// You must define a Guid attribute for your user data derived class
// in order to support serialization. Every custom user data class
// needs a custom Guid
[Guid("7098A105-CD3C-4192-A9AC-0F21017098DC")]
public class MyCustomData : Rhino.DocObjects.Custom.UserData
{
  public int IntegerData{ get; set; }
  public string StringData {get; set;}

  // Your UserData class must have a public parameterless constructor
  public MyCustomData(){}

  public MyCustomData(int i, string s)
  {
    IntegerData = i;
    StringData = s;
  }

  public override string Description
  {
    get { return "Some Custom Properties"; }
  }

  public override string ToString()
  {
    return String.Format("integer={0}, string={1}", IntegerData, StringData);
  }

  protected override void OnDuplicate(Rhino.DocObjects.Custom.UserData source)
  {
    MyCustomData src = source as MyCustomData;
    if (src != null)
    {
      IntegerData = src.IntegerData;
      StringData = src.StringData;
    }
  }

  // return true if you have information to save
  public override bool ShouldWrite
  {
    get
    {
      // make up some rule as to if this should be saved in the 3dm file
      if (IntegerData > 0 && !string.IsNullOrEmpty(StringData))
        return true;
      return false;
    }
  }

  protected override bool Read(Rhino.FileIO.BinaryArchiveReader archive)
  {
    Rhino.Collections.ArchivableDictionary dict = archive.ReadDictionary();
    if (dict.ContainsKey("IntegerData") && dict.ContainsKey("StringData"))
    {
      IntegerData = (int)dict["IntegerData"];
      StringData = dict["StringData"] as String;
    }
    return true;
  }
  protected override bool Write(Rhino.FileIO.BinaryArchiveWriter archive)
  {
    // you can implement File IO however you want... but the dictionary class makes
    // issues like versioning in the 3dm file a bit easier.  If you didn't want to use
    // the dictionary for writing, your code would look something like.
    //
    //  archive.Write3dmChunkVersion(1, 0);
    //  archive.WriteInt(IntegerData);
    //  archive.WriteString(StringData);
    var dict = new Rhino.Collections.ArchivableDictionary(1, "MyCustomData");
    dict.Set("IntegerData", IntegerData);
    dict.Set("StringData", StringData);
    archive.WriteDictionary(dict);
    return true;
  }
}

partial class Examples
{
  public static Rhino.Commands.Result Userdata(RhinoDoc doc)
  {
    Rhino.DocObjects.ObjRef objref;
    var rc = Rhino.Input.RhinoGet.GetOneObject("Select Face", false, Rhino.DocObjects.ObjectType.Surface, out objref);
    if (rc != Rhino.Commands.Result.Success)
      return rc;

    var face = objref.Face();

    // See if user data of my custom type is attached to the geomtry
    // We need to use the underlying surface in order to get the user data
    // to serialize with the file.
    var ud = face.UnderlyingSurface().UserData.Find(typeof(MyCustomData)) as MyCustomData;
    if (ud == null)
    {
      // No user data found; create one and add it
      int i = 0;
      rc = Rhino.Input.RhinoGet.GetInteger("Integer Value", false, ref i);
      if (rc != Rhino.Commands.Result.Success)
        return rc;

      ud = new MyCustomData(i, "This is some text");
      face.UnderlyingSurface().UserData.Add(ud);
    }
    else
    {
      RhinoApp.WriteLine("{0} = {1}", ud.Description, ud);
    }
    return Rhino.Commands.Result.Success;
  }
}

Partial Friend Class Examples
  Public Shared Function Userdata(ByVal doc As RhinoDoc) As Rhino.Commands.Result
	Dim objref As Rhino.DocObjects.ObjRef = Nothing
	Dim rc = Rhino.Input.RhinoGet.GetOneObject("Select Face", False, Rhino.DocObjects.ObjectType.Surface, objref)
	If rc IsNot Rhino.Commands.Result.Success Then
	  Return rc
	End If

	Dim face = objref.Face()

	' See if user data of my custom type is attached to the geomtry
	' We need to use the underlying surface in order to get the user data
	' to serialize with the file.
	Dim ud = TryCast(face.UnderlyingSurface().UserData.Find(GetType(MyCustomData)), MyCustomData)
	If ud Is Nothing Then
	  ' No user data found; create one and add it
	  Dim i As Integer = 0
	  rc = Rhino.Input.RhinoGet.GetInteger("Integer Value", False, i)
	  If rc IsNot Rhino.Commands.Result.Success Then
		Return rc
	  End If

	  ud = New MyCustomData(i, "This is some text")
	  face.UnderlyingSurface().UserData.Add(ud)
	Else
	  RhinoApp.WriteLine("{0} = {1}", ud.Description, ud)
	End If
	Return Rhino.Commands.Result.Success
  End Function
End Class

# No Python sample available