I spent some time earlier this week trying
to fix a SerializationException in some .NET code.
The fix ultimately turned out to be quite simple,
but it took a while for me to understand what the exception message
was actually telling me.
We were seeing occasional SerializationExceptions in our logs,
with callstacks like this:
System.Runtime.Serialization.SerializationException: The constructor
to deserialize an object of type 'Cozi.WebClient.AccountTagsMap'
was not found.
at System.Runtime.Serialization.ObjectManager.GetConstructor(
Type t, Type[] ctorParams)
at System.Runtime.Serialization.ObjectManager.CompleteISerializableObject(
Object obj, SerializationInfo info, StreamingContext context)
--- End of inner exception stack trace ---
at System.Runtime.Serialization.ObjectManager.CompleteISerializableObject(
Object obj, SerializationInfo info, StreamingContext context)
at System.Runtime.Serialization.ObjectManager.FixupSpecialObject(ObjectHolder holder)
at System.Runtime.Serialization.ObjectManager.DoFixups()
at System.Runtime.Serialization.Formatters.Binary.ObjectReader.Deserialize(
HeaderHandler handler,__BinaryParser serParser, Boolean fCheck,
Boolean isCrossAppDomain, IMethodCallMessage methodCallMessage)
at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Deserialize(
Stream serializationStream, HeaderHandler handler, Boolean fCheck,
Boolean isCrossAppDomain, IMethodCallMessage methodCallMessage)
at System.Web.Util.AltSerialization.ReadValueFromStream(BinaryReader reader)
at System.Web.SessionState.SessionStateItemCollection.ReadValueFromStreamWithAssert()
at System.Web.SessionState.SessionStateItemCollection.DeserializeItem(
String name, Boolean check)
at System.Web.SessionState.SessionStateItemCollection.DeserializeAllItems()
at System.Web.SessionState.SessionStateItemCollection.get_Keys()
at System.Web.SessionState.HttpSessionStateContainer.get_Keys()
at System.Web.SessionState.HttpSessionState.get_Keys()
...
where AccountTagsMap looked like this:
[Serializable]
public class AccountTagsMap : Dictionary<string, DateTime>
{
// Various irrelevant methods, but no additional data
// and no constructors
}
It needed to be marked [Serializable] as it was sometimes being stashed in
HttpContext.Current.Session.
Understandably, I assumed from the error message that simply adding
a default constructor, AccountTagsMap(), would fix the problem.
It took me some time, with the help of Reflector,
to realize that I also needed to add a serialization constructor:
[Serializable]
public class AccountTagsMap : Dictionary<string, DateTime>
{
public AccountTagsMap() : base() { }
protected AccountTagsMap(
SerializationInfo info, StreamingContext context)
: base(info, context) {}
// ...
}
This isn't a full implementation of serialization.
That's handled by the base class, Dictionary<K,V>.
We wrongly assumed that inheriting from Dictionary was all we needed to do.
Here's some test code that demonstrates the problem.
Comment out the serialization constructor to see the tests fail.
using System;
using System.Collections.Generic;
using System.Reflection;
using System.Runtime.Serialization;
using System.Runtime.Serialization.Formatters.Binary;
using System.IO;
using NUnit.Framework;
namespace Cozi.Test
{
[TestFixture]
public class MapSerializationTests
{
[Test]
public void SerializationCtor()
{
Assert.IsNotNull(
SerializationTester.GetConstructor(
typeof(AccountTagsMap)));
}
[Test]
public void SerializationRoundtrip_AccountTagsMap()
{
AccountTagsMap map1 = new AccountTagsMap();
map1["foo"] = DateTime.Now;
map1["bar"] = DateTime.Now;
AccountTagsMap map2 = SerializationTester.RoundTrip(map1);
Assert.AreEqual(map1["foo"], map2["foo"]);
Assert.AreEqual(map1["bar"], map2["bar"]);
}
}
public static class SerializationTester
{
// http://msdn.microsoft.com/en-us/library/ms182342(v=vs.80).aspx
public static T RoundTrip<T>(T value)
{
if (value == null)
throw new ArgumentNullException("value");
using (MemoryStream stream = new MemoryStream())
{
BinaryFormatter formatter = new BinaryFormatter();
formatter.Serialize(stream, value);
stream.Seek(0, SeekOrigin.Begin);
return (T)formatter.Deserialize(stream);
}
}
// Reverse-engineered from System.Runtime.Serialization
// .ObjectManager.GetConstructor with Reflector
public static ConstructorInfo GetConstructor(Type t)
{
ConstructorInfo info = t.GetConstructor(
BindingFlags.NonPublic | BindingFlags.Public
| BindingFlags.Instance,
null,
CallingConventions.Any,
new Type[] { typeof(SerializationInfo),
typeof(StreamingContext) },
null) as ConstructorInfo;
if (info == null)
{
throw new SerializationException(
String.Format(
"Serialization_ConstructorNotFound: {0}",
new object[] { t.FullName }));
}
return info;
}
}
}
I hope this saves you some time.
