在Foreach循环C＃中修改集合

Question

我在foreach循环期间更新ObservableCollection中的项目时遇到问题。 基本上，我有一个ObservableCollection员工，他们在模型中有一个字段来确定他们是否在建筑物中。

我一直在查看数据库表，以检查每位员工，以查看此状态是否有任何变化。 这就是我在C#执行此操作的方式；

public ObservableCollection<EmployeeModel> EmployeesInBuilding {get; set; }
public ObservableCollection<EmployeeModel> Employees {get; set; }

var _employeeDataService = new EmployeeDataService();
EmployeesInBuilding = _employeeDataService.GetEmployeesInBuilding();
foreach (EmployeeModel empBuild in EmployeesInBuilding)
{
    foreach (EmployeeModel emp in Employees)
    {
        if (empBuild.ID == emp.ID)
        {
            if (empBuild.InBuilding != emp.InBuilding)
            {
                emp.InBuilding = empBuild.InBuilding;
                int j = Employees.IndexOf(emp);
                Employees[j] = emp;
                employeesDataGrid.Items.Refresh();
            }
        }
    }
}

这可以正确地拾取两个ObseravbleCollections之间的ObseravbleCollections ，但是当我去更新现有的ObservableCollection我得到一个例外： Collection was modified; enumeration operation may not execute. Collection was modified; enumeration operation may not execute.

如何防止这种情况发生并仍然修改原始集合？

Answer 1

只需设置元素的属性，就无需替换Employees集合中的元素。

取而代之的是，您的EmployeeModel类应实现INotifyPropertyChanged接口，并在InBuilding属性更改时引发PropertyChanged事件：

public class EmployeeModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private bool inBuilding;
    public bool InBuilding
    {
        get { return inBuilding; }
        set
        {
            if (inBuilding != value)
            {
                inBuilding = value;
                OnPropertyChanged("InBuilding");
            }
        }
    }

    private void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    ...
}

现在可以将更新代码的内部循环简化为：

foreach (var emp in Employees)
{
    if (empBuild.ID == emp.ID)
    {
        emp.InBuilding = empBuild.InBuilding;
    }
}

或者，您可以像这样编写整个更新循环：

foreach (var empBuild in EmployeesInBuilding)
{
    var emp = Employees.FirstOrDefault(e => e.ID == empBuild.ID);

    if (emp != null)
    {
        emp.InBuilding = empBuild.InBuilding;
    }
}