Reuse the same generic EmptyMapClass instance across the project

[bahusoid]

This is simple "find and replace" kind of pull request to eliminate not needed instantiating of immutable EmptyMapClass instances.

Added CollectionHelper.EmptyDictionary<TKey, TValue>() helper mehtod that returns the same instance for given TKey and TValue and use it across the project instead of creating new instances.

[hazzik]

Please rename to EmptyMap or EmptyDictionary

[bahusoid]

@hazzik Done. I've also moved Instance to separate class - EmptyDictionaryHolder<TKey, TValue>. As EmptyMapClass<TKey, TValue> is public class and it's better to avoid unnecessary static initialization if it wasn't requested explicitly.

[bahusoid]

I didn't touch Test project. Let me know if it also needs to be updated (for consistency or whatever reasons)

[fredericDelaporte]

I didn't touch Test project. Let me know if it also needs to be updated (for consistency or whatever reasons)

Yes better update it too for consistency. Especially if doing my proposed changes, of course! (Release build would fail otherwise.)

[fredericDelaporte]

I would rather have it directly on EmptyMapClass. So here it would be

return EmptyMapClass<TKey, TValue>.Instance;

[fredericDelaporte]

I would move this into EmptyMapClass but changed to internal.

And then I would introduced an obsolete default public constructor for avoiding future code re-instanciating it directly.
So EmptyMapClass would start this way:

/// <summary>
/// A read-only dictionary that is always empty and permits lookup by <see langword="null" /> key.
/// </summary>
[Serializable]
public class EmptyMapClass<TKey, TValue> : IDictionary<TKey, TValue>
{
#pragma warning disable 618 // Constructor is obsolete, to be switched to non-obsolete but private.
	internal static readonly IDictionary<TKey, TValue> Instance = new EmptyMapClass<TKey, TValue>();
#pragma warning restore 618

	// Since v5.1. To be switched to private.
	[Obsolete("Please use CollectionHelper.EmptyDictionary<TKey, TValue>() instead.")]
	public EmptyMapClass() {}

	private static readonly EmptyEnumerator<TKey, TValue> emptyEnumerator = new EmptyEnumerator<TKey, TValue>();

...

[bahusoid]

@fredericDelaporte Ok. Did as you suggested