While working with Active Directory within SharePoint we probably don’t need to specify the domain or the root container. We can the current values. Here is a simple method from a console application just to demonstrate:

internal static void GetDomain()
{
	var context = new DirectoryContext(DirectoryContextType.Domain);
	var domain = Domain.GetDomain(context);
	Console.WriteLine("Full domain:");
	Console.WriteLine(domain.Name); //takana.local
	Console.WriteLine();
	Console.WriteLine("root container");
	var parts = domain.Name.Split(new\[\] {"."}, StringSplitOptions.RemoveEmptyEntries);
	var dcParts = parts.Select(n => "dc=" + n).ToArray();
	var d = string.Join(",", dcParts); //dc=takana, dc=local
	Console.WriteLine(d);
}

First we get get the full domain, then we split and join them again.