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Fix IndexOf upper-bound guard in ImmutableArray/List #124967

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Fix IndexOf upper-bound guard in ImmutableArray/List #124967

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a338be4
Fix IndexOf upper-bound guard in ImmutableArray/List
prozolic Feb 27, 2026
4e7130e
Merge branch 'main' into indexof
prozolic Mar 12, 2026
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2 changes: 1 addition & 1 deletion ...System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.Builder.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -796,7 +796,7 @@ public int IndexOf(T item, int startIndex, int count, IEqualityComparer<T>? equa
return -1;
}

Requires.Range(startIndex >= 0 && startIndex < this.Count, nameof(startIndex));
Requires.Range(startIndex >= 0 && startIndex <= this.Count, nameof(startIndex));
Requires.Range(count >= 0 && startIndex + count <= this.Count, nameof(count));

equalityComparer ??= EqualityComparer<T>.Default;
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2 changes: 1 addition & 1 deletion ...braries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.cs
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Expand Up @@ -173,7 +173,7 @@ public int IndexOf(T item, int startIndex, int count, IEqualityComparer<T>? equa
return -1;
}

Requires.Range(startIndex >= 0 && startIndex < self.Length, nameof(startIndex));
Requires.Range(startIndex >= 0 && startIndex <= self.Length, nameof(startIndex));
Requires.Range(count >= 0 && startIndex + count <= self.Length, nameof(count));

equalityComparer ??= EqualityComparer<T>.Default;
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10 changes: 7 additions & 3 deletions ...ies/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableList_1.Node.cs
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Expand Up @@ -759,9 +759,13 @@ internal int BinarySearch(int index, int count, T item, IComparer<T>? comparer)
/// </returns>
internal int IndexOf(T item, int index, int count, IEqualityComparer<T>? equalityComparer)
{
Requires.Range(index >= 0, nameof(index));
Requires.Range(count >= 0, nameof(count));
Requires.Range(count <= this.Count, nameof(count));
if (count == 0 && index == 0)
{
return -1;
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@MihaZupan MihaZupan Feb 27, 2026

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Why is this fast path needed?

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@prozolic prozolic Feb 27, 2026 •
edited
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I added to match the behavior of ImmutableArray.IndexOf.
This fast path can be removed because -1 is returned even without it.

}

Requires.Range(index >= 0 && index <= this.Count, nameof(index));
Requires.Range(count >= 0 && count <= this.Count, nameof(count));
Requires.Range(index + count <= this.Count, nameof(count));

equalityComparer ??= EqualityComparer<T>.Default;
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20 changes: 19 additions & 1 deletion src/libraries/System.Collections.Immutable/tests/ImmutableArrayBuilderTest.cs
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Expand Up @@ -255,7 +255,8 @@ public void IndexOf()
(b, v) => b.IndexOf(v),
(b, v, i) => b.IndexOf(v, i),
(b, v, i, c) => b.IndexOf(v, i, c),
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq),
"startIndex");
}

[Fact]
Expand All @@ -268,6 +269,23 @@ public void IndexOf_WithoutCountParam()
Assert.Equal(-1, builder.IndexOf(-5, 2, absComparer));
}

[Fact]
public void IndexOfConsistentWithArray()
{
ImmutableArray<int>.Builder builder = ImmutableArray.Create(1, 2, 3, 4).ToBuilder();
int[] array = new[] { 1, 2, 3, 4 };

Assert.Equal(-1, builder.IndexOf(2, builder.Count, 0));
Assert.Equal(-1, Array.IndexOf(array, 2, array.Length, 0));

for (int i = 0; i < array.Length; i++)
{
int builderResult = builder.IndexOf(builder[i], 0, builder.Count);
int arrayResult = Array.IndexOf(array, array[i], 0, array.Length);
Assert.Equal(builderResult, arrayResult);
}
}

[Fact]
public void LastIndexOf()
{
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20 changes: 19 additions & 1 deletion src/libraries/System.Collections.Immutable/tests/ImmutableArrayTest.cs
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Expand Up @@ -790,7 +790,25 @@ public void IndexOf()
(b, v) => b.IndexOf(v),
(b, v, i) => b.IndexOf(v, i),
(b, v, i, c) => b.IndexOf(v, i, c),
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq),
"startIndex");
}

