fix: let OrderedCollection.GetRange take a zero-length range at the end [patch]

Containers.Test/OrderedCollectionTests.cs

@@ -471,6 +471,45 @@ public void GetRange_ValidRange_ReturnsCorrectSubset()
 		Assert.AreEqual(6, range[3]);
 	}
 
+	[TestMethod]
+	public void GetRange_ZeroCountAtEnd_ReturnsEmpty()
+	{
+		// Arrange
+		OrderedCollection<int> collection = new([1, 2, 3]);
+
+		// Act: startIndex == Count is valid when count is zero, as it is for List<T>.GetRange
+		OrderedCollection<int> range = collection.GetRange(collection.Count, 0);
+
+		// Assert
+		Assert.AreEqual(0, range.Count);
+	}
+
+	[TestMethod]
+	public void GetRange_ZeroCountOnEmptyCollection_ReturnsEmpty()
+	{
+		// Arrange
+		OrderedCollection<int> collection = [];
+
+		// Act: the only range an empty collection has is the empty one at index 0
+		OrderedCollection<int> range = collection.GetRange(0, 0);
+
+		// Assert
+		Assert.AreEqual(0, range.Count);
+	}
+
+	[TestMethod]
+	public void GetRange_ZeroCountMidCollection_ReturnsEmpty()
+	{
+		// Arrange
+		OrderedCollection<int> collection = new([1, 2, 3]);
+
+		// Act
+		OrderedCollection<int> range = collection.GetRange(1, 0);
+
+		// Assert
+		Assert.AreEqual(0, range.Count);
+	}
+
 	[TestMethod]
 	public void GetRange_InvalidStartIndex_ThrowsArgumentOutOfRangeException()
 	{

Containers/OrderedCollection.cs

@@ -345,11 +345,14 @@ public int IndexOf(T item)
 	/// <param name="startIndex">The zero-based starting index of the range.</param>
 	/// <param name="count">The number of elements in the range.</param>
 	/// <returns>A new OrderedCollection containing the elements in the specified range.</returns>
+	/// <remarks>
+	/// As with <see cref="List{T}.GetRange"/>, a <paramref name="startIndex"/> equal to
+	/// <see cref="Count"/> is valid when <paramref name="count"/> is zero, and returns an empty collection.
+	/// </remarks>
 	/// <exception cref="ArgumentOutOfRangeException">Thrown when parameters are out of range.</exception>
 	public OrderedCollection<T> GetRange(int startIndex, int count)
 	{
 		ArgumentOutOfRangeException.ThrowIfNegative(startIndex);
-		ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual(startIndex, Count);
 		ArgumentOutOfRangeException.ThrowIfNegative(count);
 		ArgumentOutOfRangeException.ThrowIfGreaterThan(startIndex + count, Count);