StrataFrame Forum » StrataFrame Application Framework - V1 » Business Objects and Data Access (How do I?) » Using Nullable in BO

Using Nullable in BO

Kevin Lingofelter — Sat, 25 Mar 2006 11:26:22 GMT

Hello All!

I am setting up some fields in the mapper to Use Nullable Generic. This setting works ok if the field is a numeric type, but if the field is varchar, the code the mapper generates is causing the build to fail. Here is the code for the field:

''' <summary>
''' Test
''' </summary>
''' <remarks></remarks>
<Browsable(False), _
BusinessFieldDisplayInEditor(), _
Description("Test"), _
DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)> _
Public Property [Test]() As Nullable(Of System.String)
Get
    Dim loValue As Object
    loValue = Me.CurrentRow.Item("Test")
    If loValue Is DBNull.Value Then
        Return CType(Nothing, Nullable(Of System.String))
    Else
        Return CType(loValue, Nullable(Of System.String))
    End If
End Get
Set(ByVal value As Nullable(Of System.String))
    If value.HasValue Then
      Me.CurrentRow.Item("Test") = value.Value
    Else
      Me.CurrentRow.Item("Test") = DBNull.Value
    End If
End Set
End Property

The hilighted code is giving the error:

Error 1 Type argument 'String' does not satisfy the 'Structure' constraint for type parameter 'T' ...

Anyone know why this is? Thanks!

RE: Using Nullable in BO

Trent L. Taylor — Sat, 25 Mar 2006 11:26:22 GMT

Really the same concept of any other type of generic...saving time when applicable without having to explicitly define a specific class or value while remaining strong-typed. That is why they are dubbed "generics"...the same class can be used interchangably for different value types.

RE: Using Nullable in BO

Kevin Lingofelter — Fri, 24 Mar 2006 18:18:34 GMT

Sounds good. What's the advantage of using the nullable generic type then?

RE: Using Nullable in BO

Trent L. Taylor — Fri, 24 Mar 2006 16:19:25 GMT

Kevin,

Only structure data types support a generic nullable, and a string is not a structure data type. To determine what does and does not support a nullable generic, you can reference the object browse and look at the inheritance. Anything that inherits from a System.ValueType can be used with a nullable generic. Here is the rule of thumb, if Nothing or NULL can be assigned to it, it does not support a nullable generic.

When nullable generic is not supported, use Return Alternate on NULL option for the field and specify the exact text below that should be returned. For example, if you want to return an empty string, enter the following value: ""

RE: Using Nullable in BO

Robert Linton — Fri, 24 Mar 2006 15:02:42 GMT

Error in above post...

String are reference types so you can not assign them as nullable

RE: Using Nullable in BO

Robert Linton — Fri, 24 Mar 2006 15:01:04 GMT

Hi Kevin,

Any value type, including struct can be nullable: Except reference types. Strings are reference types so you can configure your BO string types as nullable.

I'm new to SF...so this may not be entirely correct, but I've been using the "replace..." option and returning a " " instead.

Hope that helps,

Rob