How to get selected records from list view in Salesforce?

How to get selected records from list view in Salesforce?

Visualforce page:

<apex:page standardController="Member__c" extensions="MemberExt" recordSetVar="Member">

    <apex:pageBlock >

        <apex:pageBlockTable value="{!memList}" var="M">

            <apex:column value="{!M.Name}"/>

            <apex:column value="{!M.Age__c}"/>

        </apex:pageBlockTable>

    </apex:pageBlock>

</apex:page>

Apex Controller:

public class MemberExt {

    public List<Member__c> memList {get;set;}

    private ApexPages.StandardSetController standardController;

    private Set<Id> memIds = new Set<Id>();

    public MemberExt(ApexPages.StandardSetController standardController){

        this.standardController = standardController;

        memList = new List<Member__c>();

        for (Member__c mem : (List<Member__c>)standardController.getSelected()){ 

            memIds.add(mem.Id);

        }

        memList = [SELECT Name, Age__c FROM Member__c WHERE ID IN: memIds];

    }

}

Custom Button:

Output: