IXmlSerializable Dictionary problem

Question

I was trying to create a generic Dictionary that implements IXmlSerializable (credit to Charles Feduke).

Here is my trial:

Sub Main()
    Dim z As New SerializableDictionary(Of String, String)
    z.Add("asdf", "asd")
    Console.WriteLine(z.Serialize)
End Sub

Result:

<?xml version="1.0" encoding="utf-16"?><Entry key="asdf" value="asd" />

I placed a breakpoint on top of the WriteXml method and I see that when it stops, the writer contains no data at all, and IMHO it should contain the root element and the xml declaration.

---

<Serializable()> _
Public Class SerializableDictionary(Of TKey, TValue) : Inherits Dictionary(Of TKey, TValue) : Implements IXmlSerializable
    Private Const EntryString As String = "Entry"
    Private Const KeyString As String = "key"
    Private Const ValueString As String = "value"
    Private Shared ReadOnly AttributableTypes As Type() = New Type() {GetType(Boolean), GetType(Byte), GetType(Char), GetType(DateTime), GetType(Decimal), GetType(Double), GetType([Enum]), GetType(Guid), GetType(Int16), GetType(Int32), GetType(Int64), GetType(SByte), GetType(Single), GetType(String), GetType(TimeSpan), GetType(UInt16), GetType(UInt32), GetType(UInt64)}
    Private Shared ReadOnly GetIsAttributable As Predicate(Of Type) = Function(t) AttributableTypes.Contains(t)
    Private Shared ReadOnly IsKeyAttributable As Boolean = GetIsAttributable(GetType(TKey))
    Private Shared ReadOnly IsValueAttributable As Boolean = GetIsAttributable(GetType(TValue))
    Private Shared ReadOnly GetElementName As Func(Of Boolean, String) = Function(isKey) If(isKey, KeyString, ValueString)

    Public Function GetSchema() As System.Xml.Schema.XmlSchema Implements System.Xml.Serialization.IXmlSerializable.GetSchema
        Return Nothing
    End Function

    Public Sub WriteXml(ByVal writer As XmlWriter) Implements IXmlSerializable.WriteXml
        For Each entry In Me
            writer.WriteStartElement(EntryString)

            WriteData(IsKeyAttributable, writer, True, entry.Key)
            WriteData(IsValueAttributable, writer, False, entry.Value)

            writer.WriteEndElement()
        Next
    End Sub

    Private Sub WriteData(Of T)(ByVal attributable As Boolean, ByVal writer As XmlWriter, ByVal isKey As Boolean, ByVal value As T)
        Dim name = GetElementName(isKey)

        If attributable Then
            writer.WriteAttributeString(name, value.ToString)
        Else
            Dim serializer As New XmlSerializer(GetType(T))
            writer.WriteStartElement(name)
            serializer.Serialize(writer, value)
            writer.WriteEndElement()
        End If
    End Sub

    Public Sub ReadXml(ByVal reader As XmlReader) Implements IXmlSerializable.ReadXml
        Dim empty = reader.IsEmptyElement

        reader.Read()
        If empty Then Exit Sub

        Clear()

        While reader.NodeType <> XmlNodeType.EndElement
            While reader.NodeType = XmlNodeType.Whitespace
                reader.Read()

                Dim key = ReadData(Of TKey)(IsKeyAttributable, reader, True)
                Dim value = ReadData(Of TValue)(IsValueAttributable, reader, False)

                Add(key, value)

                If Not IsKeyAttributable AndAlso Not IsValueAttributable Then reader.ReadEndElement() Else reader.Read()

                While reader.NodeType = XmlNodeType.Whitespace
                    reader.Read()
                End While
            End While

            reader.ReadEndElement()
        End While
    End Sub

    Private Function ReadData(Of T)(ByVal attributable As Boolean, ByVal reader As XmlReader, ByVal isKey As Boolean) As T
        Dim name = GetElementName(isKey)
        Dim type = GetType(T)

        If attributable Then
            Return Convert.ChangeType(reader.GetAttribute(name), type)
        Else
            Dim serializer As New XmlSerializer(type)

            While reader.Name <> name
                reader.Read()
            End While

            reader.ReadStartElement(name)
            Dim value = serializer.Deserialize(reader)
            reader.ReadEndElement()

            Return value
        End If
    End Function

    Public Shared Function Serialize(ByVal dictionary As SerializableDictionary(Of TKey, TValue)) As String
        Dim sb As New StringBuilder(1024)
        Dim sw As New StringWriter(sb)
        Dim xs As New XmlSerializer(GetType(SerializableDictionary(Of TKey, TValue)))

        xs.Serialize(sw, dictionary)
        sw.Dispose()
        Return sb.ToString
    End Function

    Public Shared Function Deserialize(ByVal xml As String) As SerializableDictionary(Of TKey, TValue)
        Dim xs As New XmlSerializer(GetType(SerializableDictionary(Of TKey, TValue)))
        Dim xr As New XmlTextReader(xml, XmlNodeType.Document, Nothing)
        xr.Close()
              Return xs.Deserialize(xr)
    End Function

    Public Function Serialize() As String
        Dim sb As New StringBuilder
        Dim xw = XmlWriter.Create(sb)
        WriteXml(xw)
        xw.Close()
        Return sb.ToString
    End Function

    Public Sub Parse(ByVal xml As String)
        Dim xr As New XmlTextReader(xml, XmlNodeType.Document, Nothing)
        ReadXml(xr)
        xr.Close()
    End Sub

End Class

Answer 1

Why would it contain a root? You aren't adding one in Serialize, where you create the XmlWriter... I wonder if you should have something more like (C#, sorry):

public string Serialize() {
    StringBuilder sb = new StringBuilder();
    XmlSerializer ser = new XmlSerializer(
        typeof(SerializableDictionary<TKey, TValue>));
    using (XmlWriter writer = XmlWriter.Create(sb)) {
        ser.Serialize(writer, this);
    }
    return sb.ToString();
}

This uses the regular XmlSerializer core for things like writing the outer element(s). Alternatively, change Serialize to include an outer element of your choosing.

Answer 2

Not an answer, but I found 2 bugs in Shimmy's code (thanks by the way) and one gotcha for those trying to use this against .Net 2.0

The bugs are related to each other. The calls to:

WriteData(IsKeyAttributable, writer, True, entry.Key)
WriteData(IsValueAttributable, writer, False, entry.Value)

should be

WriteData(IsKeyAttributable, writer, True, DirectCast(entry.Key, TKey))
WriteData(IsValueAttributable  AndAlso IsKeyAttributable, writer, False, CType(entry.Value, TValue))

i.e If the Key is not able to be an XML attribute, then the value cannot be an XML attribute. Also, need to cast the entry.Key and entry.Value to it's TKey/TValue, otherwise the XMLSerializer complains later on. Also

Similarly the call

Dim value = ReadData(Of TValue)(IsValueAttributable, reader, False)

should be

Dim value = ReadData(Of TValue)(IsValueAttributable AndAlso IsKeyAttributable, reader, False)

i.e Agin check that the key is attributale if the value is attributable

And for those targetting .Net runtime 2.0 you will need the GetIsAttributable predicate declared as

Private Shared ReadOnly GetIsAttributable As Predicate(Of Type) = Function(t) DirectCast(AttributableTypes, IList).Contains(t)