SPFieldLookup.AllowMultipleValues property

Gets or sets a Boolean value that specifies whether multiple values can be used in the lookup field.

Namespace:  Microsoft.SharePoint
Assembly:  Microsoft.SharePoint (in Microsoft.SharePoint.dll)

Public Overridable Property AllowMultipleValues As Boolean
Dim instance As SPFieldLookup
Dim value As Boolean

value = instance.AllowMultipleValues

instance.AllowMultipleValues = value

Property value

Type: System.Boolean
true to specify that multiple values can be used in the field; otherwise, false. The default is false.

You cannot set a deletion constraint on a lookup field that allows multiple values.