gistfile1.cs

public static DataTable ToDataTable<TKey, T>(this IEnumerable<IGrouping<TKey, T>> data, string keyName = "Key")
    {
        var properties = TypeDescriptor.GetProperties(typeof(T));
        var table = new DataTable();
        
        table.Columns.Add(keyName, GetRealType(typeof(TKey)));
        
        foreach (PropertyDescriptor prop in properties)
            table.Columns.Add(prop.Name, GetRealType(prop.PropertyType));
            
        foreach (IGrouping<TKey, T> group in data)
        {
          foreach (T item in group.Items)
          {
              DataRow row = table.NewRow();
              row[keyName] = group.Key ?? DBNull.Value;
              
              foreach (PropertyDescriptor prop in properties)
                   row[prop.Name] = prop.GetValue(item) ?? DBNull.Value;
                   
              table.Rows.Add(row);
          }
          
        }
        return table;
    }
    
    private static Type GetRealType(Type t)
    {
       return Nullable.GetUnderlyingType(t) ?? t;
    }