Assert.AreEqual allow IEquatable<T> for actual and expected (#2382)

src/TestFramework/TestFramework/Assertions/Assert.AreEqual.cs

@@ -201,6 +201,105 @@ public static void AreEqual<T>(T? expected, T? actual, string? message, params o
         ThrowAssertFailed("Assert.AreEqual", finalMessage);
     }
 
+    /// <summary>
+    /// Tests whether the specified values are equal and throws an exception
+    /// if the two values are not equal.
+    /// The equality is computed using the default <see cref="System.Collections.Generic.EqualityComparer{T}"/>.
+    /// </summary>
+    /// <typeparam name="T">
+    /// The type of values to compare.
+    /// </typeparam>
+    /// <param name="expected">
+    /// The first value to compare. This is the value the tests expects.
+    /// </param>
+    /// <param name="actual">
+    /// The second value to compare. This is the value produced by the code under test.
+    /// </param>
+    /// <exception cref="AssertFailedException">
+    /// Thrown if <paramref name="expected"/> is not equal to <paramref name="actual"/>.
+    /// </exception>
+    public static void AreEqual<T>(IEquatable<T>? expected, IEquatable<T>? actual)
+        => AreEqual(expected, actual, string.Empty, null);
+
+    /// <summary>
+    /// Tests whether the specified values are equal and throws an exception
+    /// if the two values are not equal.
+    /// The equality is computed using the default <see cref="System.Collections.Generic.EqualityComparer{T}"/>.
+    /// </summary>
+    /// <typeparam name="T">
+    /// The type of values to compare.
+    /// </typeparam>
+    /// <param name="expected">
+    /// The first value to compare. This is the value the tests expects.
+    /// </param>
+    /// <param name="actual">
+    /// The second value to compare. This is the value produced by the code under test.
+    /// </param>
+    /// <param name="message">
+    /// The message to include in the exception when <paramref name="actual"/>
+    /// is not equal to <paramref name="expected"/>. The message is shown in
+    /// test results.
+    /// </param>
+    /// <exception cref="AssertFailedException">
+    /// Thrown if <paramref name="expected"/> is not equal to
+    /// <paramref name="actual"/>.
+    /// </exception>
+    public static void AreEqual<T>(IEquatable<T>? expected, IEquatable<T>? actual, string? message)
+        => AreEqual(expected, actual, message, null);
+
+    /// <summary>
+    /// Tests whether the specified values are equal and throws an exception
+    /// if the two values are not equal.
+    /// The equality is computed using the default <see cref="System.Collections.Generic.EqualityComparer{T}"/>.
+    /// </summary>
+    /// <typeparam name="T">
+    /// The type of values to compare.
+    /// </typeparam>
+    /// <param name="expected">
+    /// The first value to compare. This is the value the tests expects.
+    /// </param>
+    /// <param name="actual">
+    /// The second value to compare. This is the value produced by the code under test.
+    /// </param>
+    /// <param name="message">
+    /// The message to include in the exception when <paramref name="actual"/>
+    /// is not equal to <paramref name="expected"/>. The message is shown in
+    /// test results.
+    /// </param>
+    /// <param name="parameters">
+    /// An array of parameters to use when formatting <paramref name="message"/>.
+    /// </param>
+    /// <exception cref="AssertFailedException">
+    /// Thrown if <paramref name="expected"/> is not equal to
+    /// <paramref name="actual"/>.
+    /// </exception>
+    public static void AreEqual<T>(IEquatable<T>? expected, IEquatable<T>? actual, string? message, params object?[]? parameters)
+    {
+        if (actual?.Equals(expected) == true)
+        {
+            return;
+        }
+
+        string userMessage = BuildUserMessage(message, parameters);
+        string finalMessage = actual != null && expected != null && !actual.GetType().Equals(expected.GetType())
+            ? string.Format(
+                CultureInfo.CurrentCulture,
+                FrameworkMessages.AreEqualDifferentTypesFailMsg,
+                userMessage,
+                ReplaceNulls(expected),
+                expected.GetType().FullName,
+                ReplaceNulls(actual),
+                actual.GetType().FullName)
+            : string.Format(
+                CultureInfo.CurrentCulture,
+                FrameworkMessages.AreEqualFailMsg,
+                userMessage,
+                ReplaceNulls(expected),
+                ReplaceNulls(actual));
+
+        ThrowAssertFailed("Assert.AreEqual", finalMessage);
+    }
+
     /// <summary>
     /// Tests whether the specified values are unequal and throws an exception
     /// if the two values are equal.

src/TestFramework/TestFramework/PublicAPI/PublicAPI.Shipped.txt

@@ -250,6 +250,9 @@ static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual(string? expe
 static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual(string? expected, string? actual, bool ignoreCase, System.Globalization.CultureInfo? culture) -> void
 static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual(string? expected, string? actual, bool ignoreCase, System.Globalization.CultureInfo? culture, string? message) -> void
 static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual(string? expected, string? actual, bool ignoreCase, System.Globalization.CultureInfo? culture, string? message, params object?[]? parameters) -> void
+static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual<T>(System.IEquatable<T>? expected, System.IEquatable<T>? actual) -> void
+static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual<T>(System.IEquatable<T>? expected, System.IEquatable<T>? actual, string? message) -> void
+static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual<T>(System.IEquatable<T>? expected, System.IEquatable<T>? actual, string? message, params object?[]? parameters) -> void
 static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual<T>(T? expected, T? actual) -> void
 static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual<T>(T? expected, T? actual, string? message) -> void
 static Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreEqual<T>(T? expected, T? actual, string? message, params object?[]? parameters) -> void

test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AreEqualTests.cs

@@ -381,6 +381,18 @@ public void AreEqualStringIgnoreCaseCultureInfoMessageParametersNullabilityPostC
         _ = cultureInfo.Calendar; // no warning
     }
 
+    public void AreEqualUsingCustomIEquatable()
+    {
+        var instanceOfA = new A { Id = "SomeId" };
+        var instanceOfB = new B { Id = "SomeId" };
+
+        // This call works because B implements IEquatable<A>
+        Assert.AreEqual(instanceOfA, instanceOfB);
+
+        // This one doesn't work
+        VerifyThrows(() => Assert.AreEqual(instanceOfB, instanceOfA));
+    }
+
     private CultureInfo? GetCultureInfo() => CultureInfo.CurrentCulture;
 
     private class TypeOverridesEquals
@@ -426,4 +438,32 @@ public override int GetHashCode(TypeOverridesEquals obj)
             throw new NotImplementedException();
         }
     }
+
+    private class A : IEquatable<A>
+    {
+        public string Id { get; set; } = string.Empty;
+
+        public bool Equals(A? other)
+            => other?.Id == Id;
+
+        public override bool Equals(object? obj)
+            => Equals(obj as A);
+
+        public override int GetHashCode()
+            => Id.GetHashCode() + 123;
+    }
+
+    private class B : IEquatable<A>
+    {
+        public string Id { get; set; } = string.Empty;
+
+        public override bool Equals(object? obj)
+            => Equals(obj as A);
+
+        public bool Equals(A? other)
+            => other?.Id == Id;
+
+        public override int GetHashCode()
+            => Id.GetHashCode() + 1234;
+    }
 }