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

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

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

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

var mgr = new DomainModelEntityManager();

var query = mgr.Customers.GroupBy(c => c.Country, (s, c) => new { Key=s, Count=c.Count()});
foreach (var group in query) {
  Console.WriteLine("key = " + group.Key);
  Console.WriteLine("count=" + group.Count);
}