Linq - How to query Dictionary

Question

I want to be able to query and sort a Dictionary<int, MyObj> by any of the properties inside MyObj.

class MyObj
  {
    public string Symbol { get; set; }
    public string Name { get; set; }
    public int AtomicNumber { get; set; }
    public int Id { get; set; }

    public string toString()
    {
      return Symbol + " " + Name + " " + AtomicNumber + " " + Id;
    }

  }

  class Program
  {

    private static void AddToDictionary(Dictionary<int, MyObj> elements,
    string symbol, string name, int atomicNumber, int id)
    {
      MyObj theElement = new MyObj();

      theElement.Symbol = symbol;
      theElement.Name = name;
      theElement.AtomicNumber = atomicNumber;
      theElement.Id = id;

      elements.Add(key: theElement.Id, value: theElement);
    }

    private static Dictionary<int, MyObj> BuildDictionary()
    {
      var elements = new Dictionary<int, MyObj>();

      AddToDictionary(elements, "K", "Potassium", 19, 0);
      AddToDictionary(elements, "Ca", "Calcium", 20, 1);
      AddToDictionary(elements, "Sc", "Scandium", 21, 2);
      AddToDictionary(elements, "Ti", "Titanium", 22, 3);

      return elements;
    }

    static List<T> GetListOfProperty<T>(Dictionary<int, MyObj> colBlobs, string property)
    {
      Type t = typeof(MyObj);
      PropertyInfo prop = t.GetProperty(property);
      if (prop == null)
      {
        // throw new Exception(string.Format("Property {0} not found", f.Name.ToLower()));
        Console.WriteLine(string.Format("Property {0} not found", property));
        return new List<T>();
      }
      //still need to fix this 
      return colBlobs.Values
          .Select(blob => (T)prop.GetValue(blob))
          .OrderBy(x => x)
          .ToList();
    }


    static SortedDictionary<int, MyObj> GetListOfProperty2<T>(Dictionary<int, MyObj> colBlobs, string property)
    {

      // CODE?

      return sortedDict;
    }


    static void Main(string[] args)
    {
      Dictionary<int, MyObj> myColl = BuildDictionary();

      var res = GetListOfProperty<string>(myColl, "Name");
      foreach (var rr in res)
        Console.WriteLine(rr.ToString());
      //outputs : Which is only one property, the one selected
      //--------
      //Calcium
      //Potassium
      //Scandium
      //Titanium

      var res2 = GetListOfProperty2<string>(myColl, "Name");
      //want to output the whole dictionary
      //<1, {"Ca", "Calcium", 20,1}
      //<0, {"K", "Potassium", 19, 0}
      //<2, {"Sc", "Scandium", 21, 2}
      //<3, {"Ti", "Titanium", 22, 3}
    }
  }

Since it seems to be that it is unclear what I want. I added example output. I am pretty sure there is no way to make this question more clear.

Answer 1

The problem with SortedDictionary is that it can only be sorted by Key, so you'll have to use OrderBy() in some way:

public static IOrderedEnumerable<KeyValuePair<K, T>> SortByMember<K, T>(this Dictionary<K, T> data, string memberName)
{
  Type type = typeof(T);
  MemberInfo info = type.GetProperty(memberName) ?? type.GetField(memberName) as MemberInfo;

  Func<KeyValuePair<K, T>, object> getter = kvp => kvp.Key;

  if (info is PropertyInfo pi)
    getter = kvp => pi.GetValue(kvp.Value);
  else if (info is FieldInfo fi)
    getter = kvp => fi.GetValue(kvp.Value);

  return data.OrderBy(getter);
}

This can handle both properties and fields, and if the member name is invalid, it defaults to sorting by key.

You can change that to not sort if member name is invalid by changing the return value:

public static IEnumerable<KeyValuePair<K, T>> SortByMember<K, T>(this Dictionary<K, T> data, string memberName)
{
  Type type = typeof(T);
  MemberInfo info = type.GetProperty(memberName) ?? type.GetField(memberName) as MemberInfo;

  if (info == null) return data;

  Func<KeyValuePair<K, T>, object> getter = null;

  if (info is PropertyInfo pi)
    getter = kvp => pi.GetValue(kvp.Value);
  else if (info is FieldInfo fi)
    getter = kvp => fi.GetValue(kvp.Value);

  return data.OrderBy(getter);
}

IMO it is wrong to return an empty dictionary if it fails to find the member. Alternatively you can throw an exception.

Use case:

  Dictionary<int, MyObj> myColl = BuildDictionary();

  var res = myColl.SortByMember("Name");

  foreach (var rr in res)
    Console.WriteLine(rr.Value);

Answer 2

To achieve your goal you can use expression tree to build a compiled lambda and then use this lambda in the Linq OrderBy() method:

public static class ExpressionHelper 
{
    public static Func<T, object> GetMemberExpressionFunc<T>(string memberName)
    {
        var parameter = Expression.Parameter(typeof(T));
        Expression source = Expression.PropertyOrField(parameter, memberName);  
        Expression conversion = Expression.Convert(source, typeof(object));
        return Expression.Lambda<Func<T, object>>(conversion, parameter).Compile();
    }
}   

static void Main(string[] args)
{
    Dictionary<int, MyObj> myColl = BuildDictionary();

    string property = "AtomicNumber";    // or whatever property you want your dictionary ordered by
    var func = ExpressionHelper.GetMemberExpressionFunc<MyObj>(property);
    var ordered = myColl.OrderBy(x => func(x.Value));
}