C# 委托(delegate)问题 - 这段代码到底做了什么？

Question

任何人都可以向我解释以下 C# 代码行的作用吗？

public event EventHandler<DataEventArgs<BusinessEntities.Employee>> EmployeeSelected = delegate { };

真正让我难过的是最后的 delegate { } 部分。有关更多上下文，请参阅 PRISM 2 附带的 ViewInjection 示例中的 EmployeesListView.xaml.cs 示例。完整的类定义如下所示:

/// <summary>
/// Interaction logic for EmployeesListView.xaml
/// </summary>
public partial class EmployeesListView : UserControl, IEmployeesListView
{
    public EmployeesListView()
    {
        InitializeComponent();
    }

    public ObservableCollection<BusinessEntities.Employee> Model
    {
        get { return this.DataContext as ObservableCollection<BusinessEntities.Employee>; }
        set { this.DataContext = value; }
    }

    public event EventHandler<DataEventArgs<BusinessEntities.Employee>> EmployeeSelected = delegate { };

    private void EmployeesList_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (e.AddedItems.Count > 0)
        {
            BusinessEntities.Employee selected = e.AddedItems[0] as BusinessEntities.Employee;
            if (selected != null)
            {
                EmployeeSelected(this, new DataEventArgs<BusinessEntities.Employee>(selected));
            }
        }
    }
}

Answer 1

这个位:

delegate {}

只是创建一个适当类型的“无操作”委托(delegate)。然后将该委托(delegate)分配给事件的支持变量。这是一种避免在引发事件时必须进行空值检查的简单方法 - 您始终至少有一个处理程序，即无操作处理程序。

这意味着这段代码可以很简单:

EmployeeSelected(this, new DataEventArgs<BusinessEntities.Employee>(selected));

代替:

EventHandler<DataEventArgs<BusinessEntities.Employee>> handler =EmployeeSelected;
if (handler != null)
{
    handler(this, new DataEventArgs<BusinessEntities.Employee>(selected));
}