Question

I’m writing a generic filter class that will be primarily used to filter persons from a population. I’m trying to serialize the filter class, but at runtime I get an SerializationException:

System.Runtime.Serialization.SerializationException : Type 'System.DelegateSerializationHolder+DelegateEntry' with data contract name 'DelegateSerializationHolder.DelegateEntry:http://schemas.datacontract.org/2004/07/System' 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.

My filter class looks like this:

[DataContract(Name = "Filter", Namespace = "")]
    public class Filter<T>
    {

        /// <summary>
        /// Default constructor, needed by serializers.
        /// </summary>
        public Filter()
        {
            Name = "New filter";
            Predicates = new List<Predicate<T>>();
        }

        /// <summary>
        /// ctor. Takes a list of predicates for type T
        /// to filter with.  
        /// </summary>
        public Filter(string name, IEnumerable<Predicate<T>> predicates)
        {
            Name = name;
            Predicates = predicates.ToList();
        }

        /// <summary>
        /// Name of the filter. 
        /// </summary>
        [DataMember(Order = 0)]
        public string Name
        {
            get;
            set;
        }

        [DataMember(Order = 1)]
        public List<Predicate<T>> Predicates
        {
            get;
            set;
        }

        /// <summary>
        /// Filters sequence of type T. 
        /// </summary>
        public IEnumerable<T> ApplyFilter(IEnumerable<T> input)
        {
            var result = new List<T>(input);
            return Predicates.Aggregate(result, (current, predicate) => current.FindAll(predicate));
        }
    }

When serializing the filter class, I get the above exception. If I don’t mark the Predicate as a DataMember, then it works. But obviously I want to serialize that property as well.

I’ve been at this for a couple of hours now, and I can’t figure it out. Any helps would be really appreciated!

Answer 1

DataContractSerializer is not intended for serializing delegates; a multi-cast delegate is a set of arbitrary target objects and methods; this is not a well-defined data-oriented contract. DataContractSerializer is intended for data.

Either serialize some form of expression as a string (or some simple tree), or use a different serializer.