<ExtensionAttribute()>
Public Overloads Shared Function GroupBy
    (Of TSource,TKey)( _
   ByVal source As IEntityQuery(Of TSource), _
   ByVal keySelector As Expression(Of Func(Of TSource,TKey)) _
) As IEntityQuery(Of IGrouping(Of TKey,TSource))

Dim source As IEntityQuery(Of TSource)
Dim keySelector As Expression(Of Func(Of TSource,TKey))
Dim value As IEntityQuery(Of IGrouping(Of TKey,TSource))
 
value = EntityQueryExtensions.GroupBy(Of TSource, TKey)(source, keySelector)

[ExtensionAttribute()]
public static IEntityQuery<IGrouping<TKey,TSource>> GroupBy<TSource,TKey>( 
   IEntityQuery<TSource> source,
   Expression<Func<TSource,TKey>> keySelector
)

[ExtensionAttribute()]
public:
static IEntityQuery<IGrouping<TKey^,TSource^>^>^ GroupBygeneric<typename TSource>
generic<typename TKey>
( 
   IEntityQuery<TSource^>^ source,
   Expression<Func<TSource^,TKey^>^>^ keySelector
) 

var mgr = new DomainModelEntityManager();

var query = mgr.Customers.GroupBy(c => c.Country);
foreach (var group in query) {
  Console.WriteLine("key = " + group.Key);
  foreach (var cust in group) {
    Console.WriteLine("Cust = " + cust.CompanyName);
  }
}