'Declaration
 
Public Overloads Function Add(Of TKey)( _
   ByVal keySelector As System.Linq.Expressions.Expression(Of Func(Of T,TKey)), _
   ByVal keyIsUnique As System.Boolean, _
   ByVal onlyOnce As System.Boolean _
) As Subindex(Of T,TKey)

public Subindex<T,TKey> Add<TKey>( 
   System.Linq.Expressions.Expression<Func<T,TKey>> keySelector,
   System.bool keyIsUnique,
   System.bool onlyOnce
)

Parameters

keySelector

Key selector expression of the subindex, see KeySelector.

keyIsUnique

Specifies whether the key used in this subindex is unique for any given value of the parent key (default: false).

onlyOnce

Specifies whether it is required that the subindex does not exist prior to this method call (default: false). If a subindex with this keySelector already exists, an exception is thrown if it is true, and this method call is ignored if it is false.

Type Parameters

TKey

The type of the subindex key.

Return Value

A unique index occupies less memory and performs better than a non-unique index (although the difference isn't dramatic). Therefore, for unique keys, it's recommended to specify the corresponding index as unique.

But do that only if you are sure that the key is indeed unique, as it imposes a uniqueness constraint on the indexed collection. An attempt to modify the indexed collection violating the uniqueness throws an System.InvalidOperationException.

For a subindex, uniqueness means that any given pair of parent key and subindex key values uniquely determines an item in the indexed collection.

Reference

SubindexCollection<T> Class
SubindexCollection<T> Members
Overload List