Make HitSampleInfo immutable

osu.Game/Utils/Optional.cs

@@ -0,0 +1,45 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+#nullable enable
+
+namespace osu.Game.Utils
+{
+    /// <summary>
+    /// A wrapper over a value and a boolean denoting whether the value is valid.
+    /// </summary>
+    /// <typeparam name="T">The type of value stored.</typeparam>
+    public readonly ref struct Optional<T>
+    {
+        /// <summary>
+        /// The stored value.
+        /// </summary>
+        public readonly T Value;
+
+        /// <summary>
+        /// Whether <see cref="Value"/> is valid.
+        /// </summary>
+        /// <remarks>
+        /// If <typeparamref name="T"/> is a reference type, <c>null</c> may be valid for <see cref="Value"/>.
+        /// </remarks>
+        public readonly bool HasValue;
+
+        private Optional(T value)
+        {
+            Value = value;
+            HasValue = true;
+        }
+
+        /// <summary>
+        /// Returns <see cref="Value"/> if it's valid, or a given fallback value otherwise.
+        /// </summary>
+        /// <remarks>
+        /// Shortcase for: <c>optional.HasValue ? optional.Value : fallback</c>.
+        /// </remarks>
+        /// <param name="fallback">The fallback value to return if <see cref="HasValue"/> is <c>false</c>.</param>
+        /// <returns></returns>
+        public T GetOr(T fallback) => HasValue ? Value : fallback;
+
+        public static implicit operator Optional<T>(T value) => new Optional<T>(value);
+    }
+}