItem,Element=listBox1}" />

Update

Looking at this article, it appears that there is a generic version of DelegateCommand which accepts a parameter in a similar way. You might want to try changing your SaveCommand to a DelegateCommand<MyObject> and change your Save and CanSave methods so that they take a MyObject parameter.

Answer 2

In your VM:

private DelegateCommand<string> _saveCmd = new DelegateCommand<string>(Save);

public ICommand SaveCmd{ get{ return _saveCmd } }

public void Save(string s) {...}

In you View, use CommandParameter like Matt's example.

Answer 3

You're asking about passing data via the button Command.

What you actually want, I think, is to bind your Textbox's text to a public property in your ViewModel:

<!-- View: TextBox's text is bound to the FirstName property in your ViewModel -->
<TextBox Text="{Binding Path=FirstName}" />
<Button Command="{Binding SaveCommand}"/>

<!-- ViewModel: Expose a property for the TextBox to bind to -->
public string FirstName{ get; set; }
...
private void Save()
{
    //textBox's text is bound to --> this.FirstName;
}

Answer 4

Hi, here is the elegant way.

Give a name to your textbox, then bind the CommandParameter in the button to it's Text property:

<StackPanel HorizontalAlignment="Left" VerticalAlignment="Top">
    <TextBlock Text="{Binding FirstName}"/>
    <TextBox x:Name="ParameterText" Text="Save this text to the database."/>
    <Button Content="Save" Command="{Binding SaveCommand}"
            CommandParameter="{Binding Text, ElementName=ParameterText}"/>
</StackPanel>

Answer 5

I'm not allowed to make comments yet, I guess. I'm responding to Carlos' suggestion because I tried it out. While it's a great idea, DelegateCommand would need to be modified in some way because otherwise you'll get this error: A field initializer cannot reference the non-static field, method, or property 'MyViewModel.Save(string)'.