@isTest
public class AddAttachmentToProjectTest {
     @isTest static void testAddAttachments() {
        // Created test data
        Opportunity opp = new Opportunity(Name = 'New Opp', StageName = 'Prospecting', CloseDate = Date.today());
        insert opp;
        
        ProjectDemo__c proj = new ProjectDemo__c(Opportunity_Name__c = opp.Id);
        insert proj;
        
        ContentVersion cv = new ContentVersion(
            Title = 'New Content version',
            PathOnClient = 'Test.txt',
            VersionData = Blob.valueOf('Test Content')
        );
        insert cv;
        
        Id cdId = [SELECT ContentDocumentId FROM ContentVersion WHERE Id = :cv.Id].ContentDocumentId;
        
        ContentDocumentLink cdl = new ContentDocumentLink(
            ContentDocumentId = cdId,
            LinkedEntityId = opp.Id,
            ShareType = 'V',
            Visibility = 'AllUsers'
        );
        insert cdl;
        
        // Calling the method to test
        Test.startTest();
        AddAttachementToProjectHandler.AddAttachments(new List<ProjectDemo__c>{proj});
        Test.stopTest();
        
        // Verifing the results
        List<ContentDocumentLink> resultCdlList = [SELECT Id FROM ContentDocumentLink WHERE LinkedEntityId = :proj.Id];
        System.assertEquals(1, resultCdlList.size());
         
         
         
    }
    
}