Modify the inherited properties of a field
One of my colleagues found a very interesting thing in SharePoint:: It's forbidden to modify the inherited field's properties (for example AllowMultipleValues defined by SPFieldLookup class) via properties! Doing this you start a back process
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Read the complete post at http://www.sharepointblogs.com/aghy/archive/2007/11/19/modify-the-inherited-properties-of-a-field.aspx