使用 C# LINQ 从字典中查找值属性为 null 的值

Question

In the below code, the dictionary will have a lot of Person data, with some missing information about Name property. I need to get the list of Person ID s, where the Name property is Null or Empty.

Note: I am expecting List for the missing Names

public class Person
{
    public int Id {get; set;}
    public string Name {get; set;}
    public int Age {get; set;}
}  

public class Persons
{
   private Dictionary<int, Person> m_dic =new Dictionary<int, Person>();
   public Dictionary<int, Person> Data 
   {
       get { return m_dic; }
   }

   public void AddData()
   {
       //Code to fill m_dic 
       m_dic.Add(1, new Person() {101, "John", 30 });
       m_dic.Add(2, new Person() {102, "", 40 });
       m_dic.Add(3, new Person() {103, "Peter", 20 });
       m_dic.Add(4, new Person() {104, "", 46});
   }

   public List<int> FindData()
   {
       var list = from p in m_dic
                  where p.Value.Name == Null
                  select new { p.value.Id };
       return list;
   }
}

So the output will have List with values like 102, 104. Need to improve the LINQ query. Can someone help?

Answer 1

You can use .Where() clause to filter dictionary elements based on condition and use .Select() to get list of Ids ,

var result = m_dic
      .Where(x => String.IsNullOrEmpty(x.Value.Name))  //Filter where name is null or empty
      .Select(y => y.Value.Id)  //Get only Ids
      .ToList();

Answer 2

1. You should be check for where string.IsNullOrEmpty(p.Value.Name) instead of where p.Value.Name == Null . So it will handle null value as well as blank "" .
2. Update select statement and not use new {... } because it will create IEnumerable<Object> not IEnumerable<int> . You can just use select p.Value.Id to return list of id s.
3. In return statement use return list.ToList(); so it will convert IEnumerable<int> to List<int> .

Updated function will be like below.

public static List<int> FindData()
{
     var list = from p in m_dic
              where string.IsNullOrEmpty(p.Value.Name)
              select p.Value.Id;
     return list.ToList();
}

Answer 3

Why don't you use Lambda expression in your find function. I'm rewriting your function with lambda.

public List<int> FindData()
{
    var list = m_dic.Where(x => x.Value.Name.Equals("")).Select(x => x.Value.Id).ToList();
    return list;
}

You'll get your desired result.