Question

我需要从IDictionary<int,string>的{{1}}匹配键中选择一个数据子集，然后返回一个新的IEnumerable<T>。我陷入语法错误并得到错误：

方法IEnumerable<T>的类型参数不能从用法。尝试显式指定类型参数。

这是我的代码：

Extensions.GetValues<K,
  V>(IDictionary<K, V>, IEnumerable<K>)

我想我需要告诉它使用public class Subject { public short SubjectID { get; set; } public byte CategoryID { get; set; } public string Title { get; set; } } public static class Extensions { public static IEnumerable<V> GetValues<K, V>(this IDictionary<K, V> dict, IEnumerable<K> keys) { return keys.Select((x) => dict[x]); } } private IEnumerable<Subject> Translate() { IEnumerable<Subject> selectedSubjects = new Subject[] { new Subject { SubjectID = 1, CategoryID = 2, Title = null } }; // this is given externally as an IDictionary var dict = new Dictionary<int, string> { { 1, "Hello" }, { 2, "Goodbye" } }; // this line produces the error: IEnumerable<Subject> data = dict.GetValues(selectedSubjects); // would like to return IEnumerable<Subject> containing SubjectID = 1, CategoryID and Title = "Hello" return data; }作为dict来过滤SubjectID吗？

Answer 1

好，因此这将返回与字典匹配的主题列表（其中dict.key == Subject.SubjectID），并更新Subject.Title = dict.value：

return dict.Keys.Join(
    selectedSubjects, 
    k => k, 
    s => (Int32)s.SubjectID, 
    (k, s) => new Subject
    {
        SubjectID = s.SubjectID, 
        CategoryID = s.CategoryID, 
        Title = dict[k] 
    });

Answer 2

尝试一个简单的yield IEnumerable。 EG

public static IEnumerable<V> GetValues<K, V>(this IDictionary<K, V> dict, IEnumerable<K> keys)
{
    foreach (var key in keys)
    {
        if (dict.TryGetValue(key, out V value))
        {
            yield return value;
        }
    }            
}

然后类似：

var selectedSubjectIds = selectedSubjects.Select(s => s.SubjectID);
IEnumerable<Subject> data = dict.GetValues(selectedSubjectIds);

根据IEnumerable <t>从IDictionary中选择子集，然后返回IEnumerable <t>

2 个答案: