'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 IIndexScanner(Of T,TKey)

public IIndexScanner<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 index, see KeySelector.

keyIsUnique

Specifies whether the key used in this index is a unique key for the indexed collection (default: false).

onlyOnce

Specifies whether it is required that the index does not exist prior to this method call (default: false). If an index 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 index 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.

Reference

ScannerCollection<T> Class
ScannerCollection<T> Members
Overload List