diff --git a/src/JsonMergePatch.Abstractions/ITypeRepositoryExntesions.cs b/src/JsonMergePatch.Abstractions/ITypeRepositoryExntesions.cs
deleted file mode 100644
index 8061fc7..0000000
--- a/src/JsonMergePatch.Abstractions/ITypeRepositoryExntesions.cs
+++ /dev/null
@@ -1,13 +0,0 @@
-namespace LaDeak.JsonMergePatch.Abstractions
-{
-    public static class ITypeRepositoryExntesions
-    {
-        public static ITypeRepository Extend(this ITypeRepository repository, ITypeRepository other)
-        {
-            foreach (var item in other.GetAll())
-                if (!repository.TryGet(item.Key, out _))
-                    repository.Add(item.Key, item.Value);
-            return repository;
-        }
-    }
-}
\ No newline at end of file
diff --git a/src/JsonMergePatch.Abstractions/ITypeRepositoryExtensions.cs b/src/JsonMergePatch.Abstractions/ITypeRepositoryExtensions.cs
new file mode 100644
index 0000000..ba70e9f
--- /dev/null
+++ b/src/JsonMergePatch.Abstractions/ITypeRepositoryExtensions.cs
@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace LaDeak.JsonMergePatch.Abstractions
+{
+    public static class ITypeRepositoryExtensions
+    {
+        public static ITypeRepository Extend(this ITypeRepository target, ITypeRepository source)
+        {
+            _ = target ?? throw new ArgumentNullException(nameof(target));
+            _ = source ?? throw new ArgumentNullException(nameof(source));
+
+            foreach (var item in source.GetAll() ?? Enumerable.Empty<KeyValuePair<Type, Type>>())
+                if (!target.TryGet(item.Key, out _))
+                    target.Add(item.Key, item.Value);
+            return target;
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/JsonMergePatch.SourceGenerator/NameBuilder.cs b/src/JsonMergePatch.SourceGenerator/NameBuilder.cs
index 32042b2..fbff545 100644
--- a/src/JsonMergePatch.SourceGenerator/NameBuilder.cs
+++ b/src/JsonMergePatch.SourceGenerator/NameBuilder.cs
@@ -4,7 +4,7 @@ namespace LaDeak.JsonMergePatch.SourceGenerator
 {
     public static class NameBuilder
     {
-        internal const string Namespace = "LaDeak.JsonMergePatch.Generated";
+        private const string Namespace = "LaDeak.JsonMergePatch.Generated";
 
         public static string GetName(ITypeSymbol typeInfo) => $"{typeInfo.Name}Wrapped";
 
diff --git a/test/JsonMergePatch.SourceGenerator.Abstractions.Tests/ITypeRepositoryExtensionTests.cs b/test/JsonMergePatch.SourceGenerator.Abstractions.Tests/ITypeRepositoryExtensionTests.cs
new file mode 100644
index 0000000..83f057c
--- /dev/null
+++ b/test/JsonMergePatch.SourceGenerator.Abstractions.Tests/ITypeRepositoryExtensionTests.cs
@@ -0,0 +1,68 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using LaDeak.JsonMergePatch.Abstractions;
+using NSubstitute;
+using Xunit;
+
+namespace LaDeak.JsonMergePatch.SourceGenerator
+{
+    public class ITypeRepositoryExtensionTests
+    {
+        [Fact]
+        public void WithTypesInOther_ExtendAddsAllTo_TargetTypeRepository()
+        {
+            var testData = new[] { new KeyValuePair<Type, Type>(typeof(object), typeof(string)), new KeyValuePair<Type, Type>(typeof(object), typeof(int)) };
+            var targetRespository = Substitute.For<ITypeRepository>();
+            var sourceRepository = Substitute.For<ITypeRepository>();
+            sourceRepository.GetAll().Returns(testData);
+
+            var result = targetRespository.Extend(sourceRepository);
+
+            Assert.Same(targetRespository, result);
+            targetRespository.Received().Add(testData[0].Key, testData[0].Value);
+            targetRespository.Received().Add(testData[1].Key, testData[1].Value);
+        }
+
+        [Fact]
+        public void WithNoTypesInOther_ExtendAddsAllTo_TargetTypeRepository()
+        {
+            var testData = Enumerable.Empty<KeyValuePair<Type, Type>>();
+            var targetRespository = Substitute.For<ITypeRepository>();
