Introduced new "ReadOnlySet" class and "ReadOnlySet.Empty<T>" static …

…method

Similar to the "Enumerable.Empty<T>" and "Array.Empty" method in .NET, Skybrud.Essentials now offers a method to get an empty immutable instance of "IReadOnlySet". As the "IReadOnlySet" interface is only supported from .NET 5 and up, the new class and method is also only available for .NET 5 and up, but not .NET Framework and .NET Standard.

src/Skybrud.Essentials/Collections/ReadOnlySet.cs

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+#if NET5_0_OR_GREATER
+
+using System.Collections.Generic;
+using System.Collections.Immutable;
+
+namespace Skybrud.Essentials.Collections;
+
+/// <summary>
+/// Static class for working with <see cref="IReadOnlySet{T}"/>.
+/// </summary>
+public static class ReadOnlySet {
+
+    /// <summary>
+    /// Returns an empty instance of <see cref="IReadOnlySet{T}"/>.
+    /// </summary>
+    /// <typeparam name="T">The item type of the list.</typeparam>
+    /// <returns>An instance of <see cref="IReadOnlySet{T}"/>.</returns>
+    public static IReadOnlySet<T> Empty<T>() {
+        return EmptyHashSet<T>.Value;
+    }
+
+    private static class EmptyHashSet<T> {
+
+        internal static readonly IReadOnlySet<T> Value = ImmutableHashSet.Create<T>();
+
+    }
+
+}
+
+#endif

src/TestProject1/Collections/ReadOnlySetTests.cs

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+using Skybrud.Essentials.Collections;
+
+namespace TestProject1.Collections;
+
+[TestClass]
+public class ReadOnlySetTests {
+
+    [TestMethod]
+    public void Empty() {
+
+        var a = ReadOnlySet.Empty<string>();
+        var b = ReadOnlySet.Empty<string>();
+
+        Assert.AreEqual(a, b);
+
+        Assert.AreEqual(a.GetHashCode(), b.GetHashCode());
+
+    }
+
+}