Added various extensions.

Added coding style helpers.
Unit tests.

SystemExtensions.NetStandard/Extensions/Optional.cs

@@ -0,0 +1,178 @@
+namespace System.Extensions
+{
+    public static class Optional
+    {
+        public static Optional<T> None<T>()
+        {
+            return new Optional<T>.None();
+        }
+
+        public static Optional<T> FromValue<T>(T value)
+        {
+            if (value == null)
+            {
+                return new Optional<T>.None();
+            }
+
+            return new Optional<T>.Some(value);
+        }
+    }
+
+    public abstract class Optional<T> : IEquatable<Optional<T>>
+    {
+        public Optional(T value)
+        {
+            this.Value = value;
+        }
+
+        private T Value { get; }
+
+        public T ExtractValue()
+        {
+            if (this is None)
+            {
+                return default;
+            }
+            else
+            {
+                return Value;
+            }
+        }
+        public Optional<T> Do(Action<T> onSome, Action onNone)
+        {
+            if (onSome is null) throw new ArgumentNullException(nameof(onSome));
+            if (onNone is null) throw new ArgumentNullException(nameof(onNone));
+
+            if (this is Some)
+            {
+                onSome.Invoke(this.Value);
+            }
+            else
+            {
+                onNone.Invoke();
+            }
+            return this;
+        }
+        public Optional<T> DoAny(Action<T> onSome = null, Action onNone = null)
+        {
+            if (this is Some)
+            {
+                onSome?.Invoke(this.Value);
+            }
+            else
+            {
+                onNone?.Invoke();
+            }
+            return this;
+        }
+        public Optional<V> Switch<V>(Func<T, V> onSome, Func<V> onNone)
+        {
+            if (onSome is null) throw new ArgumentNullException($"{nameof(onSome)}");
+            if (onNone is null) throw new ArgumentNullException($"{nameof(onNone)}");
+
+            if (this is Some)
+            {
+                return Optional.FromValue(onSome.Invoke(this.Value));
+            }
+            else
+            {
+                return Optional.FromValue(onNone.Invoke());
+            }
+        }
+        public Optional<V> SwitchAny<V>(Func<T, V> onSome = null, Func<V> onNone = null)
+        {
+            if (this is Some)
+            {
+                return onSome != null ? Optional.FromValue(onSome.Invoke(this.Value)) : Optional.FromValue(default(V));
+            }
+            else
+            {
+                return onNone != null ? Optional.FromValue(onNone.Invoke()) : Optional.FromValue(default(V));
+            }
+        }
+
+        public override bool Equals(object obj)
+        {
+            if (obj is Optional<T>)
+            {
+                return this.Equals(obj);
+            }
+
+            return base.Equals(obj);
+        }
+
+        public bool Equals(Optional<T> other)
+        {
+            if (this is Some && other is Some)
+            {
+                return this.As<Some>().Equals(other.As<Some>());
+            }
+
+            return false;
+        }
+
+        public static implicit operator Optional<T>(T value)
+        {
+            return Optional.FromValue(value);
+        }
+
+        public override int GetHashCode()
+        {
+            return this.Value == null ? base.GetHashCode() : this.Value.GetHashCode();
+        }
+
+        internal class Some : Optional<T>, IEquatable<Some>
+        {
+            public Some(T value) : base(value)
+            {
+            }
+
+            public override bool Equals(object obj)
+            {
+                if (obj is Some)
+                {
+                    return this.Equals(obj.As<Some>());
+                }
+
+                return base.Equals(obj);
+            }
+
+            public bool Equals(Some other)
+            {
+                if (other is None)
+                {
+                    return false;
+                }
+
+                if (other is Some)
+                {
+                    return this.Value.Equals(other.Value);
+                }
+
+                return false;
+            }
+
+            public static bool operator ==(Some left, Some right)
+            {
+                return left?.Equals(right) == true;
+            }
+
+            public static bool operator !=(Some left, Some right)
+            {
+                return left?.Equals(right) != true;
+            }
+
+            public override int GetHashCode()
+            {
+                return this.Value is object ? this.Value.GetHashCode() : this.As<object>().GetHashCode();
+            }
+        }
+
+        internal class None : Optional<T>
+        {
+            public None() : base(default)
+            {
+            }
+        }
+    }
+}