Hi Mattias, thanks for your reply.

I forgot to post one important fact: the private property I'm looking for is
not defined in a base class of the object passed to the SetProperty() method.

Type.GetProperty() does not return private properties declared in base
types. So my modified (and working) method now also checks the base types:

void SetProperty(object obj, string propertyName, object newValue)
{
 Type type = obj.GetType();
 PropertyInfo pi = null;
 while ((pi == null) && (type != typeof(object)))
 {
   pi = type.GetProperty(propertyName,
     BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance |
BindingFlags.FlattenHierarchy);
   type = type.BaseType;
 }
 if (pi != null)
   pi.SetValue(obj, newValue, null);
}

Best regards,
Martin