Put a new index before deleting the old one

RavenIndexHandler.cs

public interface IIndexHandler
{
	void CreateIndexes();
}

public class RavenIndexHandler : IIndexHandler
{
	private readonly IDocumentSession _session;
	private IndexNameMap _map;

	public RavenIndexHandler(IDocumentSession session, IndexNameMap map)
	{
		_session = session;
		_map = map;
	}

	public void CreateIndexes()
	{
		var productIndex = new ProductIndex();
		var productIndexName = productIndex.IndexName;
		var catalog = new CompositionContainer(new AssemblyCatalog(typeof(ProductIndex).Assembly));
		var tasks = catalog.GetExportedValues<AbstractIndexCreationTask>();
		foreach (var task in tasks)
		{
			//skip just the index(es) that need special attention
			if (task.IndexName == productIndexName)
				continue;

			task.Execute(_session.Advanced.DocumentStore);
		}

		var newName = "";
		var definition = productIndex.CreateIndexDefinition();
		using (var md5 = MD5.Create())
		{
			var bytes = md5.ComputeHash(definition.GetIndexHash());
			var hash = HttpServerUtility.UrlTokenEncode(bytes);
			newName = string.Format("{0}.{1}", productIndexName, hash);
		}
		
		var oldName = "";
		_map.IndexNames.TryGetValue(productIndexName, out oldName);

		if (oldName == newName) return; //no updates; we're done
		
		_session.Advanced.DocumentStore.DatabaseCommands.PutIndex(newName, definition, true); ;
		
		_map.IndexNames.AddOrUpdate(productIndexName, s => newName, (s, s1) => newName);
		_session.Store(_map); //probably not the same session it was loaded from
		_session.SaveChanges();

		if (!string.IsNullOrWhiteSpace(oldName))
		{
			_session.Advanced.DocumentStore.DatabaseCommands.DeleteIndex(oldName);
		}
	}
}

public interface IIndexNameProvider
{
	string GetIndexName<T>() where T : AbstractIndexCreationTask, new();
}

public class IndexNameMap : IIndexNameProvider
{
	public static string DocumentId { get { return "admin/indexes/map"; } }
	public string Id { get { return DocumentId; } }
	public ConcurrentDictionary<string, string> IndexNames { get; set; }

	public string GetIndexName<T>() where T : AbstractIndexCreationTask, new()
	{
		var name = new T().IndexName;
		return IndexNames.GetOrAdd(name, s => name);
	}

	public IndexNameMap()
	{
		IndexNames = new ConcurrentDictionary<string, string>();
	}
}