方法传递ViewModel与传递绑定变量

Question

I have a ComboBox bound to a ViewModel string Quality_SelectedItem .

And I have a Method named Quality, which inside accesses the value of the SelectedItem in an if statement.

I have two ways of accessing the value, by passing the ViewModel through the Method, or by passing the string Quality_SelectedItem .

Which way should I be using it and which performs faster?

---

XAML

<ComboBox x:Name="cboQuality" 
          ItemsSource="{Binding Quality_Items}"
          SelectedItem="{Binding Quality_SelectedItem, Mode=TwoWay}"
          HorizontalAlignment="Left" 
          Margin="0,2,0,0" 
          VerticalAlignment="Top" 
          Width="105" 
          Height="22"/>

ViewModel Class

public class ViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged = delegate { };
    private void OnPropertyChanged(string prop)
    {
        PropertyChangedEventHandler handler = PropertyChanged;

        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(prop));
        }
    }

    // Quality Selected Item
    private string _Quality_SelectedItem { get; set; }
    public string Quality_SelectedItem
    {
        get { return _Quality_SelectedItem; }
        set
        {
            if (_Quality_SelectedItem == value)
            {
                return;
            }

            _Quality_SelectedItem = value;
            OnPropertyChanged("Quality_SelectedItem");
        }
    }

    ...

---

Example 1 - Passing ViewModel

In the Quality Method, I access vm.Quality_SelectedItem directly from the if statement.

public ViewModel vm = new ViewModel();

// MainWindow
public MainWindow() 
{
    InitializeComponent();
    DataContext = vm;

    // Quality Method
    Quality(vm); // <---
}


// Quality Method
public static void Quality(ViewModel vm) 
{
    if (vm.Quality_SelectedItem == "High") 
    {
        // do something
    }
    else if (vm.Quality_SelectedItem == "Low") 
    {
        // do something
    }
}

---

Example 2 - Passing String SelectedItem

I pass vm.Quality_SelectedItem through the Quality Method and give it the string name quality .

public ViewModel vm = new ViewModel();

// MainWindow
public MainWindow() 
{
    InitializeComponent();
    DataContext = vm;

    // Quality Method
    Quality(vm.Quality_SelectedItem); // <---
}


// Quality Method
public static void Quality(string quality) 
{
    if (quality == "High") 
    {
        // do something
    }
    else if (quality == "Low") 
    {
        // do something
    }
}

Answer 1

As a general rule, you should make your code as simple as possible. Remember the KISS principle. This also plays well with SOLID ("simple" is a good way to achieve Single responsibility and Interface segregation).

Avoid reaching into one object to get another.

If you need only a string value in the method, only pass that string value. Don't force your method to dig into object hierarchies and dependencies to get that value.

If the method needs to modify a string property value, then pass the object where to modify the property.

From the performance point of view, you will not notice any change. Accessing an object by-reference is a very cheap operation. (Unless you're implementing loops with billions of iterations.)

From the design point of view, keeping the things simple makes your code SOLID and easily allows re-usage.

Answer 2

It depends on what is //do something .

• If you have to handle/interact your viewmodel -object, then pass viewmodel as parameter
• otherwise use a string for less dependencies and possibility of common use.
• If you have your viewmodel as singleton, then it doesn't matter which way you go.