SPField.FieldValueType Property
Gets the value type of the field when the field type has its own value type.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
Available in Sandboxed Solutions: Yes
Available in SharePoint Online
Use the FieldValueType property when the field type has its own custom value type object, such as the one that is returned through the GetFieldValue method. For more information about field value objects, see How to: Create a Custom Field Value Class.