Using Dynamic Properties to Access Values in a Dynamic Entity

Answer:

Dynamic Properties!

Here is a NUnit test I wrote to confirm my approach:

First, the C#:

/// <summary>AggregateQuery Test</summary>

/// <remarks>

/// Demonstrates an aggregate query using DynamicTypes

/// and creation of a dynamic property so that we can

/// data bind to the column (field) returned from the query.

/// </remarks>

[Test]

 public void AggregateQueryTest() {

   Type DynType;

   DynType = DynamicEntityTypeBuilder.CreateType("DynTypeTest", "Default");

   // Query for count of Employees with a LastName before 'K'

   IEntityQuery qry;

   qry = new PassthruRdbQuery(DynType,

     "SELECT count(*) as TotalEmps FROM Employee WHERE LastName < 'K'");

   EntityList<DynamicEntity> entities =

     PersistenceManager.DefaultManager.GetEntities<DynamicEntity>(qry);

   // Add a property descriptor to get a column we want through data binding

   AddTotalEmpsPropertyDescriptor(DynType, entities);

 }

 /// <summary>

 /// Add a "TotalEmps" dynamic property for a dynamic type

 /// with an integer "TotalEmps" field.

 /// </summary>

 private void AddTotalEmpsPropertyDescriptor(Type pDynType,

   EntityList<DynamicEntity> pEntities) {

   // This MUST become a private property list

   // because the next time an instance of

   // DynamicEntity will have completely different columns

   // and we don't want the pd we're about to add to be

   // hanging around!

   pEntities.UsesGlobalPropertyDescriptors = false;

   // Make a dynamic property descriptor for TotalEmps

   AdaptedPropertyDescriptor pd =

     new AdaptedPropertyDescriptor("TotalEmps", pDynType, typeof(int), _get_TotalEmps, null);

   // Add it to the list's PropertyDescriptors

   pEntities.PropertyDescriptors.Add(pd);

   // Confirm that the "getter" delegate returns the same value

   // as the field inside the entity.

   // That means it will work when we data bind to it.

   Assert.AreEqual(pEntities[0]["TotalEmps"], _get_TotalEmps(pEntities[0]));

 }

 /// <summary>

 /// The "getterdelegate" used within a dynamic property for a dynamic type

 /// with an integer "TotalEmps" field.

 /// </summary>

 private object _get_TotalEmps(object pEntity) {

   return (int) ((DynamicEntity) pEntity)["TotalEmps"];

 }

Now the VB (which I have converted but not tested):

''' <summary>AggregateQuery Test</summary>

''' <remarks>

''' Demonstrates an aggregate query using DynamicTypes

''' and creation of a dynamic property so that we can

''' data bind to the column (field) returned from the query.

''' </remarks>

<Test> _

Public Sub AggregateQueryTest()

   Dim DynType As Type

   DynType = DynamicEntityTypeBuilder.CreateType("DynTypeTest", "Default")

   ' Query for count of Employees with a LastName before 'K'

   Dim qry As IEntityQuery

   qry = New PassthruRdbQuery(DynType, _

    "SELECT count(*) as TotalEmps FROM Employee WHERE LastName < 'K'")

  Dim entities As EntityList(Of DynamicEntity) = _

    PersistenceManager.DefaultManager.GetEntities(Of DynamicEntity)(qry)

   ' Add a property descriptor to get a column we want through data binding

   AddTotalEmpsPropertyDescriptor(DynType, entities)

End Sub

 ''' <summary>

 ''' Add a "TotalEmps" dynamic property for a dynamic type

 ''' with an integer "TotalEmps" field.

 ''' </summary>

 Private Sub AddTotalEmpsPropertyDescriptor( _

   ByVal pDynType As Type, ByVal pEntities As EntityList(Of DynamicEntity))

   ' This MUST become a private property list

   ' because next time an instance of

   ' DynamicEntity will have completely different columns

   ' and we don't want the pd we're about to add to be

   ' hanging around!

   pEntities.UsesGlobalPropertyDescriptors = False

   ' Make a dynamic property descriptor for TotalEmps

   Dim pd As New AdaptedPropertyDescriptor( _

    "TotalEmps", pDynType, GetType(Integer), AddressOf _get_TotalEmps, Nothing)

   ' Add it to the list's PropertyDescriptors

   pEntities.PropertyDescriptors.Add(pd)

   ' Confirm that the "getter" delegate returns the same value

   ' as the field inside the entity.

   ' That means it will work when we data bind to it.

   Assert.AreEqual(pEntities(0)("TotalEmps"), _get_TotalEmps(pEntities(0)))

 End Sub

 ''' <summary>

 ''' The "getterdelegate" used within a dynamic property for a dynamic type

 ''' with an integer "TotalEmps" field.

 ''' </summary>

 Private Function _get_TotalEmps(ByVal pEntity As Object) As Object

   Return CInt(Fix((CType(pEntity, DynamicEntity))("TotalEmps")))

 End Function

Enjoy !