从递归中丢失值

Question

我对递归比较陌生。我遇到了一个奇怪的情况。正如您在下面看到的，我正在为一个复杂的部门填充员工。复杂的部门可以拥有一个或多个简单的部门和复杂的部门。它还有一个雇员的集合（列表）。当我通过这个调试时，第一个部门是复杂的，所以它进行递归，在那里我看到员工正确填充，员工收集计数是2，但是当它来自递归调用的员工集合计数再次设置为零。关于我在这里做错了什么的想法？

private void PopulateEmployees(ComplexDepartment complex)
{            
    foreach (var dep in complex.Departments)
    {                
        if (dep is SimpleDepartment)
        {
             var simple = dep as SimpleDepartment;
             complex.employees.Add(GetEmployee(simple));
        }
        else if (dep is ComplexDepartment) 
        {
            PopulateEmployees(dep as ComplexDepartment);
        }                
    }            
}

private Employee GetEmployee(SimpleDapartment simple)
{
    var employee = new Employee();
    // some code here

    return employee;
}

Answer 1

让代码正常工作（快速和脏）：

private void PopulateEmployees(ComplexDepartment complex)
{            
    foreach (var dep in complex.Departments)
    {                
        if (dep is SimpleDepartment)
        {
             var simple = dep as SimpleDepartment;
             complex.employees.Add(GetEmployee(simple));
        }
        else if (dep is ComplexDepartment) 
        {
            PopulateEmployees(dep as ComplexDepartment);
            // you need this here, because you collect the additional employees in dep
            complex.employees.AddRange(dep.employees);
        }                
    }            
}

让代码正常工作（更干净）：

private void PopulateEmployees(ComplexDepartment complex, ComplexDepartment addTo)
{            
    foreach (var dep in complex.Departments)
    {                
        if (dep is SimpleDepartment)
        {
             var simple = dep as SimpleDepartment;
             addTo.employees.Add(GetEmployee(simple));
        }
        else if (dep is ComplexDepartment) 
        {
            PopulateEmployees(dep as ComplexDepartment, addTo);
        }                
    }            
}

// Call it like
PopulateEmployees(myDep, myDep);

为什么不做非破坏性的：

   private IEnumerable<Employee> EnumerateEmployees(ComplexDepartment complexDepartment)
    {            
        foreach (var department in complexDepartment.Departments)
        {                
            if (department is SimpleDepartment)
                yield return GetEmployee((SimpleDepartment) department);
            else if (department is ComplexDepartment) 
            {
                foreach(var e in EnumerateEmployees(department))
                   yield return e;
            }                
        }            
    }

但是如果您的结构非常深，那么您可能会遇到StackOverflows的问题 - 所以我建议使用累加器或仅使用LINQ。

代码只是为了演示如何使用递归来实现;）