c# - 无法将 System.Collections.Generic List 转换为 T

Question

我有一个字典，其值是一个列表。我需要阅读每个列表来提取一个字段。但是在foreach循环中出现如下错误

cannot convert type System.Collections.Generic.List<UserQuery.Employee> to UserQuery.Employee

如果我在 var iValue 的注释行中使用 firstorDefault，它就会起作用。但是我将有多个记录来检查我正在阅读的字典(尽管在这个例子中，我硬编码 1，以测试我的代码)。

void Main()
{
    var svalue =1;
    List<Employee> emplist = new List<Employee>()
    {
         new Employee{ EID=10, Ename="John"},
         new Employee{ EID=11, Ename="Adam"},
         new Employee{ EID=12, Ename="Ram"}
    };
    List<Employee> emplist1 = new List<Employee>()
    {
         new Employee{ EID=1, Ename="Jo"},
         new Employee{ EID=1, Ename="Ad"},
         new Employee{ EID=1, Ename="Ra"}
    };

    var dict1 = new Dictionary<int, List<Employee>>();
    dict1.Add(1, emplist);
    dict1.Add(2, emplist1);

    //var ivalue = dict1.FirstOrDefault(x => x.Key == svalue).Value.ToList();
    var ivalue = dict1.Where(kvp => dict1.ContainsKey(1)).Select(x => x.Value);

    Console.WriteLine(ivalue);
    foreach (Employee emp in ivalue)
    {
        Console.WriteLine(emp.EID);
    }
}

class Employee
{
    public int EID { get; set;}
    public string Ename {get; set;}
}

Answer 1

您当前的代码

 var ivalue = dict1
   .Where(kvp => dict1.ContainsKey(1)) //TODO: Very strange condition: check it
   .Select(x => x.Value);

返回 IEnumerable<List<Employee>>结果 ( ivalue )。既然你想要 IEnumerable<Employee>只是，您必须展平结果:

 var ivalue = dict1
   .Where(kvp => dict1.ContainsKey(1))
   .SelectMany(kvp => kvp.Value); // kvp: we still have key-value pair

然而，这不是我们处理字典的方式；您可能正在寻找这样的东西(让我们保留一些 Linq:- .Select(emp => emp.EID) ):

 if (dict1.TryGetValue(1, out var ivalue)) // do we have corresponding value?
   Console.WriteLine(String.Join(Environment.NewLine, ivalue // if yes, print them out
     .Select(emp => emp.EID)));