Verifies that the value of Value meets all restrictions on field content such as length, format, and data type.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
Value is the value of the child drop-down list in the UI, not the underlying value (which is ItemFieldValue) of the SPFieldLookup object that has the LookupField as its FieldRenderingControl property. Use only when you need validation of the UI value. To validate ItemFieldValue, use GetValidatedString().