fixed the Issue_Register_null_string

src/DryIoc/Container.cs

@@ -11819,7 +11819,8 @@ public override Expression CreateExpressionOrDefault(Request request)
             // otherwise just return a constant
             var instanceExpr = request.Container.GetConstantExpression(Instance);
             var serviceType = request.GetActualServiceType();
-            if (serviceType.GetTypeInfo().IsAssignableFrom(ImplementationType.GetTypeInfo()))
+            var implType = ImplementationType;
+            if (implType != null && serviceType.GetTypeInfo().IsAssignableFrom(implType.GetTypeInfo()))
                 return instanceExpr;
             return Convert(instanceExpr, serviceType);
         }

test/DryIoc.IssuesTests/Issue_Register_null_string.cs

@@ -0,0 +1,57 @@
+using System;
+using NUnit.Framework;
+
+namespace DryIoc.IssuesTests
+{
+    [TestFixture]
+    public class Issue_Register_null_string
+    {
+        [Test]
+        public void Test()
+        {
+            var rules = Rules.Default.WithConcreteTypeDynamicRegistrations(reuse: Reuse.Transient)
+                .With(Made.Of(FactoryMethod.ConstructorWithResolvableArguments))
+                .WithDefaultIfAlreadyRegistered(IfAlreadyRegistered.Replace)
+                .WithFuncAndLazyWithoutRegistration()
+                .WithTrackingDisposableTransients()
+                .WithoutFastExpressionCompiler()
+                .WithFactorySelector(Rules.SelectLastRegisteredFactory());
+
+            var c = new Container(rules);
+
+            c.Register<A>();
+
+            // Both variants work with DryIoc v5 (preview) but failing with the v4.8.4
+            //c.RegisterDelegate<string>(() => null, setup: Setup.With(asResolutionCall: true));
+            c.RegisterInstance<string>(null, setup: Setup.With(asResolutionCall: true));
+
+            var s = c.Resolve<string>();
+            var a = c.Resolve<A>();
+
+            var a2 = c.Resolve<A>(new object[] { "Hello World" });
+
+            c.RegisterInstance<string>(null);
+            var a3 = c.Resolve<A>();
+
+            c.RegisterInstance<string>("Hello New Registration");
+            var a4 = c.Resolve<A>();
+
+            c.RegisterDelegate<string>(() => "Hello Another Registration");
+            var a5 = c.Resolve<A>();
+
+            Console.WriteLine("Hello s: " + (s ?? "null"));
+            Console.WriteLine("Hello a.S: " + (a.S ?? "null"));
+            Console.WriteLine("Hello a2.S: " + (a2.S ?? "null"));
+            Console.WriteLine("Hello a3.S: " + (a3.S ?? "null"));
+
+            Console.WriteLine("Hello a4.S: " + (a4.S ?? "null"));
+            Console.WriteLine("Hello a5.S: " + (a5.S ?? "null"));
+        }
+
+        class A
+        {
+            public string S;
+            public A(string s) => S = s;
+        }
+    }
+}