在 WPF mvvm 中将项目添加到 ComboBox

Question

I'm dealing with a ComboBox in WPF and MVVM but I can't find how to populate the ComboBox in real time.

private ObservableCollection<AudioListaDevice> _audiodevicesitem = new ObservableCollection<AudioListaDevice>{
//Adding my first Item to test it
        new AudioListaDevice { AudioDeviceItem = "Test 01"},
        new AudioListaDevice { AudioDeviceItem = "Test 02"},
};
        public IEnumerable<AudioListaDevice> AudioDevicesItem { get { return _audiodevicesitem; } }
        private AudioListaDevice _AudioDevice;
        public AudioListaDevice AudioDevice
        {
            get
            { return _AudioDevice; }
            set
            {
                _AudioDevice = value; OnPropertyChanged("AudioDevice");
                //On Selection 
                Debug.WriteLine(_AudioDevice.AudioDeviceItem.ToString());
            }
        }

This is the class

public class AudioListaDevice : INotifyPropertyChanged
    {
        private string _AudioDeviceItem;
        public string AudioDeviceItem
        {
            get { return _AudioDeviceItem; }
            set { _AudioDeviceItem = value; OnPropertyChanged("AudioDeviceItem"); }
        }

        public void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

    }

What I'm trying to do is add a new item in the collection later on from anywhere and refresh the ComboBox.

public MainWindowVM() 
        {
            _audiodevicesitem.Add(new AudioListaDevice("Test03"));

//not relevant code continues here...
}

This is the Xaml Binding

<ComboBox x:Name="listaoutputdevices" 
          ItemsSource="{Binding AudioDevicesItem}"
          DisplayMemberPath="AudioDeviceItem"
          SelectedItem="{Binding AudioDevice}" 
          Grid.Row="0" Grid.Column="0" Margin="60,5,0,5" HorizontalAlignment="Left"  Width="352" >

The error says I don't have a contructor with an argument to pass the String via AudioListaDevice, but If I declare it in the Class AudioListaDevice : INotifyPropertyChanged

    private string item;
    public AudioListaDevice(string item)
        {
            this.item = item;
        }

Then

private ObservableCollection<AudioListaDevice> _audiodevicesitem = new ObservableCollection<AudioListaDevice>{
//Adding my first Item to test it
        new AudioListaDevice { AudioDeviceItem = "Test 01"},
        new AudioListaDevice { AudioDeviceItem = "Test 02"},
};

Gives an error when adding the previous items and the ComboBox doesn't reflect the

_audiodevicesitem.Add(new AudioListaDevice("Test03"));

Thanks!:)

Answer 1

The point is, the parameterless constructor is optional, only if there is no other constructor defined at all. It will be auto-generated in this special case, so that the compiler is able to create instances of the type. But as soon as you define a constructor, the compiler assumes that every constructor which wasn't defined was left out deliberately. This means once you have defined the constructor, which takes the string parameter, you also have to explicitly define the parameterless default constructor (if you want to) - which apparently you haven't. That's why the initialization list fails, as it invokes an undefined parameterless constructor.

// Invokes the parameterless default constructor
new AudioListaDevice { AudioDeviceItem = "Test 01"}

The solution is to define a default constructor:

public class AudioListaDevice : INotifyPropertyChanged
{ 
  // Parameterless default constructor
  public AudioListaDevice() : this(string.Empty)
  {
  }

  public AudioListaDevice(string audioDeviceItem)
  {
    this.AudioDeviceItem = audioDeviceItem;
  }

  private string _AudioDeviceItem;
  public string AudioDeviceItem
  {
    get { return _AudioDeviceItem; }
    set { _AudioDeviceItem = value; OnPropertyChanged("AudioDeviceItem"); }
  }

  public void OnPropertyChanged(string propertyName)
  {
    if (PropertyChanged != null)
    {
      PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
  }

  public event PropertyChangedEventHandler PropertyChanged;
}

Alternatively, if you don't want to define a parameterless constructor, then use the existing constructor for instantiation properly:

private ObservableCollection<AudioListaDevice> _audiodevicesitem = new ObservableCollection<AudioListaDevice>
{
  //Adding my first Item to test it
  new AudioListaDevice("Test 01"),
  new AudioListaDevice("Test 02")
};

Recommended read:

• Microsoft Docs: Constructors (C# Programming Guide)

• Microsoft Docs: Using Constructors (C# Programming Guide)