diff --git a/framework/src/Volo.Abp.Ddd.Application/Volo.Abp.Ddd.Application.csproj b/framework/src/Volo.Abp.Ddd.Application/Volo.Abp.Ddd.Application.csproj
index fc0586ce8e..03351adb7f 100644
--- a/framework/src/Volo.Abp.Ddd.Application/Volo.Abp.Ddd.Application.csproj
+++ b/framework/src/Volo.Abp.Ddd.Application/Volo.Abp.Ddd.Application.csproj
@@ -5,6 +5,8 @@
 
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;netstandard2.1;net7.0</TargetFrameworks>
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors>Nullable</WarningsAsErrors>
     <AssemblyName>Volo.Abp.Ddd.Application</AssemblyName>
     <PackageId>Volo.Abp.Ddd.Application</PackageId>
     <AssetTargetFallback>$(AssetTargetFallback);portable-net45+win8+wp8+wpa81;</AssetTargetFallback>
diff --git a/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/AbstractKeyCrudAppService.cs b/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/AbstractKeyCrudAppService.cs
index 6b6ad975d7..b3a2af6663 100644
--- a/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/AbstractKeyCrudAppService.cs
+++ b/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/AbstractKeyCrudAppService.cs
@@ -70,11 +70,11 @@ public abstract class AbstractKeyCrudAppService<TEntity, TGetOutputDto, TGetList
 {
     protected IRepository<TEntity> Repository { get; }
 
-    protected virtual string CreatePolicyName { get; set; }
+    protected virtual string? CreatePolicyName { get; set; }
 
-    protected virtual string UpdatePolicyName { get; set; }
+    protected virtual string? UpdatePolicyName { get; set; }
 
-    protected virtual string DeletePolicyName { get; set; }
+    protected virtual string? DeletePolicyName { get; set; }
 
     protected AbstractKeyCrudAppService(IRepository<TEntity> repository)
         : base(repository)
diff --git a/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/AbstractKeyReadOnlyAppService.cs b/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/AbstractKeyReadOnlyAppService.cs
index e56e3ff042..8602792701 100644
--- a/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/AbstractKeyReadOnlyAppService.cs
+++ b/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/AbstractKeyReadOnlyAppService.cs
@@ -40,9 +40,9 @@ public abstract class AbstractKeyReadOnlyAppService<TEntity, TGetOutputDto, TGet
 {
     protected IReadOnlyRepository<TEntity> ReadOnlyRepository { get; }
 
-    protected virtual string GetPolicyName { get; set; }
+    protected virtual string? GetPolicyName { get; set; }
 
-    protected virtual string GetListPolicyName { get; set; }
+    protected virtual string? GetListPolicyName { get; set; }
 
     protected AbstractKeyReadOnlyAppService(IReadOnlyRepository<TEntity> repository)
     {
@@ -107,7 +107,7 @@ public abstract class AbstractKeyReadOnlyAppService<TEntity, TGetOutputDto, TGet
         {
             if (!sortInput.Sorting.IsNullOrWhiteSpace())
             {
-                return query.OrderBy(sortInput.Sorting);
+                return query.OrderBy(sortInput.Sorting!);
             }
         }
 
diff --git a/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/ApplicationService.cs b/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/ApplicationService.cs
index b24e4b2900..ba82e5bdc2 100644
--- a/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/ApplicationService.cs
+++ b/framework/src/Volo.Abp.Ddd.Application/Volo/Abp/Application/Services/ApplicationService.cs
@@ -36,10 +36,10 @@ public abstract class ApplicationService :
     IGlobalFeatureCheckingEnabled,
     ITransientDependency
 {
-    public IAbpLazyServiceProvider LazyServiceProvider { get; set; }
+    public IAbpLazyServiceProvider LazyServiceProvider { get; set; } = default!;
 
     [Obsolete("Use LazyServiceProvider instead.")]
-    public IServiceProvider ServiceProvider { get; set; }
+    public IServiceProvider ServiceProvider { get; set; } = default!;
 
     public static string[] CommonPostfixes { get; set; } = { "AppService", "ApplicationService", "IntService", "IntegrationService", "Service" };
 
@@ -49,7 +49,7 @@ public abstract class ApplicationService :
 
     protected IAsyncQueryableExecuter AsyncExecuter => LazyServiceProvider.LazyGetRequiredService<IAsyncQueryableExecuter>();
 
-    protected Type ObjectMapperContext { get; set; }
+    protected Type? ObjectMapperContext { get; set; }
     protected IObjectMapper ObjectMapper => LazyServiceProvider.LazyGetService<IObjectMapper>(provider =>
         ObjectMapperContext == null
             ? provider.GetRequiredService<IObjectMapper>()
@@ -85,34 +85,34 @@ public abstract class ApplicationService :
             return _localizer;
         }
     }
-    private IStringLocalizer _localizer;
+    private IStringLocalizer? _localizer;
 
-    protected Type LocalizationResource {
+    protected Type? LocalizationResource {
         get => _localizationResource;
         set {
             _localizationResource = value;
             _localizer = null;
         }
     }
-    private Type _localizationResource = typeof(DefaultResource);
+    private Type? _localizationResource = typeof(DefaultResource);
 
-    protected IUnitOfWork CurrentUnitOfWork => UnitOfWorkManager?.Current;
+    protected IUnitOfWork? CurrentUnitOfWork => UnitOfWorkManager?.Current;
 
-    protected ILogger Logger => LazyServiceProvider.LazyGetService<ILogger>(provider => LoggerFactory?.CreateLogger(GetType().FullName) ?? NullLogger.Instance);
+    protected ILogger Logger => LazyServiceProvider.LazyGetService<ILogger>(provider => LoggerFactory?.CreateLogger(GetType().FullName!) ?? NullLogger.Instance);
 
     /// <summary>
     /// Checks for given <paramref name="policyName"/>.
     /// Throws <see cref="AbpAuthorizationException"/> if given policy has not been granted.
     /// </summary>
     /// <param name="policyName">The policy name. This method does nothing if given <paramref name="policyName"/> is null or empty.</param>
-    protected virtual async Task CheckPolicyAsync([CanBeNull] string policyName)
+    protected virtual async Task CheckPolicyAsync(string? policyName)
     {
         if (string.IsNullOrEmpty(policyName))
         {
             return;
         }
 
-        await AuthorizationService.CheckAsync(policyName);
+        await AuthorizationService.CheckAsync(policyName!);
     }
 
     protected virtual IStringLocalizer CreateLocalizer()