MultiKeyDictionary.cs

using System;
using System.Collections.Generic;
using System.Reflection;
using System.Linq;

public class MyDictionary<T, V>: Dictionary<(string, T), V>
{
    private IEnumerable<(string Name, T Value)> GetMembers(object obj)
    {
        var properties = obj.GetType().GetProperties(BindingFlags.Public).Select(o => (o.Name, Value: (T)o.GetValue(obj)));
        var fields = obj.GetType().GetFields(BindingFlags.Public).Select(o => (o.Name, Value: (T)o.GetValue(obj)));
        var rtFields = obj.GetType().GetRuntimeFields().Select(o => (o.Name, Value: (T)o.GetValue(obj)));
        return properties.Concat(fields).Concat(rtFields);
    }

    public void Add(T key, V value)
    {
        foreach(var x in GetMembers(key))
        {
            base.Add((x.Name, x.Value), value);
        }
    }

    public V this[T key]
    {
        get
        {
            V value = default;
            foreach(var x in GetMembers(key))
            {
                value = base[(x.Name, x.Value)];
            }
            return value;
        }
    }

    public List<V> this[string key]
    {
        get
        {
            var keys = base.Keys.Where(o => o.Item1 == key || o.Item1.StartsWith($"<{key}>"));
            if (keys.Equals(default)) throw new KeyNotFoundException();
            var values = new List<V>();
            foreach(var _key in keys)
            {
                values.Add(base[_key]);
            }
            return values;
        }
    }

    public List<V> this[string key]
    {
        get
        {
            var keys = base.Keys.Where(o => o.Item1 == key || o.Item1.StartsWith($"<{key}>"));
            if (keys.Equals(default)) throw new KeyNotFoundException();
            var values = new List<V>();
            foreach(var _key in keys)
            {
                values.Add(base[_key]);
            }
            return values;
        }
    }

    public bool ContainsKey(object obj)
    {
        foreach(var x in GetMembers(obj))
        {
            if (!base.ContainsKey((x.Name, (T)x.Value))) return false;
        }
        return true;
    }

    public bool ContainsKey(string key)
    {
        var keys = base.Keys.Where(o => o.Item1 == key || o.Item1.StartsWith($"<{key}>"));
        if (!keys.Any()) return false;
        return true;
    }
}

public class MyFoo
{
    public int a;
    public string b;
    public double c;
}

public static class ConsoleExt
{
    public static void Dump(this object obj)
    {
       Console.WriteLine(obj);
    }
}

class Program
{
    public static void Main (string[] args)
    {
        var fooStr = "foo";
        var dict = new MyDictionary<object, object>
        {
            { new { a = "A", b = "B", c = "C" }, fooStr },
            { new { c = "D" }, fooStr },
            { new MyFoo { a = 1, b = "two", c = 3.3 }, fooStr }
        };

        dict.ContainsKey(new { a="A" }).Dump(); // true
        dict.ContainsKey(new { a="A", b="B" }).Dump(); // true
        dict.ContainsKey(new { a="A", b="B", c="C" }).Dump(); // true
        dict.ContainsKey(new { c="D" }).Dump(); // true
        dict.ContainsKey(new { a="A", b="B", c="D" }).Dump(); // true
        dict.ContainsKey(new { a = "A", b = "B", c = "Z" }).Dump(); // false
        dict.ContainsKey(new { a = "A", b = "B", c = "C", d = "D" }).Dump(); // false
        dict.ContainsKey(new MyFoo { a = 1, b = "two", c = 3.3 }).Dump(); // true
        dict.ContainsKey(new MyFoo { a = 1, b = "two", c = 9.9 }).Dump(); // false
        dict[new { a = "A" }].Dump();
        dict[new { a = "A", b = "B" }].Dump();
        dict[new { a = "A", b = "B", c = "C" }].Dump();
        dict[new { c = "D" }].Dump();
        dict[new MyFoo { a = 1, b = "two", c = 3.3 }].Dump();
        dict["c"].ForEach(o => o.Dump());
        dict.ContainsKey("c").Dump(); // true
        dict.ContainsKey("z").Dump(); // false
        //dict[new { c = 3.3 }].Dump(); // KeyNotFoundException
        //dict[new {a="A", b="B", c="C", d="D"}].Dump(); // KeyNotFoundException
    }
}