There's no to use Group (and probably other aggregations) with anonymous types to generate a projection for the discriminator (i.e. _t).

For example this works:

collection.Aggregate.Group(x => new { Name = x.Name }, g => new { Count = g.Count() }

But grouping on the document type:

collection.Aggregate.Group(x => new { Type = x.GetType }, g => new { Count = g.Count() }

throws an exception with the following message:

GetType of type System.Object is not supported in the expression tree document.GetType()