Add new argument matcher for comparing objects by reference

src/NSubstitute/Arg.cs

@@ -65,6 +65,15 @@ public static ref T Is<T>(IArgumentMatcher matcher) =>
     public static ref T Is<T>(IArgumentMatcher<T> matcher) =>
         ref ArgumentMatcher.Enqueue(matcher);
 
+    /// <summary>
+    /// Match argument that has the same reference as <paramref name="value"/>.
+    /// </summary>
+    public static ref T Same<T>(T value)
+        where T : class
+    {
+        return ref ArgumentMatcher.Enqueue(new ReferenceArgumentMatcher<T>(value));
+    }
+
     /// <summary>
     /// Invoke any <see cref="Action"/> argument whenever a matching call is made to the substitute.
     /// </summary>

src/NSubstitute/Compatibility/Arg.Compat.cs

@@ -59,6 +59,13 @@ public static class Compat
         /// </summary>
         public static T Is<T>(IArgumentMatcher<T> matcher) => ArgumentMatcher.Enqueue(matcher);
 
+        /// <summary>
+        /// Match argument that has the same reference as <paramref name="value"/>.
+        /// This is provided for compatibility with older compilers --
+        /// if possible use <see cref="Arg.Same{T}(T)" /> instead.
+        /// </summary>
+        public static T Same<T>(T value) where T : class => Arg.Same(value);
+
         /// <summary>
         /// Invoke any <see cref="Action"/> argument whenever a matching call is made to the substitute.
         /// This is provided for compatibility with older compilers --

src/NSubstitute/Compatibility/CompatArg.cs

@@ -68,6 +68,13 @@ private CompatArg() { }
     /// </summary>
     public static T Is<T>(IArgumentMatcher<T> matcher) => ArgumentMatcher.Enqueue(matcher);
 
+    /// <summary>
+    /// Match argument that has the same reference as <paramref name="value"/>.
+    /// This is provided for compatibility with older compilers --
+    /// if possible use <see cref="Arg.Same{T}(T)" /> instead.
+    /// </summary>
+    public T Same<T>(T value) where T : class => Arg.Same(value);
+
     /// <summary>
     /// Invoke any <see cref="Action"/> argument whenever a matching call is made to the substitute.
     /// This is provided for compatibility with older compilers --

src/NSubstitute/Core/Arguments/ReferenceArgumentMatcher.cs

@@ -0,0 +1,17 @@
+namespace NSubstitute.Core.Arguments;
+
+public class ReferenceArgumentMatcher<T>(T value) : IArgumentMatcher<T>, IDescribeNonMatches
+    where T : class
+{
+    public override string ToString() => ArgumentFormatter.Default.Format(value, false);
+
+    public string DescribeFor(object? argument)
+    {
+        var expected = ArgumentFormatter.Default.Format(value, false);
+        var actual = ArgumentFormatter.Default.Format(argument, false);
+
+        return $"Reference of {expected} does not match reference of {actual}.";
+    }
+
+    public bool IsSatisfiedBy(T? argument) => ReferenceEquals(argument, value);
+}

tests/NSubstitute.Acceptance.Specs/ArgumentMatching.cs

@@ -350,6 +350,15 @@ public void Should_add_list_of_all_pending_specifications_to_ambiguous_exception
         Assert.That(exception.Message, Contains.Substring("42"));
     }
 
+    [Test]
+    public void Should_match_identical_objects()
+    {
+        var obj = new object();
+        _something.Anything(Arg.Same(obj)).Returns(1);
+        Assert.That(_something.Anything(obj), Is.EqualTo(1));
+        Assert.That(_something.Anything(new object()), Is.EqualTo(0));
+    }
+
     [Test]
     public void Returns_should_work_with_params()
     {

tests/NSubstitute.Acceptance.Specs/ArgumentMatchingCompat.cs

@@ -175,6 +175,15 @@ public void Should_add_list_of_all_pending_specifications_to_ambiguous_exception
         Assert.That(exception.Message, Contains.Substring("42"));
     }
 
+    [Test]
+    public void Should_match_identical_objects()
+    {
+        var obj = new object();
+        _something.Anything(Arg.Compat.Same(obj)).Returns(1);
+        Assert.That(_something.Anything(obj), Is.EqualTo(1));
+        Assert.That(_something.Anything(new object()), Is.EqualTo(0));
+    }
+
     [Test]
     public void Returns_should_work_with_params()
     {