如何在列表中添加多个项目以获取修复字典键

Question

我有一个修复字典键X，我想用这个键X 创建一个List<Student> 作为finalResult，其中我有来自某个外部来源的data。

在下面的代码中，我收到了错误，An item with the same key has already been added. Key: X'。如何解决？

 const string dictKey = "X";
        var finalResult = new Dictionary<string, List<Student>>();

        var data = new Dictionary<string, int> {{"A1!D1", 10}, {"A2!D2", 20}};

        foreach (var (key, value) in data)
        {
            finalResult.Add(dictKey, new List<Student>
            {
                new Student
                {
                    Name = key.Split('!')[0],
                    Section = key.Split('!')[1],
                    Age = value
                }
            });
        }

Answer 1

您可以通过以下两种方式之一进行操作。

1. 首先创建您的列表，然后将其添加到字典中。
2. 检查密钥是否已经存在。如果没有，添加它，否则更新列表。

先创建列表。

var data = new Dictionary<string, int> { { "A1!D1", 10 }, { "A2!D2", 20 } };
List<Student> allStudents = data.Select(x => new Student()
{
    Name = x.Key.Split('!')[0],
    Section = x.Key.Split('!')[1],
    Age = x.Value
}).ToList(); // Need to convert to List from IEnumerable.

finalResult.Add(dictKey, allStudents);

使用相同的键添加/更新字典。

var data = new Dictionary<string, int> { { "A1!D1", 10 }, { "A2!D2", 20 } };
foreach (var (key, value) in data)
{
    // Create Student object first otherwise repeating code twice.
    var student = new Student
    {
        Name = key.Split('!')[0],
        Section = key.Split('!')[1],
        Age = value
    };

    if (!finalResult.ContainsKey(dictKey))
        finalResult.Add(dictKey, new List<Student> { student }); // new list
    else
        finalResult[dictKey].Add(student); // Adding new item to existing list.
}

Answer 2

据我所见，您正在尝试将学生添加到分配给特定键的现有列表中，请尝试这样做：

const string dictKey = "X";

var finalResult = new Dictionary<string, List<Student>>();

var data = new Dictionary<string, int> {{"A1!D1", 10}, {"A2!D2", 20}};

foreach (var (key, value) in data)
{
    // check if key exists in the dictionary, and return the list assigned to it if it does
    if (!finalResult.TryGetValue(dictKey, out var list))
    {
        // if the key doesn't exist we assign a new List<Student> to the variable "list"
        list = new List<Student>();

        // We Add it to the dictionary, now when we call TryGetValue(dictKey) it will return true and the resulting value will be the List<Student> we assigned to "list".
        finalResult.Add(dictKey, list);
    }
    // Add the student to the list.
    list.Add(new Student
    {
        Name = key.Split('!')[0],
        Section = key.Split('!')[1],
        Age = value
    });
}

Answer 3

所以您想要字典中的单个元素，其中键设置为“X”，值设置为学生列表？

var students = data
    .Select(d => new Student
    {
        Name = d.Key.Split('!')[0],
        Section = d.Key.Split('!')[1],
        Age = d.Value
    })
    .ToList();

finalResult.Add(dictKey, students);