How to add new ObservableCollection from Icommand command?

问题

I have a simple MVVM project I am learning on. I am trying to add to an ObservableCollection through an ICommand command, but I am unable to?

MainWindow.cs I haven't added anything*

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:Local="clr-namespace:WpfApplication1"
    Title="MainWindow" Height="350" Width="525">
<Grid>
    <Grid.DataContext>
        <Local:ViewModel></Local:ViewModel>
    </Grid.DataContext>

    <ListView Grid.Row="0" x:Name="lstNames" Margin="5,5,5,5" Grid.Column="1" ItemsSource="{Binding View_}">
        <ListView.View>
            <GridView x:Name="Setting_Items">
                <GridViewColumn Header="Setting_A"  DisplayMemberBinding="{Binding View_String}"/>
            </GridView>
        </ListView.View>
    </ListView>

   <TextBox Height="23" 
            HorizontalAlignment="Left" 
            Margin="145,195,0,0" 
            Name="textBox1" 
            VerticalAlignment="Top" 
            Width="120" />
  <ComboBox Height="23" 
            HorizontalAlignment="Left" 
            Margin="269,195,0,0" 
            Name="My_ComboBox" 
            VerticalAlignment="Top" 
            Width="222"     
            ItemsSource="{Binding View_}"/>
    <Button Content="Message Text" 
            Height="23" 
            HorizontalAlignment="Left"
            Margin="52,166,0,0" 
            Name="button1" 
            VerticalAlignment="Top" 
            Width="75" 
            CommandParameter="{Binding Text, ElementName=textBox1}"
            Command="{Binding Print_Line}"/>
    <Button Content="Add To Drop"
            Height="23" 
            HorizontalAlignment="Left"
            Margin="52,195,0,0" 
            Name="button2" 
            VerticalAlignment="Top" 
            Width="75" 
            />


</Grid>

    public class View
{
    public string View_String {get; set;}
}

    public class SimpleDelegateCommand : ICommand
{
    Action<object> _executeDelegate;

    public SimpleDelegateCommand(Action<object> executeDelegate)
    {
        _executeDelegate = executeDelegate;
    }

    public void Execute(object parameter)
    {
        _executeDelegate(parameter);
    }

    public bool CanExecute(object parameter) { return true; }
    public event EventHandler CanExecuteChanged;
}

 public class ViewModel
{
    private ObservableCollection<View> _View;

    public string _View_String { get; set; }

    public ObservableCollection<View> View_
    {
        get { return _View; }
        set { _View = value; }
    }

    ICommand _Print_Line = new SimpleDelegateCommand((x) => MessageBox.Show(x.ToString()));

    ICommand _Add_Line = new SimpleDelegateCommand((x) =>
         View_ = new ObservableCollection<View>() /////////Error HERE
        {
            new View(){View_String = x.ToString()}
        }
        );


    public ViewModel()
    {
        View_ = new ObservableCollection<View>()
        {
            new View(){View_String = "Setting 1"},
            new View(){View_String = "Setting 2"}
        };
    }

    public ICommand Print_Line { get { return _Print_Line; } }
    public ICommand Add_Line { get { return _Add_Line; } }
}

How do I go about adding to my ObservableCollection using an ICommand command? or how do I go about it?

Also, how do you perform multiple tasks using an ICommand command, for example: ICommand _Print_Line = new SimpleDelegateCommand((x) => MessageBox.Show(x.ToString()); MessageBox.Show("Second task"));

回答1:

perform multiple tasks:

_Print_Line = new SimpleDelegateCommand((x) => {
   MessageBox.Show(x.ToString());  
   MessageBox.Show("Second task");
 });

Add private set to your command's field, for access it just from your class

private ICommand print_Line;
public ICommand Print_Line { 
                             get { return print_Line; } 
                             private set { print_Line = value; } 
                           }

private ICommand add_Line;
public ICommand Add_Line  { 
                             get { return add_Line; } 
                             private set { add_Line = value; } 
                          }

maybe can help this way too:

private ICommand print_Line;
public ICommand Print_Line { get { return print_Line; } }

private ICommand add_Line;
public ICommand Add_Line{ get { return add_Line; } }

回答2:

Move your Commands Initialization code inside the constructor of your ViewModel class. You are trying to access View_ which is not yet constructed yet and hence giving error. Object initializer outside constructor should not call the instance methods/properties as the class is not constructed till that time.

ICommand _Print_Line;

ICommand _Add_Line;


public ViewModel()
{
    _Print_Line = new SimpleDelegateCommand((x) => MessageBox.Show(x.ToString()));

    _Add_Line = new SimpleDelegateCommand((x) =>
     View_ = new ObservableCollection<View>() /////////Error HERE
    {
        new View(){View_String = x.ToString()}
    }
    );

    View_ = new ObservableCollection<View>()
    {
        new View(){View_String = "Setting 1"},
        new View(){View_String = "Setting 2"}
    };
}

回答3:

First of all, your code is barely readable. The error is pretty straight forward. When you initialize a field, you cannot reference a non-static field. ICommand _Add_Line is an instance field. Just as _View is. If you want to reference it, initialize the ICommand _Add_Line within the constructor of the class. Every time you call a non-static field or method, you need an instance of that class to get its value. There are several answers on this issue on stackoverflow

来源：https://stackoverflow.com/questions/19457120/how-to-add-new-observablecollection-from-icommand-command

标签

.net

wpf

mvvm

observablecollection