Default Property Resolution through Helper

Visual Basic 6 presents early, late and/or dynamic binding for objects and variables; these concepts refer to the time when that specific object or variable type is defined. As per Early Binding it can be said that an object is early bound when it is assigned to a variable declared to be of a specific object type.In contrast to this concept Late Binding is the linking of a routine or object at runtime based on the conditions at the moment. A common issue found by the VB6 AI Migrator is the inability to resolve default properties in late binding cases.

In a nutshell, VB6 is able to resolve during runtime the type of any variable, and if it is necessary, to expand the default property of that type in order to get the correct value to be used. In the following example you can see that the variable ‘o’ is an Object, which must have a default property; at runtime VB6 expands its corresponding default property and gets or sets the value.

The VB6 AI Migrator is able to solve the late binding conflicted default properties by means of a helper class called “DefaultPropHelper”

Original VB6 Code:

Private Sub Foo(ByVal o As Object)
    Dim v As Variant
    o = "Value for the Default Property"
    v = o
End Sub

Resulting VB.NET Code (no DefaultPropHelper):

Public Sub Foo(ByVal o As Object)
    'UPGRADE_WARNING: (1037) Couldn't resolve default property of object o.
    o = "Value for the Default Property"
    'UPGRADE_WARNING: (1068) o of type Object is being forced to Scalar.
    Dim v As Object = o
End Sub

Resulting VB.NET Code (using DefaultPropHelper):

Public Sub Foo(ByVal o As Object)
    DefaultPropHelper.SetDefaultProperty(o, "Value for the Default Property")
    Dim v As Object = DefaultPropHelper.GetDefaultProperty(o)
End Sub

Resulting C#.NET Code (no DefaultPropHelper):

private void  Foo( object o)
{
    //UPGRADE_WARNING: (1037) Couldn't resolve default property of object o.
    o = " Value for the Default Property";
    //UPGRADE_WARNING: (1068) o of type object is being forced to string.
    string v = o;
}

Resulting .NET Code (using DefaultPropHelper):

private void  Foo( object o)
{
    DefaultPropHelper.SetDefaultProperty(ref o, "Value for the Default Property");
    string v = DefaultPropHelper.GetDefaultProperty<string>(o);
}