Can I Create a Dictionary of Generic Types?

I'd like to create a Dictionary object, with string Keys, holding values which are of a generic type. I imagine that it would look something like this:

Dictionary<string, List<T>> d = new Dictionary<string, List<T>>();  

And enable me to add the following:

d.Add("Numbers", new List<int>());
d.Add("Letters", new List<string>());

I know that I can do it for a list of strings, for example, using this syntax:

Dictionary<string, List<string>> d = new Dictionary<string, List<string>>();
d.Add("Key", new List<string>());

but I'd like to do it for a generic list if possible...

2 questions then:

1. Is it possible?
2. What's the syntax?

EDIT: Now I've reread the question...

You can't do this, but a custom collection would handle it to some extent. You'd basically have a generic Add method:

public void Add<T>(string key, List<T> list)

(The collection itself wouldn't be generic - unless you wanted to make the key type generic.)

You couldn't extract values from it in a strongly typed manner though, because the compiler won't know which type you've used for a particular key. If you make the key the type itself, you end with a slightly better situation, but one which still isn't supported by the existing collections. That's the situation my original answer was responding to.

EDIT: Original answer, when I hadn't quite read the question correctly, but which may be informative anyway...

No, you can't make one type argument depend on another, I'm afraid. It's just one of the things one might want to express in a generic type system but which .NET's constraints don't allow for. There are always going to be such problems, and the .NET designers chose to keep generics relatively simple.

However, you can write a collection to enforce it fairly easily. I have an example in a blog post which only keeps a single value, but it would be easy to extend that to use a list.

Would something like this work?

public class GenericDictionary
{
    private Dictionary<string, object> _dict = new Dictionary<string, object>();

    public void Add<T>(string key, T value) where T : class
    {
        _dict.Add(key, value);
    }

    public T GetValue<T>(string key) where T : class
    {
        return _dict[key] as T;
    }
}

Basically it wraps all the casting behind the scenes for you.

How about Dictionary<string, dynamic>? (assuming you're on C# 4)

Dictionary<string, dynamic> Dict = new Dictionary<string, dynamic>();

Source: https://stackoverflow.com/a/5038029/3270733

I prefer this way of putting generic types into a collection:

interface IList
{
  void Add (object item);
}

class MyList<T> : List<T>, IList
{
  public void Add (object item)
  {
    base.Add ((T) item); // could put a type check here
  }
}

class Program
{
  static void Main (string [] args)
  {
    SortedDictionary<int, IList>
      dict = new SortedDictionary<int, IList> ();

    dict [0] = new MyList<int> ();
    dict [1] = new MyList<float> ();

    dict [0].Add (42);
    dict [1].Add ("Hello"); // Fails! Type cast exception.
  }
}

But you do lose the type checks at compile time.

I came to a type safe implementation using ConditionalWeakTable.

public class FieldByType
{
    static class Storage<T>
        where T : class
    {
        static readonly ConditionalWeakTable<FieldByType, T> table = new ConditionalWeakTable<FieldByType, T>();

        public static T GetValue(FieldByType fieldByType)
        {
            table.TryGetValue(fieldByType, out var result);
            return result;
        }

        public static void SetValue(FieldByType fieldByType, T value)
        {
            table.Remove(fieldByType);
            table.Add(fieldByType, value);
        }
    }

    public T GetValue<T>()
        where T : class
    {
        return Storage<T>.GetValue(this);
    }

    public void SetValue<T>(T value)
        where T : class
    {
        Storage<T>.SetValue(this, value);
    }
}

It can be used like this:

/// <summary>
/// This class can be used when cloning multiple related objects to store cloned/original object relationship.
/// </summary>
public class CloningContext
{
    readonly FieldByType dictionaries = new FieldByType();

    public void RegisterClone<T>(T original, T clone)
    {
        var dictionary = dictionaries.GetValue<Dictionary<T, T>>();
        if (dictionary == null)
        {
            dictionary = new Dictionary<T, T>();
            dictionaries.SetValue(dictionary);
        }
        dictionary[original] = clone;
    }

    public bool TryGetClone<T>(T original, out T clone)
    {
        var dictionary = dictionaries.GetValue<Dictionary<T, T>>();
        if (dictionary == null)
        {
            clone = default(T);
            return false;
        }

        return dictionary.TryGetValue(original, out clone);
    }
}

See also this question where the type of the values is stored in as a generic parameter of the keys.

