Added various extensions.

Added coding style helpers.
Unit tests.

SystemExtensions.NetStandard/Extensions/ObjectExtensions.cs

@@ -0,0 +1,32 @@
+namespace System.Extensions
+{
+    public static class ObjectExtensions
+    {
+        public static T Cast<T>(this object obj)
+        {
+            return (T)obj;
+        }
+
+        public static T As<T>(this object obj) where T : class
+        {
+            return obj as T;
+        }
+
+        public static Optional<T> ToOptional<T>(this T obj)
+        {
+            return Optional.FromValue(obj);
+        }
+
+        public static T ThrowIfNull<T>([ValidatedNotNull] this T obj, string name) where T : class
+        {
+            if (obj is null) throw new ArgumentNullException(name);
+
+            return obj;
+        }
+
+        [AttributeUsage(AttributeTargets.Parameter)]
+        sealed class ValidatedNotNullAttribute : Attribute
+        {
+        }
+    }
+}