I have generated a signed OCX wrapper for a licenced ActiveX control using
aximp with the /source option, as recommended in Adam Nathan's interop
guide. However, whenever I add an instance of this control to a form and
then save the form I receive the following error:

Code generation for property 'OcxState' failed.
Error was: 'Found a high surrogate char without a following low surrogate at
index: 7.  The input may not be in this encoding, or may not contain valid
Unicode (UTF-16) characters.
Parameter name: chars'

It turns out that this is a problem with the licence string, which contains
the following sequence of characters at the specified index:

0xDAFB, 0xDA8A

Since 0xDAFB is in the Unicode high surrogate range (0xD800-0xDBFF), to be a
well-formed Unicode text string, this should be followed by a low-surrogate
character, i.e. a character in the range 0xDC00-0xDFFF. Unfortunately 0xDA8A
doesn't meet this requirement, and it seems like this is why the exception
is being thrown. I can work around this issue by manually building the
correct licence string with a StringBuilder and then explicitly setting the
licenseKey member using Reflection as part of my initialisation:

 Dim info As FieldInfo = GetType(AxHost).GetField("licenseKey",
BindingFlags.Instance Or BindingFlags.NonPublic)
 AxCtrl = New AxInterop.LibName.AxMyControl()
 info.SetValue(AxCtrl , licenseBuilder.ToString())

Unfortunately this means I cannot add or manipulate the controls at
design-time using the designer, so it's not really a workable solution.

I guess my question (yes, there is a question <g>) is why is the licence
string being validated in this way? It seems like an unnecessary restriction
to me.