feat(DependencyInjector): throw ArgumentNullException in SetDependency

Core/DependencyInjector.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using JetBrains.Annotations;
 using UnityEngine;
 
 namespace UIComponents.Core
@@ -63,11 +64,14 @@ private static object CreateInstance(Type dependencyType)
             return instance;
         }
 
-        public void SetDependency<T>(T instance) where T : class
+        public void SetDependency<T>([NotNull] T instance) where T : class
         {
+            if (instance == null)
+                throw new ArgumentNullException("Dependency can not be set as null.");
+
             DependencyDictionary[typeof(T)] = instance;
         }
-        
+
         public T Provide<T>() where T : class
         {
             var type = typeof(T);

Tests/DependencyInjectorTests.cs

@@ -0,0 +1,44 @@
+using System;
+using NUnit.Framework;
+using UIComponents.Core;
+
+namespace UIComponents.Tests
+{
+    [TestFixture]
+    public class DependencyInjectorTests
+    {
+        private interface IDependency {}
+
+        public class DependencyOne : IDependency {}
+
+        public class DependencyTwo : IDependency {}
+
+        [TestFixture]
+        public class SetDependency
+        {
+            [Test]
+            public void Switches_The_Dependency()
+            {
+                var injector = new DependencyInjector();
+
+                injector.SetDependency<IDependency>(new DependencyOne());
+
+                Assert.That(injector.Provide<IDependency>(), Is.InstanceOf<DependencyOne>());
+
+                injector.SetDependency<IDependency>(new DependencyTwo());
+
+                Assert.That(injector.Provide<IDependency>(), Is.InstanceOf<DependencyTwo>());
+            }
+
+            [Test]
+            public void Throws_Exception_If_Null_Is_Given_As_Parameter()
+            {
+                var injector = new DependencyInjector();
+
+                Assert.Throws<ArgumentNullException>(
+                    () => injector.SetDependency<IDependency>(null)
+                );
+            }
+        }
+    }
+}