DataObject.GetData() returns an object of type MemoryStream anyway?

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C#

WinForms

clipboard

I don't know why the method GetData() requires a parameter of Type, I thought the Type is to specify that of which class/type the object should be.
I have a structure called GraphicsPathWrap, it's made serializable by implementing ISerializable. I tried the following copy function:

private void Copy(GraphicsPathWrap gpw){
  DataObject obj = new DataObject();
  obj.SetData(typeof(GraphicsPathWrap), gpw);
  Clipboard.SetDataObject(obj);
}

Then tried the following paste function:

private GraphicsPathWrap Paste()
{
  return (GraphicsPathWrap)Clipboard.GetDataObject().GetData(typeof(GraphicsPathWrap));
}

It should work, but the GetData(...) returns an object of type MemoryStream and an InvalidCastException was thrown. I don't understand why it's type of MemoryStream. I thought it should be able to be casted to GraphicsPathWrap?
I can work around this by using a BinaryFormatter to deserialize the MemoryStream but it's too ridiculous when Clipboard can't do all the thing for me?

Thanks!