ConditionalWeakTable, what does it do?

C#'s ConditionalWeakTable, what does it do?

C# has many lesser known features, some more useful than others, and one of them is the ConditionalWeakTable<TKey, TValue> (keep in mind that TKey and TValue must be reference types).
You can think of this type as a dictionary where the keys are weakly referenced, meaning that they won’t count when the GC checks if the object has to be collected. Additionally, when the keys do eventually get collected by the GC, that entry will get removed from the dictionary. This means that you can attach arbitrary objects to any object, allowing you to do something like this:

public static class Extensions
{
    private static ConditionalWeakTable<object, dynamic> Table = new ConditionalWeakTable<object, dynamic>();

    public static dynamic Data(this object obj)
    {
        if (!Table.TryGetValue(obj, out var dyn))
            Table.Add(obj, dyn = new ExpandoObject());

        return dyn;
    }
}

...

var myObject = "hello";
myObject.Data().Foo = "bar";

Assert.AreEqual(myObject.Data().Foo, "bar");

As you can see, this is a really powerful feature allowing you to add any data you want to any object, although it may also lead to some ugly code so beware! If you’re still not convinced, take a look at a StringBuilder extension class I built for a project:

public static class StringBuilderExtensions
{
    private class StringBuilderData
    {
        public int Indentation;
    }

    private static ConditionalWeakTable<StringBuilder, StringBuilderData> DataTable = new ConditionalWeakTable<StringBuilder, StringBuilderData>();

    public static StringBuilder AppendIndentation(this StringBuilder builder)
    {
        return builder.Append(new string(' ', DataTable.GetOrCreateValue(builder).Indentation * 4));
    }

    public static StringBuilder Indent(this StringBuilder builder)
    {
        DataTable.GetOrCreateValue(builder).Indentation++;
        return builder;
    }

    public static StringBuilder Unindent(this StringBuilder builder)
    {
        DataTable.GetOrCreateValue(builder).Indentation--;
        return builder;
    }
}

Which you would use like:

var builder = new StringBuilder();

builder.AppendIndentation().AppendLine("This is unindented")
       .Indent()
       .AppendIndentation().AppendLine("This is indented by one level")
       .Unindent()
       .AppendIndentation().AppendLine("This is unindented again");

Yes, you could just write your own StringBuilder-esque class, but where’s the fun in that!

This type is clearly not meant for day-to-day use but I’m sure there’s at least one situation where using this type is the best solution. One place where I think it could be useful is if you’re patching methods at runtime with something like pardeike’s Harmony and you want to store additional info on a patched method’s instance object.