Add `Nullable` package.

framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs

@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Security.Cryptography;
 using System.Text;
@@ -48,7 +47,7 @@ public static class AbpStringExtensions
     /// Indicates whether this string is null or an System.String.Empty string.
     /// </summary>
     [ContractAnnotation("str:null => true")]
-    public static bool IsNullOrEmpty([NotNullWhen(false)]this string? str)
+    public static bool IsNullOrEmpty([System.Diagnostics.CodeAnalysis.NotNullWhen(false)]this string? str)
     {
         return string.IsNullOrEmpty(str);
     }
@@ -57,7 +56,7 @@ public static class AbpStringExtensions
     /// indicates whether this string is null, empty, or consists only of white-space characters.
     /// </summary>
     [ContractAnnotation("str:null => true")]
-    public static bool IsNullOrWhiteSpace([NotNullWhen(false)]this string? str)
+    public static bool IsNullOrWhiteSpace([System.Diagnostics.CodeAnalysis.NotNullWhen(false)]this string? str)
     {
         return string.IsNullOrWhiteSpace(str);
     }

framework/src/Volo.Abp.Core/Volo.Abp.Core.csproj

@@ -33,6 +33,10 @@
     <PackageReference Include="JetBrains.Annotations" Version="2022.1.0" />
     <PackageReference Include="Nito.AsyncEx.Coordination" Version="5.1.2" />
     <PackageReference Include="Nito.AsyncEx.Context" Version="5.1.2" />
+    <PackageReference Include="Nullable" Version="1.3.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETStandard' And $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '2.1')) ">
     <PackageReference Include="System.ComponentModel.Annotations" Version="5.0.0" />