We're using lots of reflection to create an extensible administration tool. We needed a way to register items in the global search in the module definition. Each search would return results in a consistent way, but each one had different dependencies. Here's an example of us registering search for a single module:

public void ConfigureSearch(ISearchConfiguration config)
	{
		config.AddGlobalSearchCallback<IEmploymentDataContext>((query, ctx) =>
		{
			return ctx.Positions.Where(p => p.Name.Contains(query)).ToList().Select(p =>
				new SearchResult("Positions", p.Name, p.ThumbnailUrl,
					new UrlContext("edit", "position", new RouteValueDictionary(new { Id = p.Id }))
					));
		});
	}

In the background during module registration, we iterate over every module and add the Func to a SearchTable with an instance of:

public class GenericFuncCollection : IEnumerable<Tuple<Type, Type, Object>>
{
	private List<Tuple<Type, Type, Object>> objects = new List<Tuple<Type, Type, Object>>();

	/// <summary>
	/// Stores a list of Func of T where T is unknown at compile time.
	/// </summary>
	/// <typeparam name="T1">Type of T</typeparam>
	/// <typeparam name="T2">Type of the Func</typeparam>
	/// <param name="func">Instance of the Func</param>
	public void Add<T1, T2>(Object func)
	{
		objects.Add(new Tuple<Type, Type, Object>(typeof(T1), typeof(T2), func));
	}

	public IEnumerator<Tuple<Type, Type, object>> GetEnumerator()
	{
		return objects.GetEnumerator();
	}

	System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
	{
		return objects.GetEnumerator();
	}
}

Then when we finally call it, we do it with reflection:

var dependency = DependencyResolver.Current.GetService(search.Item1);
var methodInfo = search.Item2.GetMethod("Invoke");
return (IEnumerable<SearchResult>)methodInfo.Invoke(search.Item3, new Object[] { query, dependency });

I didn't find what I was looking for here but after reading I think it might be what is being asked for so an attempt to answer.

The problem is that when you use Dictionary it is a closed constructed type and all elements must be of the TValue type. I see this question in a number of places without a good answer.

Fact is that I want indexing but each element to have a different type and based on the value of TKey we already know the type. Not trying to get around the boxing but trying to simply get more elegant access something like DataSetExtensions Field. And don't want to use dynamic because the types are known and it is just not wanted.

A solution can be to create a non generic type that does not expose T at the class level and therefore cause the TValue part of the dictionary to be closed constructed. Then sprinkle in a fluent method to help initialization.

public class GenericObject
{
    private object value;

    public T GetValue<T>()
    {
        return (T)value;
    }

    public void SetValue<T>(T value)
    {
        this.value = value;
    }

    public GenericObject WithValue<T>(T value)
    {
        this.value = value;
        return this;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Dictionary<string, GenericObject> dict = new Dictionary<string, GenericObject>();

        dict["mystring"] = new GenericObject().WithValue<string>("Hello World");
        dict["myint"] = new GenericObject().WithValue<int>(1);

        int i = dict["myint"].GetValue<int>();
        string s = dict["mystring"].GetValue<string>();
    }
}

Other posibility it's to use the variable dynamic.

For example:

Dictionary<string, List<dynamic>> d = new Dictionary<string, List<dynamic>>(); d.Add("Key", new List<dynamic>());

the variable dynamic resolve the type on runtime.

No, but you can use object instead of generic type.

Long answer: The current version of C# will not allow you to make entries of generic type in a dictionary. Your options are either a) create a custom class that is the same as a dictionary except allow it to accept generic types, or b) make your Dictionary take values of type object. I find option b to be the simpler approach.

If you send lists of specific types, then when you go to process the lists you will have to test to see what kind of list it is. A better approach is to create lists of objects; this way you can enter integers, strings, or whatever data type you want and you don't necessarily have to test to see what type of object the List holds. This would (presumably) produce the effect you're looking for.

Here is a short console program that does the trick:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace dictionary
{
class Program
{
    static void Main(string[] args)
    {
        Dictionary<string, object> dic = new Dictionary<string, object>();
        var lstIntList = new List<object>();
        var lstStrings = new List<object>();
        var lstObjects = new List<object>();
        string s = "";

        lstIntList.Add(1);
        lstIntList.Add(2);
        lstIntList.Add(3);

        lstStrings.Add("a");
        lstStrings.Add("b");
        lstStrings.Add("c");

        dic.Add("Numbers", lstIntList);
        dic.Add("Letters", lstStrings);

        foreach (KeyValuePair<string, object> kvp in dic)
        {
            Console.WriteLine("{0}", kvp.Key);
            lstObjects = ((IEnumerable)kvp.Value).Cast<object>().ToList();

            foreach (var obj in lstObjects)
               {s = obj.ToString(); Console.WriteLine(s);}
            Console.WriteLine("");
        }


        Console.WriteLine("");
        Console.WriteLine("press any key to exit");
        Console.ReadKey();
    }//end main
}
}

One of the way is to create a Dictionary value with type "object" like:

Dictionary<string, object> d = new Dictionary<string, object>();

So, here object datatype is used as a generic datatype, you can put anything in this as a value.

Or it's possible to use generic Type like this

public static void SafeUpdateInDictionary<T, L>(T DictionaryToBeUpdated, string Key, L Value) where T : Dictionary<string, L>
    {
        if (DictionaryToBeUpdated != null)
        {
            if(Value != null)
            {
                if (!DictionaryToBeUpdated.ContainsKey(Key))
                    DictionaryToBeUpdated.Add(Key, Value);
                else
                    DictionaryToBeUpdated[Key] = Value;
            }
        }
    }