[Fact]
public void IndexOfConsistentWithArray()
{
ImmutableArray<int> immutableArray = ImmutableArray.Create(1, 2, 3, 4);
int[] array = new[] { 1, 2, 3, 4 };

Assert.Equal(-1, immutableArray.IndexOf(2, immutableArray.Length, 0));
Assert.Equal(-1, Array.IndexOf(array, 2, array.Length, 0));

for (int i = 0; i < array.Length; i++)
{
int immutableArrayResult = immutableArray.IndexOf(immutableArray[i], 0, immutableArray.Length);
int arrayResult = Array.IndexOf(array, array[i], 0, array.Length);
Assert.Equal(immutableArrayResult, arrayResult);
}
}

[Fact]
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3 changes: 2 additions & 1 deletion src/libraries/System.Collections.Immutable/tests/ImmutableListBuilderTest.cs
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Expand Up @@ -333,7 +333,8 @@ public void IndexOf()
(b, v) => b.IndexOf(v),
(b, v, i) => b.IndexOf(v, i),
(b, v, i, c) => b.IndexOf(v, i, c),
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq),
"index");
}

[Fact]
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23 changes: 21 additions & 2 deletions src/libraries/System.Collections.Immutable/tests/ImmutableListTest.cs
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Expand Up @@ -494,13 +494,32 @@ public void IndexOf()
(b, v) => b.IndexOf(v),
(b, v, i) => b.IndexOf(v, i),
(b, v, i, c) => b.IndexOf(v, i, c),
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq),
"index");
IndexOfTests.IndexOfTest(
seq => (IImmutableList<int>)ImmutableList.CreateRange(seq),
(b, v) => b.IndexOf(v),
(b, v, i) => b.IndexOf(v, i),
(b, v, i, c) => b.IndexOf(v, i, c),
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.IndexOf(v, i, c, eq),
"index");
}

[Fact]
public void IndexOfConsistentWithArray()
{
ImmutableList<int> list = ImmutableList.Create(1, 2, 3, 4);
int[] array = new[] { 1, 2, 3, 4 };

Assert.Equal(-1, list.IndexOf(2, list.Count, 0));
Assert.Equal(-1, Array.IndexOf(array, 2, array.Length, 0));

for (int i = 0; i < array.Length; i++)
{
int listResult = list.IndexOf(list[i], 0, list.Count);
int arrayResult = Array.IndexOf(array, array[i], 0, array.Length);
Assert.Equal(listResult, arrayResult);
}
}

[Fact]
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42 changes: 32 additions & 10 deletions src/libraries/System.Collections.Immutable/tests/IndexOfTests.cs
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Expand Up @@ -14,26 +14,48 @@ public static void IndexOfTest<TCollection>(
Func<TCollection, int, int> indexOfItem,
Func<TCollection, int, int, int> indexOfItemIndex,
Func<TCollection, int, int, int, int> indexOfItemIndexCount,
Func<TCollection, int, int, int, IEqualityComparer<int>, int> indexOfItemIndexCountEQ)
Func<TCollection, int, int, int, IEqualityComparer<int>, int> indexOfItemIndexCountEQ,
string indexParameterName)
{
TCollection emptyCollection = factory(new int[0]);
TCollection singleCollection = factory(new[] { 10 });
TCollection collection1256 = factory(new[] { 1, 2, 5, 6 });

