void Main()
{
	var mongo = new MongoDB.Driver.MongoClient("mongodb://test.test.local");
	var db = mongo.GetDatabase("TEST");
	var coll = db.GetCollection<TestClass>("TestColl");


	//Try #1
	Console.WriteLine(coll.Aggregate().Group(g => g.P1, p => new OutTestClass1()
	{
		P1 = p.Key,
		P2 = p.Max(pp => pp.P2),
		P3 = p.Select(i => i.P3).Distinct().ToArray()
	}).ToString());
	
	//Try #2
	Console.WriteLine(coll.Aggregate().Group(g => g.P1, p => new OutTestClass1()
	{
		P1 = p.Key,
		P2 = p.Max(pp => pp.P2),
		P3 = new HashSet<string>(p.Select(i => i.P3)).ToArray()
	}).ToString());

	//Try #3
	Console.WriteLine(coll.Aggregate().Group(g => g.P1, p => new OutTestClass2()
	{
		P1 = p.Key,
		P2 = p.Max(pp => pp.P2),
		P3 = new HashSet<string>(p.Select(i => i.P3))
	}).ToString());
}

public class TestClass
{
	public string P1 { get; set; }
	public string P2 { get; set; }
	public string P3 { get; set; }
}

public class OutTestClass1
{
	public string P1 { get; set; }
	public string P2 { get; set; }
	public string[] P3 { get; set; }
}

public class OutTestClass2
{
	public string P1 { get; set; }
	public string P2 { get; set; }
	public HashSet<string> P3 { get; set; }
}