Send Email error

I have a custom entity A that has one to many releation with custom entity B, Custom entity A record and Custom Entity B records are in inactive state, When user clicks on custom ribbon button in javascript I change state of custom entity A and updates a datetime attribute e.g. activated childs on. which triggers a plugin which is running under service account, in the plugin I have logic to ignore which Custom entity B records to activate, as I am using solutions concept, this logic is on top level component, to pass these values I used shared variable and passed it list of guids of custom entity B to ignore.

on custom entity a, when plugin trigger it looks for shared variable and ignores the custom entity b records which are mentioned and then activates rest of the records.

on activate of custom entity b, i have another plugin which sends email.

and I was getting this error in this case. when whole above scenario is executed. when I simple use activate OOB button email sent is fine, but not when whole custom process is executed.

The whole above process works fine in my dev env but when deployed into system test I got following error.

The Web Service plug-in failed in OrganizationId: xxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx; SdkMessageProcessingStepId: xxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx; EntityName: email; Stage: 30; MessageName: Send; AssemblyName: Microsoft.Crm.Extensibility.InternalOperationPlugin, Microsoft.Crm.ObjectModel, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35; ClassName: Microsoft.Crm.Extensibility.InternalOperationPlugin; Exception: Unhandled Exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
   at System.RuntimeMethodHandle._InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeType typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Web.Services.Protocols.LogicalMethodInfo.Invoke(Object target, Object[] values)
   at Microsoft.Crm.Extensibility.InternalOperationPlugin.Execute(IServiceProvider serviceProvider)
   at Microsoft.Crm.Extensibility.V5PluginProxyStep.ExecuteInternal(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.VersionedPluginProxyStepBase.Execute(PipelineExecutionContext context)
Inner Exception: System.Runtime.Serialization.SerializationException: Type 'System.Collections.Generic.List`1[[System.Guid, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]' with data contract name 'ArrayOfguid:http://schemas.microsoft.com/2003/10/Serialization/Arrays' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiType(XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle objectTypeHandle, Type objectType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerializeReference(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at WriteKeyValuePairOfstringanyTypeToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , ClassDataContract )
   at System.Runtime.Serialization.ClassDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at WriteParameterCollectionToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , CollectionDataContract )
   at System.Runtime.Serialization.CollectionDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerializeReference(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at WriteAsyncOperationDataToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , ClassDataContract )
   at System.Runtime.Serialization.ClassDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerializeReference(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at WriteAsyncOperationDataToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , ClassDataContract )
   at System.Runtime.Serialization.ClassDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerializeReference(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at WriteAsyncOperationDataToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , ClassDataContract )
   at System.Runtime.Serialization.ClassDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
   at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
   at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
   at System.Runtime.Serialization.DataContractSerializer.WriteObject(XmlWriter writer, Object graph)
   at Microsoft.Crm.Extensibility.AsynchronousStep.SerializeAsyncData(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.AsynchronousStep.Execute(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.SystemUserProxy.Execute(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.Pipeline.Execute(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.MessageProcessor.Execute(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.InternalMessageDispatcher.Execute(PipelineExecutionContext context)
   at Microsoft.Crm.Extensibility.ExtensiblePlatformMessageDispatcher.Execute(PipelineExecutionContext pluginContext)
   at Microsoft.Crm.Extensibility.ExtensiblePlatformMessageDispatcher.UpdateWithInvocationSource(BusinessEntity entity, FilterExpression filter, Int32 invocationSource, ExecutionContext context)
   at Microsoft.Crm.Extensibility.ExtensiblePlatformMessageDispatcher.Update(BusinessEntity entity, FilterExpression filter, ExecutionContext context)
   at Microsoft.Crm.BusinessEntities.BusinessProcessObject.UpdateWithPipelineAndExtensions(IBusinessEntity entity, ExecutionContext context)
   at Microsoft.Crm.ObjectModel.GenericActivityServiceBase.UpdateInternal(IBusinessEntity entityInterface, ExecutionContext context)
   at Microsoft.Crm.ObjectModel.CommunicationActivityServiceBase.Update(IBusinessEntity entityInterface, ExecutionContext context)
   at Microsoft.Crm.ObjectModel.EmailService.UpdateHelper(IBusinessEntity entityInterface, ExecutionContext context, Boolean internalUpdateCall)
   at Microsoft.Crm.ObjectModel.EmailService.Send(Guid emailId, Boolean issueSend, String trackingToken, ExecutionContext context)

Resolution: Changed the Generic Collection from List<Guid> to array of []Guid, solution seems simple but it took me more than a day to track down and figure out that this was causing problem.