+            var sourceRepository = Substitute.For<ITypeRepository>();
+            sourceRepository.GetAll().Returns(testData);
+
+            var result = targetRespository.Extend(sourceRepository);
+
+            Assert.Same(targetRespository, result);
+            targetRespository.DidNotReceiveWithAnyArgs().Add(null, null);
+        }
+
+        [Fact]
+        public void WithNullTypesInOther_ExtendAddsAllTo_TargetTypeRepository()
+        {
+            var targetRespository = Substitute.For<ITypeRepository>();
+            var sourceRepository = Substitute.For<ITypeRepository>();
+            sourceRepository.GetAll().Returns((IEnumerable<KeyValuePair<Type, Type>>)null);
+
+            var result = targetRespository.Extend(sourceRepository);
+
+            Assert.Same(targetRespository, result);
+            targetRespository.DidNotReceiveWithAnyArgs().Add(null, null);
+        }
+
+        [Fact]
+        public void NullTarget_ThrowsArgumetNullExcepion()
+        {
+            ITypeRepository targetRespository = null;
+            Assert.Throws<ArgumentNullException>(() => targetRespository.Extend(Substitute.For<ITypeRepository>()));
+        }
+
+        [Fact]
+        public void NullSource_ThrowsArgumetNullExcepion()
+        {
+            ITypeRepository targetRespository = Substitute.For<ITypeRepository>();
+            Assert.Throws<ArgumentNullException>(() => targetRespository.Extend(null));
+        }
+    }
+}
diff --git a/test/JsonMergePatch.SourceGenerator.Abstractions.Tests/NameBuilderTests.cs b/test/JsonMergePatch.SourceGenerator.Abstractions.Tests/NameBuilderTests.cs
new file mode 100644
index 0000000..59ec057
--- /dev/null
+++ b/test/JsonMergePatch.SourceGenerator.Abstractions.Tests/NameBuilderTests.cs
@@ -0,0 +1,59 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis;
+using NSubstitute;
+using Xunit;
+
+namespace LaDeak.JsonMergePatch.SourceGenerator.Tests
+{
+    public class NameBuilderTests
+    {
+        [Fact]
+        public void GetName_ReturnsTypeName_WithWrappedSuffix()
+        {
+            var typeSymbol = Substitute.For<ITypeSymbol>();
+            typeSymbol.Name.Returns("TestName");
+            var result = NameBuilder.GetName(typeSymbol);
+            Assert.Equal("TestNameWrapped", result);
+        }
+
+        [Fact]
+        public void GetNamespaceExtension_Returns_SafeExtension()
+        {
+            var typeSymbol = Substitute.For<ITypeSymbol>();
+            typeSymbol.ContainingNamespace.ToDisplayString().Returns("Test.Namespace");
+            var result = NameBuilder.GetNamespaceExtension(typeSymbol);
+            Assert.Equal("SafeTest.Namespace", result);
+        }
+
+        [Fact]
+        public void GetNamespace_Returns_Namespace_SafeExtension()
+        {
+            var typeSymbol = Substitute.For<ITypeSymbol>();
+            typeSymbol.ContainingNamespace.ToDisplayString().Returns("Test.Namespace");
+            var result = NameBuilder.GetNamespace(typeSymbol);
+            Assert.Equal("LaDeak.JsonMergePatch.Generated.SafeTest.Namespace", result);
+        }
+
+
+        [Fact]
+        public void GetNamespaceOnString_Returns_Namespace_SafeExtension()
+        {
+            var result = NameBuilder.GetNamespace("Test.Namespace");
+            Assert.Equal("LaDeak.JsonMergePatch.Generated.SafeTest.Namespace", result);
+        }
+
+        [Fact]
+        public void GetGetFullTypeName_Returns_Namespace_SafeExtension_TypeNameWrapped()
+        {
+            var typeSymbol = Substitute.For<ITypeSymbol>();
+            typeSymbol.Name.Returns("TestName");
+            typeSymbol.ContainingNamespace.ToDisplayString().Returns("Test.Namespace");
+            var result = NameBuilder.GetFullTypeName(typeSymbol);
+            Assert.Equal("LaDeak.JsonMergePatch.Generated.SafeTest.Namespace.TestNameWrapped", result);
+        }
+    }
+}