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)
Available in Sandboxed Solutions: Yes
Available in SharePoint Online

public virtual bool AllowMultipleValues { get; set; }

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.