How to validate the checkbox in trigger?

Hi Harshal,

 It's an Apex trigger for contact, so I don't think you need to search from your account.

Here is an example of an Apex trigger for a contact created with other requirements.

trigger AccOnePrimaryCon on Contact (before insert, before update) {
	 Map<Id,Account> accMap = new Map<Id,Account> ([Select Id,Name, (Select Id, name, Is_Primary__c from Contacts) from Account
                                                 Where Id in (Select AccountId from Contact where Id in :Trigger.new)]);
    List<Contact> conList = [Select Id, AccountId, Is_Primary__c from Contact where Id in :Trigger.new]; 
    for(Contact c: conList){
        if(c.Is_Primary__c == true && accMap.get(c.AccountId).Contacts.size()>0){
            for(Contact c2:accMap.get(c.AccountId).Contacts){
                if(c2.Is_Primary__c == true){
                    c.addError('Account Already Can\'t have more than one Primary Contact! Already have One!');                    
                }
            } 
       }
    }
}