The KeyFieldAttribute should specify the field name that uniquely identifies each object within the collection. As opposed to the ordinal/index position or distinct field, the key field provides another method of accessing a particular object within the collection. The key field must be unique within the collection. If the SubsetAttribute has been specified then the key field only needs to be unique within the subset, not the entire table. Specifying this attribute is optional.

public class KeyFieldAttribute : Attribute

Public Class KeyFieldAttribute _
	Inherits Attribute

public ref class KeyFieldAttribute : public Attribute

<KeyField("CustomerName")>
Public Class Customers
    ...