When using ISerializable with DataContractSerializer, how do I stop the serializer from outputting type information?

Do you need the ISerializable here? What was the regular DataContractSerializer not giving you? If you switch back to this, it should work fine.

Basically, by implementing custom serialization, the data is no longer contract based - so it has to include this extra information to guarantee that it is able to understand it later.

So: is there a reason to implement ISerializable in this case?

If you want full control over serialization to xml, you can use XmlSerializer

public class Test{    [XmlIgnore]    public Nullable<int> NullableNumber = 7;    [XmlElement("NullableNumber")]    public int NullableNumberValue    {        get { return NullableNumber.Value; }        set { NullableNumber = value; }    }    public bool ShouldSerializeNullableNumberValue()    {        return NullableNumber.HasValue;    }    [XmlElement]    public int Number = 5;}

sample serialization code:

static void Main(string[] args){    XmlSerializer serializer = new XmlSerializer(typeof(Test));    serializer.Serialize(Console.Out, new Test());}

results:

<Test xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">  <Number>5</Number>  <NullableNumber>7</NullableNumber></Test>

Starting in .Net Framework 4.5 (and .Net Core 1.0), this is possible using the DataContractJsonSerializerSettings class:

DataContractJsonSerializerSettings settings = new DataContractJsonSerializerSettings{    EmitTypeInformation = EmitTypeInformation.Never};var dcs = new DataContractSerializer(typeof(Test), settings);

The EmitTypeInformation settings tells the serializer not to output the (annoying?) __type parameter during serialization.

There are a range of other useful settings available. Here is the docs page for DataContractJsonSerializerSettings.