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CA2239: Provide deserialization methods for optional fields

TypeName

ProvideDeserializationMethodsForOptionalFields

CheckId

CA2239

Category

Microsoft.Usage

Breaking Change

Non Breaking

A type has a field that is marked with the System.Runtime.Serialization.OptionalFieldAttribute attribute and the type does not provide de-serialization event handling methods.

The OptionalFieldAttribute attribute has no effect on serialization; a field marked with the attribute is serialized. However, the field is ignored on de-serialization and retains the default value associated with its type. De-serialization event handlers should be declared to set the field during the de-serialization process.

To fix a violation of this rule, add de-serialization event handling methods to the type.

It is safe to suppress a warning from this rule if the field should be ignored during the de-serialization process.

The following example shows a type with an optional field and de-serialization event handling methods.

Imports System
Imports System.Reflection
Imports System.Runtime.Serialization

<Assembly: AssemblyVersionAttribute("2.0.0.0")>
Namespace UsageLibrary

   <SerializableAttribute> _ 
   Public Class SerializationEventHandlers

      <OptionalFieldAttribute(VersionAdded := 2)> _ 
      Dim optionalField As Integer = 5

      <OnDeserializingAttribute> _ 
      Private Sub OnDeserializing(context As StreamingContext)
         optionalField = 5
      End Sub

      <OnDeserializedAttribute> _ 
      Private Sub OnDeserialized(context As StreamingContext)
         ' Set optionalField if dependent on other deserialized values. 
      End Sub 

   End Class 

End Namespace
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