Assert.Throws<ArgumentOutOfRangeException>(() => indexOfItemIndexCountEQ(emptyCollection, 100, 1, 1, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => indexOfItemIndexCountEQ(emptyCollection, 100, -1, 1, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => indexOfItemIndexCountEQ(collection1256, 100, 1, 20, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => indexOfItemIndexCountEQ(collection1256, 100, 1, -1, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => indexOfItemIndexCountEQ(emptyCollection, 100, 1, 1, new CustomComparer(50)));
Assert.Throws<ArgumentOutOfRangeException>(() => indexOfItemIndexCountEQ(emptyCollection, 100, -1, 1, new CustomComparer(50)));
Assert.Throws<ArgumentOutOfRangeException>(() => indexOfItemIndexCountEQ(collection1256, 100, 1, 20, new CustomComparer(1)));
Assert.Throws<ArgumentOutOfRangeException>(() => indexOfItemIndexCountEQ(collection1256, 100, 1, -1, new CustomComparer(1)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(indexParameterName, () => indexOfItemIndexCountEQ(emptyCollection, 100, 1, 1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(indexParameterName, () => indexOfItemIndexCountEQ(emptyCollection, 100, -1, 1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>("count", () => indexOfItemIndexCountEQ(singleCollection, 100, 1, 1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(indexParameterName, () => indexOfItemIndexCountEQ(singleCollection, 100, -1, 1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>("count", () => indexOfItemIndexCountEQ(collection1256, 100, 1, 20, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>("count", () => indexOfItemIndexCountEQ(collection1256, 100, 1, -1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(indexParameterName, () => indexOfItemIndexCountEQ(emptyCollection, 100, 1, 1, new CustomComparer(50)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(indexParameterName, () => indexOfItemIndexCountEQ(emptyCollection, 100, -1, 1, new CustomComparer(50)));
AssertExtensions.Throws<ArgumentOutOfRangeException>("count", () => indexOfItemIndexCountEQ(singleCollection, 100, 1, 1, new CustomComparer(50)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(indexParameterName, () => indexOfItemIndexCountEQ(singleCollection, 100, -1, 1, new CustomComparer(50)));
AssertExtensions.Throws<ArgumentOutOfRangeException>("count", () => indexOfItemIndexCountEQ(collection1256, 100, 1, 20, new CustomComparer(1)));
AssertExtensions.Throws<ArgumentOutOfRangeException>("count", () => indexOfItemIndexCountEQ(collection1256, 100, 1, -1, new CustomComparer(1)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(indexParameterName, () => indexOfItemIndex(collection1256, 2, 5));
AssertExtensions.Throws<ArgumentOutOfRangeException>("count", () => indexOfItemIndexCountEQ(collection1256, 6, 2, 4, EqualityComparer<int>.Default));

Assert.Equal(-1, indexOfItem(emptyCollection, 5));
Assert.Equal(-1, indexOfItemIndex(emptyCollection, 5, 0));
Assert.Equal(2, indexOfItemIndex(collection1256, 5, 1));
Assert.Equal(-1, indexOfItemIndexCount(emptyCollection, 5, 0, 0));
Assert.Equal(-1, indexOfItemIndexCountEQ(emptyCollection, 5, 0, 0, EqualityComparer<int>.Default));

Assert.Equal(0, indexOfItem(singleCollection, 10));
Assert.Equal(0, indexOfItemIndex(singleCollection, 10, 0));
Assert.Equal(0, indexOfItemIndexCount(singleCollection, 10, 0, 1));
Assert.Equal(0, indexOfItemIndexCountEQ(singleCollection, 10, 0, 1, EqualityComparer<int>.Default));
Assert.Equal(-1, indexOfItemIndexCountEQ(singleCollection, 100, 0, 1, EqualityComparer<int>.Default));
Assert.Equal(-1, indexOfItemIndexCountEQ(singleCollection, 10, 1, 0, EqualityComparer<int>.Default));

Assert.Equal(2, indexOfItem(collection1256, 5));
Assert.Equal(2, indexOfItemIndex(collection1256, 5, 1));
Assert.Equal(-1, indexOfItemIndexCount(collection1256, 5, 1, 1));
Assert.Equal(2, indexOfItemIndexCount(collection1256, 5, 1, 2));
Assert.Equal(2, indexOfItemIndexCountEQ(collection1256, 5, 0, 4, EqualityComparer<int>.Default));
Assert.Equal(3, indexOfItemIndexCountEQ(collection1256, 6, 0, 4, EqualityComparer<int>.Default));
Assert.Equal(-1, indexOfItemIndexCountEQ(collection1256, 100, 0, 4, EqualityComparer<int>.Default));
Assert.Equal(-1, indexOfItemIndexCountEQ(collection1256, 100, 4, 0, EqualityComparer<int>.Default));

// Create a list with contents: 100,101,102,103,104,100,101,102,103,104
ImmutableList<int> list = ImmutableList<int>.Empty.AddRange(Enumerable.Range(100, 5).Concat(Enumerable.Range(100, 5)));
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