Fix SourceTextComparer IEqualityComparer contract violation

src/Compilers/Core/CodeAnalysisTest/Text/SourceTextTests.cs

@@ -466,5 +466,147 @@ public void WriteWithSpanEndingAfterEndThrowsOutOfRange()
 
             Assert.Equal("span", ex.ParamName);
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/71006")]
+        public void Comparer_EqualityContract_SameContent_DifferentEncodings()
+        {
+            // Test case from the issue: two SourceText instances with same content but different encodings
+            // should be equal and have the same hash code when using content-based comparison
+            const string content = "Hello, World!";
+            var utf8 = Encoding.UTF8;
+            var unicode = Encoding.Unicode;
+
+            var text1 = SourceText.From(content, utf8);
+            var text2 = SourceText.From(content, unicode);
+
+            var comparer = SourceTextComparer.Instance;
+
+            // Both should be considered equal by the comparer (content-based)
+            Assert.True(comparer.Equals(text1, text2));
+
+            // They must have the same hash code to satisfy IEqualityComparer contract
+            Assert.Equal(comparer.GetHashCode(text1), comparer.GetHashCode(text2));
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/71006")]
+        public void Comparer_EqualityContract_SameContent_WithAndWithoutBOM()
+        {
+            // Test the case described in the issue comments:
+            // Create source texts from byte arrays where one has a BOM and the other doesn't
+            const string content = "Test content";
+
+            var utf8WithBOM = new UTF8Encoding(encoderShouldEmitUTF8Identifier: true);
+            var utf8NoBOM = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false);
+
+            // Create byte arrays with and without BOM
+            var preamble = utf8WithBOM.GetPreamble();
+            var contentBytes = utf8WithBOM.GetBytes(content);
+            var bytesWithBOM = new byte[preamble.Length + contentBytes.Length];
+            preamble.CopyTo(bytesWithBOM, 0);
+            contentBytes.CopyTo(bytesWithBOM, preamble.Length);
+
+            var bytesNoBOM = utf8NoBOM.GetBytes(content);
+
+            // Both pass Encoding.UTF8 as the encoding parameter
+            var textWithBOM = SourceText.From(bytesWithBOM, bytesWithBOM.Length, Encoding.UTF8);
+            var textNoBOM = SourceText.From(bytesNoBOM, bytesNoBOM.Length, Encoding.UTF8);
+
+            var comparer = SourceTextComparer.Instance;
+
+            // They should be equal based on content
+            Assert.True(comparer.Equals(textWithBOM, textNoBOM));
+
+            // They must have the same hash code
+            Assert.Equal(comparer.GetHashCode(textWithBOM), comparer.GetHashCode(textNoBOM));
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/71006")]
+        public void Comparer_EqualityContract_DifferentContent()
+        {
+            var text1 = SourceText.From("content1");
+            var text2 = SourceText.From("content2");
+
+            var comparer = SourceTextComparer.Instance;
+
+            // Different content should not be equal
+            Assert.False(comparer.Equals(text1, text2));
+
+            // Hash codes may or may not be equal (no requirement for unequal objects)
+            // but we just verify they don't throw
+            _ = comparer.GetHashCode(text1);
+            _ = comparer.GetHashCode(text2);
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/71006")]
+        public void Comparer_EqualityContract_NullHandling()
+        {
+            var text = SourceText.From("content");
+            var comparer = SourceTextComparer.Instance;
+
+            // null equals null
+            Assert.True(comparer.Equals(null, null));
+
+            // null does not equal non-null
+            Assert.False(comparer.Equals(null, text));
+            Assert.False(comparer.Equals(text, null));
+
+            // null hash code
+            Assert.Equal(0, comparer.GetHashCode(null));
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/71006")]
+        public void Comparer_EqualityContract_SameContentFromStream()
+        {
+            const string content = "Stream content";
+            var bytes = Encoding.UTF8.GetBytes(content);
+
+            // Create from stream
+            var stream1 = new MemoryStream(bytes);
+            var text1 = SourceText.From(stream1, Encoding.UTF8);
+
+            // Create from string
+            var text2 = SourceText.From(content, Encoding.UTF8);
+
+            var comparer = SourceTextComparer.Instance;
+
+            // Should be equal based on content
+            Assert.True(comparer.Equals(text1, text2));
+
+            // Must have same hash code
+            Assert.Equal(comparer.GetHashCode(text1), comparer.GetHashCode(text2));
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/71006")]
+        public void Comparer_ContentEquals_MatchesComparerEquals()
+        {
+            // Verify that SourceTextComparer.Equals aligns with SourceText.ContentEquals
+            const string content = "Test";
+
+            var text1 = SourceText.From(content, Encoding.UTF8);
+            var text2 = SourceText.From(content, Encoding.Unicode);
+
+            var comparer = SourceTextComparer.Instance;
+
+            // SourceTextComparer.Equals should match SourceText.ContentEquals
+            Assert.Equal(text1.ContentEquals(text2), comparer.Equals(text1, text2));
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/71006")]
+        public void Comparer_GetContentHash_MatchesComparerHashCode()
+        {
+            // Verify that SourceTextComparer uses content-based hashing
+            const string content = "Hash test content";
+
+            var text1 = SourceText.From(content, Encoding.UTF8);
+            var text2 = SourceText.From(content, Encoding.Unicode);
+
+            // Both should have the same content hash
+            Assert.True(text1.GetContentHash().SequenceEqual(text2.GetContentHash()));
+
+            var comparer = SourceTextComparer.Instance;
+
+            // And therefore the same hash code from the comparer
+            Assert.Equal(comparer.GetHashCode(text1), comparer.GetHashCode(text2));
+        }
     }
 }

src/Compilers/Core/Portable/Text/SourceTextComparer.cs

@@ -3,7 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
-using Roslyn.Utilities;
+using System.Runtime.InteropServices;
 
 namespace Microsoft.CodeAnalysis.Text
 {
@@ -32,13 +32,9 @@ public int GetHashCode(SourceText? obj)
                 return 0;
             }
 
-            var checksum = obj.GetChecksum();
-            var contentsHash = !checksum.IsDefault ? Hash.CombineValues(checksum) : 0;
-            var encodingHash = obj.Encoding != null ? obj.Encoding.GetHashCode() : 0;
-
-            return Hash.Combine(obj.Length,
-                Hash.Combine(contentsHash,
-                Hash.Combine(encodingHash, ((int)obj.ChecksumAlgorithm).GetHashCode())));
+            // GetContentHash returns a 16-byte hash, so reading the first 4 bytes as an int is always safe.
+            var contentHash = obj.GetContentHash();
+            return MemoryMarshal.Read<int>(contentHash.AsSpan());
         }
     }
 }