System.InvalidCastException "Object must implement IConvertible." when deserializing a Dictionary using a BinaryFormatter

Question

[Serializable]
public class CustomFields : MyObject, ICustomFields // MyObject is marked as ISerializable
{
    private IDictionary<string, string> _customFieldDictionary;

    public CustomFields( ) {
        this._customFieldDictionary = new Dictionary<string, string>( );
    }

    protected CustomFields( SerializationInfo info, StreamingContext context )
        : base( info, context )
    {
        if( info == null )
            throw new System.ArgumentNullException( "info" );

        // Following line throws
        this.CustomFieldDictionary = ( Dictionary<string, string> ) info.GetValue( "CustomFieldDictionary", typeof( Dictionary<string, string> ) );
    }

    [SecurityPermission( SecurityAction.LinkDemand,
        Flags = SecurityPermissionFlag.SerializationFormatter )]
    public override void GetObjectData( SerializationInfo info, StreamingContext context )
    {
        if( info == null )
            throw new ArgumentNullException( "info" );

        base.GetObjectData( info, context );

        info.AddValue( "CustomFieldDictionary", this.CustomFieldDictionary, typeof( Dictionary<string, string> ) );
    }

    public virtual IDictionary<string, string> CustomFieldDictionary
    {
        get { return this._customFieldDictionary; }
        set { this._customFieldDictionary = value; }
    }
}

This class is referenced by another Serializable class.

[Serializable]
public class UserOfCustomFields : MyObject, IUserOfCustomFields {

    public UserOfCustomFields( ) {
        this.CustomFields = new CustomFields( );
    }

    protected UserOfCustomFields( SerializationInfo info, StreamingContext context ) : base(info, context)
    {
        if( info == null )
            throw new System.ArgumentNullException( "info" );
        this.CustomFields = ( CustomFields ) info.GetValue( "CustomFields", typeof( CustomFields ) );
    }

    [SecurityPermission(SecurityAction.LinkDemand,
        Flags = SecurityPermissionFlag.SerializationFormatter)]
    public override void GetObjectData( SerializationInfo info, StreamingContext context )
    {
        if( info == null )
            throw new ArgumentNullException( "info" );

        base.GetObjectData( info, context );

        info.AddValue( "CustomFields", this.CustomFields, typeof( CustomFields ) );
    }

    public virtual ICustomFields CustomFields { get; set; }
}

Here's the code that calls the serialization/deserialization code:

    public bool SaveObjects( IEnumerable<IMyObject> objects ) {
        // If we have null objects fail
        if ( objects == null ) { return false; }

        var local_objects = objects.ToList( );
        var new_list = new List< IDBObject< Guid > >( );
        foreach ( var obj in local_objects ) {
            var clone = this.DeepCopy( obj );
            new_list.Add( clone );
        }

        return true;
    }

And the actual serialization/deserialization takes place here:

    public virtual T DeepCopy<T>( T obj )
    {
        if( !obj.GetType( ).IsSerializable )
        {
            throw new Exception( "The source object must be serializable" );
        }

        if( Object.ReferenceEquals( obj, null ) )
        {
            throw new Exception( "The source object must not be null" );
        }

        T result = default( T );

        using( var memoryStream = new MemoryStream( ) )
        {
            var formatter = new BinaryFormatter( );
            formatter.Serialize( memoryStream, obj );
            memoryStream.Seek( 0, SeekOrigin.Begin );

            result = ( T ) formatter.Deserialize( memoryStream );

            memoryStream.Close( );
        }

        return result;
    }

As noted in the title I get System.InvalidCastException "Object must implement IConvertible." when deserializing. Is Dictionary a special case? I've had no problems with any other types (up to now).

Answer 1

Turns out that since I'm using NHibernate that it injects it's own implementation of collections. In order to make this work I must use the interface IDictionary in the serialization/deserialization:

    protected CustomFields( SerializationInfo info, StreamingContext context )
        : base( info, context )
    {
        if( info == null )
            throw new ArgumentNullException( "info" );

        this.CustomFieldDictionary = ( IDictionary<string, string> ) info.GetValue( "CustomFieldDictionary", typeof( IDictionary<string, string> ) );
    }

    [SecurityPermission( SecurityAction.LinkDemand,
        Flags = SecurityPermissionFlag.SerializationFormatter )]
    public override void GetObjectData( SerializationInfo info, StreamingContext context )
    {
        if( info == null )
            throw new ArgumentNullException( "info" );

        base.GetObjectData( info, context );

        // Following line throws
        info.AddValue( "CustomFieldDictionary", this.CustomFieldDictionary, typeof( IDictionary<string, string> ) );
    }