Fix overflow in LengthBuckets for large strings

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/String/LengthBuckets.cs

@@ -27,18 +27,20 @@ internal static class LengthBuckets
                 return null;
             }
 
-            int arraySize = spread * MaxPerLength;
+            long expectedArraySize = (long)spread * MaxPerLength;
 #if NET
-            if (arraySize > Array.MaxLength)
+            if (expectedArraySize > Array.MaxLength)
 #else
-            if (arraySize > 0X7FFFFFC7)
+            if (expectedArraySize > 0X7FFFFFC7)
 #endif
             {
                 // In the future we may lower the value, as it may be quite unlikely
                 // to have a LOT of strings of different sizes.
                 return null;
             }
 
+            int arraySize = (int)expectedArraySize;
+
             // Instead of creating a dictionary of lists or a multi-dimensional array
             // we rent a single dimension array, where every bucket has five slots.
             // The bucket starts at (key.Length - minLength) * 5.

src/libraries/System.Collections.Immutable/tests/Frozen/FrozenDictionaryTests.cs

@@ -419,6 +419,19 @@ public void ContainsKey_WithNonAscii(int percentageWithNonAscii)
 
             Assert.All(expected, kvp => actual.ContainsKey(kvp.Key));
         }
+
+        [Fact]
+        [OuterLoop]
+        public void ToFrozenDictionary_WithExtremelyLargeStrings()
+        {
+            // Test case with extremely large strings that exceed length bucket boundaries.
+            var keys = new[] { "", new string('a', 0X7FFFFFC7 / 4) };
+            var frozen = keys.ToFrozenDictionary(s => s, GetKeyIEqualityComparer());
+            Assert.Equal(keys.Length, frozen.Count);
+            Assert.Equal(keys.Length, frozen.Keys.Length);
+            Assert.Equal(keys.Length, frozen.Values.Length);
+            Assert.All(keys, key => frozen.ContainsKey(key));
+        }
     }
 
     public class FrozenDictionary_Generic_Tests_string_string_Default : FrozenDictionary_Generic_Tests_string_string

src/libraries/System.Collections.Immutable/tests/Frozen/FrozenSetTests.cs

@@ -376,6 +376,17 @@ public void Contains_WithNonAscii(int percentageWithNonAscii)
 
             Assert.All(expected, s => actual.Contains(s));
         }
+
+        [Fact]
+        [OuterLoop]
+        public void ToFrozenSet_WithExtremelyLargeStrings()
+        {
+            // Test case with extremely large strings that exceed length bucket boundaries.
+            var keys = new[] { "", new string('a', 0X7FFFFFC7 / 4) };
+            var frozen = keys.ToFrozenSet(GetIEqualityComparer());
+            Assert.Equal(keys.Length, frozen.Count);
+            Assert.All(keys, key => frozen.Contains(key));
+        }
     }
 
     public class FrozenSet_Generic_Tests_string_Default : FrozenSet_Generic_Tests_string