Changed from == null to is null, better for null checking.

SecureSharing.Business/Infrastructure/StartupConfiguration.cs

     public static TConfig ConfigureStartupConfig<TConfig>(IServiceCollection services, IConfiguration configuration)
         where TConfig : class, new()
     {
-        if (services == null)
+        if (services is null)
             throw new ArgumentNullException(nameof(services));
 
-        if (configuration == null)
+        if (configuration is null)
             throw new ArgumentNullException(nameof(configuration));
 
         //create instance of config

SecureSharing.Business/Services/MessageService.cs

     public async Task<int> Create(MessageDto messageDto, PeriodOfValidity chosenPeriod)
     {
         var message = _mapper.Map<Message>(messageDto);
-        switch (chosenPeriod)
+        message.ExpiryDate = chosenPeriod switch
         {
-            case PeriodOfValidity.ONE_TIME:
-                message.ExpiryDate = null;
-                break;
-            case PeriodOfValidity.ONE_DAY:
-                message.ExpiryDate = DateTime.UtcNow.AddMinutes(1);
-                break;
-            case PeriodOfValidity.ONE_HOUR:
-                message.ExpiryDate = DateTime.UtcNow.AddHours(1);
-                break;
-            case PeriodOfValidity.ONE_WEEK:
-                message.ExpiryDate = DateTime.UtcNow.AddDays(7);
-                break;
-            default:
-                message.ExpiryDate = null;
-                break;
-        }
+            PeriodOfValidity.ONE_TIME => null,
+            PeriodOfValidity.ONE_DAY => DateTime.UtcNow.AddMinutes(1),
+            PeriodOfValidity.ONE_HOUR => DateTime.UtcNow.AddHours(1),
+            PeriodOfValidity.ONE_WEEK => DateTime.UtcNow.AddDays(7),
+            _ => null
+        };
 
         await _dbContext.Messages.AddAsync(message);
         await _dbContext.SaveChangesAsync();
     public async Task<bool> Delete(int messageId)
     {
         var messageDto = await GetById(messageId);
-        if (messageDto == null) return false;
+        if (messageDto is null) return false;
         _dbContext.Set<Message>().Remove(_mapper.Map<Message>(messageDto));
         try
         {

SecureSharing/Areas/Identity/Pages/Account/ResetPassword.cshtml.cs

 
     public IActionResult OnGet(string code = null)
     {
-        if (code == null)
+        if (code is null)
         {
             return BadRequest("A code must be supplied for password reset.");
         }
         if (!ModelState.IsValid) return Page();
 
         var user = await _userManager.FindByEmailAsync(Input.Email);
-        if (user == null)
+        if (user is null)
             // Don't reveal that the user does not exist
             return RedirectToPage("./ResetPasswordConfirmation");
 

SecureSharing/Infrastructure/ModelFactory.cs

-using System;
-using System.Threading.Tasks;
-using SecureSharing.Business.Interfaces;
+using SecureSharing.Business.Interfaces;
 using SecureSharing.Models;
 
 namespace SecureSharing.Infrastructure;