CA2239: Provide deserialization methods for optional fields
Applies to: 
Visual Studio 
Visual Studio for Mac
Note
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| Item | Value |
|---|---|
| RuleId | CA2239 |
| Category | Microsoft.Usage |
| Breaking change | Non-breaking |
Cause
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.
Rule description
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.
How to fix violations
To fix a violation of this rule, add de-serialization event handling methods to the type.
When to suppress warnings
It is safe to suppress a warning from this rule if the field should be ignored during the de-serialization process.
Example
The following example shows a type with an optional field and de-serialization event handling methods.
using System;
using System.Reflection;
using System.Runtime.Serialization;
[assembly: AssemblyVersionAttribute("2.0.0.0")]
namespace UsageLibrary
{
[SerializableAttribute]
public class SerializationEventHandlers
{
[OptionalFieldAttribute(VersionAdded = 2)]
int optionalField = 5;
[OnDeserializingAttribute]
void OnDeserializing(StreamingContext context)
{
optionalField = 5;
}
[OnDeserializedAttribute]
void OnDeserialized(StreamingContext context)
{
// Set optionalField if dependent on other deserialized values.
}
}
}
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
Related rules
CA2236: Call base class methods on ISerializable types
CA2240: Implement ISerializable correctly
CA2229: Implement serialization constructors
CA2238: Implement serialization methods correctly
CA2235: Mark all non-serializable fields