Resolve “Type not expected” error

Question

I’m using DataContractSerializer for xml serialization. While It tries to serialize SubClass, throws the below error

'SubClass:http://schemas.datacontract.org/2004/07/SubClass ' is not expected. Consider using a DataContractResolver if you are using DataContractSerializer 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 the serializer

So I have added “SubClass” in KnownTypeAttribute to resolve this. But it does not work.

I have visited the below article for this issue, "Type not expected", using DataContractSerializer - but it's just a simple class, no funny stuff?

Please anyone let me know how to resolve this?

[DataContract(Name = "BaseClass"]
[KnownType("Types")]
public class BaseClass
{
    [DataMember]
    public bool IsSerialized{get;set;}

     public static Type[] Types()
     {
         return MainClass.Types();
     }
}
[DataContract(Name = "SubClass"]
public class SubClass:BaseClass //Getting error for this class
{
    [DataMember]
    public string Format {get;set;}
}

public class MainClass
{
    public MainClass()
    {
        var ser = new DataContractSerializer(typeof(BaseClass));
        MemoryStream stream = new MemoryStream();
        ser.WriteObject(stream,new BaseClass());
    }
    public static Type[] Types()
    {
         return new Type[]{typeof(BaseClass),typeof(SubClass)};
    }
}