DefaultTestCaseOrderer.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Xunit.Internal;
using Xunit.Sdk;

namespace Xunit.v3
{
	/// <summary>
	/// Default implementation of <see cref="ITestCaseOrderer"/>. Orders tests in
	/// an unpredictable but stable order, so that repeated test runs of the
	/// identical test assembly run tests in the same order.
	/// </summary>
	public class DefaultTestCaseOrderer : ITestCaseOrderer
	{
		readonly _IMessageSink diagnosticMessageSink;

		/// <summary>
		/// Initializes a new instance of the <see cref="DefaultTestCaseOrderer"/> class.
		/// </summary>
		/// <param name="diagnosticMessageSink">The message sink which receives <see cref="_DiagnosticMessage"/> messages.</param>
		public DefaultTestCaseOrderer(_IMessageSink diagnosticMessageSink)
		{
			this.diagnosticMessageSink = Guard.ArgumentNotNull(nameof(diagnosticMessageSink), diagnosticMessageSink);
		}

		/// <inheritdoc/>
		public IEnumerable<TTestCase> OrderTestCases<TTestCase>(IEnumerable<TTestCase> testCases)
			where TTestCase : _ITestCase
		{
			var result = testCases.ToList();

			try
			{
				result.Sort(Compare);
			}
			catch (Exception ex)
			{
				diagnosticMessageSink.OnMessage(new _DiagnosticMessage { Message = $"Exception thrown in DefaultTestCaseOrderer.OrderTestCases(); falling back to random order.{Environment.NewLine}{ex}" });
				result = Randomize(result);
			}

			return result;
		}

		List<TTestCase> Randomize<TTestCase>(List<TTestCase> testCases)
		{
			var result = new List<TTestCase>(testCases.Count);
			var randomizer = new Random();

			while (testCases.Count > 0)
			{
				var next = randomizer.Next(testCases.Count);
				result.Add(testCases[next]);
				testCases.RemoveAt(next);
			}

			return result;
		}

		int Compare<TTestCase>(TTestCase x, TTestCase y)
			where TTestCase : _ITestCase
		{
			Guard.ArgumentNotNull(nameof(x), x);
			Guard.ArgumentNotNull(nameof(y), y);
			Guard.ArgumentValid(nameof(x), $"Could not compare test case {x.DisplayName} because it has a null UniqueID", x.UniqueID != null);
			Guard.ArgumentValid(nameof(y), $"Could not compare test case {y.DisplayName} because it has a null UniqueID", y.UniqueID != null);

			return string.CompareOrdinal(x.UniqueID, y.UniqueID);
		}
	}
}