'Declaration
 
Public Overloads Shadows Function Add(Of TKey)( _
   ByVal keySelector As Expression(Of Func(Of T,TKey)), _
   ByVal keyIsUnque As Boolean _
) As Index(Of T,TKey)

public new Index<T,TKey> Add<TKey>( 
   Expression<Func<T,TKey>> keySelector,
   bool keyIsUnque
)

Parameters

keySelector

Key selector expression of the index, see IndexDefinition<T>.KeySelector.

keyIsUnque

true if a unique index must be created.

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

IndexCollection<T> Class
IndexCollection<T> Members
Overload List