Support for IContext.Tag with a value of null in factories.

src/Pure.DI.Core/Core/Code/FactoryRewriter.cs

@@ -218,6 +218,11 @@ public override SyntaxNode VisitExpressionStatement(ExpressionStatementSyntax no
             {
                 return SyntaxFactory.LiteralExpression(SyntaxKind.StringLiteralExpression, token);
             }
+
+            if (token.IsKind(SyntaxKind.NullKeyword))
+            {
+                return SyntaxFactory.LiteralExpression(SyntaxKind.StringLiteralExpression, token);
+            }
         }
 
         return base.VisitMemberAccessExpression(node);

tests/Pure.DI.IntegrationTests/FactoryTests.cs

@@ -792,6 +792,78 @@ public static void Main()
         result.StdOut.ShouldBe(ImmutableArray.Create("123", "Created"), result);
     }
 
+    [Fact]
+    public async Task ShouldReplaceContextTagWhenTagIsNull()
+    {
+        // Given
+
+        // When
+        var result = await """
+using System;
+using Pure.DI;
+
+namespace Sample
+{
+    interface IDependency {}
+
+    class Dependency: IDependency {}
+
+    interface IService
+    {
+        IDependency Dep { get; }
+    }
+
+    class Service: IService 
+    {
+        public Service(IDependency dep)
+        { 
+            Dep = dep;
+            Console.WriteLine("Created");           
+        }
+
+        public IDependency Dep { get; }
+    }
+
+    internal partial class Composition
+    {
+        private partial T OnDependencyInjection<T>(in T value, object? tag, Lifetime lifetime)            
+        {
+            return value;                  
+        }
+    }
+
+    static class Setup
+    {
+        private static void SetupComposition()
+        {
+            // OnDependencyInjection = On
+            DI.Setup("Composition")
+                .Bind<IDependency>(123).To(ctx => new Dependency())
+                .Bind<IService>().To(ctx => {
+                    System.Console.WriteLine(ctx.Tag ?? "null");
+                    ctx.Inject<IDependency>(123, out var dependency);
+                    return new Service(dependency);
+                })    
+                .Root<IService>("Service");
+        }
+    }
+
+    public class Program
+    {
+        public static void Main()
+        {
+            var composition = new Composition();      
+            var service = composition.Service;                                                 
+        }
+    }                
+}
+""".RunAsync(new Options(LanguageVersion.CSharp9));
+
+        // Then
+        result.Success.ShouldBeTrue(result);
+        result.StdOut.ShouldBe(ImmutableArray.Create("null", "Created"), result);
+    }
+
     [Fact]
     public async Task ShouldSupportFactoryWithInjectWhenGenericTypeIsNotSpecifiedAsTypeArgument()
     {