public void AddValidate( 
   Func<IEnumerable<Attribute.ValidationError>> f
)

var insp = new Inspector();
insp.LoadSchema(featLayer);
var attrib = insp.Where(a => a.FieldName == "Mineral").First();

attrib.AddValidate(() =>
{
  if (attrib.CurrentValue.ToString() == "Salt")
    return Enumerable.Empty<ArcGIS.Desktop.Editing.Attributes.Attribute.ValidationError>();
  else return new[] { ArcGIS.Desktop.Editing.Attributes.Attribute.ValidationError.Create("Error", ArcGIS.Desktop.Editing.Attributes.Severity.Low) };
});