Exception: DataAnalysis.Reference+<>c__DisplayClass4 is not serializable

I am trying to serialize objects of class Reference at the end of my program. A serialization exception is thrown, which complains that "DataAnalysis.Reference+<>c__DisplayClass4" is not marked as serializable.

Initially I had the two delegates without the Serializable attribute, so I gave it a try, but it didn't change anything. The classes Cacheable and Operation are already marked as Serializable - and in fact the serialization of the both of them worked perfectly fine before I introduced the Reference class.

I don't even know what c__DisplayClass4 means. So I am sorry, but I don't know what other parts of my 1 megabytes+ source code to post here to help you solve the problem, because in the end I would be posting everything.

As I said, everything worked fine before introducing the Reference class. So I am hoping the problem is somehow localized to it.

using System;
using System.Reflection;

namespace DataAnalysis
{
    /// <summary>
    /// Description of Reference.
    /// </summary>
    [Serializable]
    public class Reference
    {
        [Serializable]
        public delegate void ReferenceSetter(Operation op, Cacheable c);

        [Serializable]
        public delegate Cacheable ReferenceGetter(Operation op);

        readonly ReferenceGetter refGetter;
        readonly ReferenceSetter refSetter;

        public Reference(ReferenceGetter getter, ReferenceSetter setter)
        {
            refGetter = getter;
            refSetter = setter;
        }

        public Reference(FieldInfo operationField)
        {
            refGetter = (op => (Cacheable)operationField.GetValue(op));
            refSetter = ((op, value) => operationField.SetValue(op, value));
        }

        public Cacheable this[Operation op]
        {
            get {return refGetter(op);}
            set {refSetter(op, value);}
        }
    }
}