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Use Requires.Range in LastIndexOf #124560

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b1ee226
Use Requires.Range in LastIndexOf
prozolic Feb 18, 2026
35b481e
Fix test case in FindLastIndexTest
prozolic Feb 18, 2026
8cd336b
Fix test case to also validate the parameter name
prozolic Feb 19, 2026
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2 changes: 1 addition & 1 deletion ...ies/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableList_1.Node.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -808,7 +808,7 @@ internal int LastIndexOf(T item, int index, int count, IEqualityComparer<T>? equ

Requires.Range(index >= 0 && index < this.Count, nameof(index));
Requires.Range(count >= 0 && count <= this.Count, nameof(count));
Requires.Argument(index - count + 1 >= 0);
Requires.Range(index - count + 1 >= 0, nameof(count));
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Copilot AI Feb 18, 2026

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The change to use nameof(count) correctly aligns with ImmutableArray<T>.LastIndexOf and follows the established pattern in this codebase where range validation errors involving index + count or index - count are attributed to the count parameter (see line 765 for a similar pattern). However, there's an inconsistency with the FindLastIndex method at line 1249, which uses nameof(startIndex) for the equivalent validation check. Consider updating FindLastIndex in a follow-up change to use nameof(count) for consistency across both methods.

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@prozolic prozolic Feb 18, 2026 •
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Should we also change nameof(startIndex) in FindLastIndex(int, int, Predicate<T>) to nameof(count)?
If it should be changed, I'll also update the tests to validate the parameter as well.


equalityComparer ??= EqualityComparer<T>.Default;
using (var enumerator = new Enumerator(this, startIndex: index, count: count, reversed: true))
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4 changes: 3 additions & 1 deletion src/libraries/System.Collections.Immutable/tests/ImmutableArrayBuilderTest.cs
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Expand Up @@ -277,7 +277,9 @@ public void LastIndexOf()
(b, v, eq) => b.LastIndexOf(v, b.Count > 0 ? b.Count - 1 : 0, b.Count, eq),
(b, v, i) => b.LastIndexOf(v, i),
(b, v, i, c) => b.LastIndexOf(v, i, c),
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq),
"startIndex",
"count");
}

[Fact]
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4 changes: 3 additions & 1 deletion src/libraries/System.Collections.Immutable/tests/ImmutableArrayTest.cs
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Expand Up @@ -802,7 +802,9 @@ public void LastIndexOf()
(b, v, eq) => b.LastIndexOf(v, eq),
(b, v, i) => b.LastIndexOf(v, i),
(b, v, i, c) => b.LastIndexOf(v, i, c),
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq),
"startIndex",
"count");
}

[Theory]
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4 changes: 3 additions & 1 deletion src/libraries/System.Collections.Immutable/tests/ImmutableListBuilderTest.cs
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Expand Up @@ -345,7 +345,9 @@ public void LastIndexOf()
(b, v, eq) => b.LastIndexOf(v, b.Count > 0 ? b.Count - 1 : 0, b.Count, eq),
(b, v, i) => b.LastIndexOf(v, i),
(b, v, i, c) => b.LastIndexOf(v, i, c),
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq),
"index",
"count");
}

[Fact]
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8 changes: 6 additions & 2 deletions src/libraries/System.Collections.Immutable/tests/ImmutableListTest.cs
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Expand Up @@ -512,14 +512,18 @@ public void LastIndexOf()
(b, v, eq) => b.LastIndexOf(v, eq),
(b, v, i) => b.LastIndexOf(v, i),
(b, v, i, c) => b.LastIndexOf(v, i, c),
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq),
"index",
"count");
IndexOfTests.LastIndexOfTest(
seq => (IImmutableList<int>)ImmutableList.CreateRange(seq),
(b, v) => b.LastIndexOf(v),
(b, v, eq) => b.LastIndexOf(v, eq),
(b, v, i) => b.LastIndexOf(v, i),
(b, v, i, c) => b.LastIndexOf(v, i, c),
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq));
(b, v, i, c, eq) => b.LastIndexOf(v, i, c, eq),
"index",
"count");
}

[Fact]
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13 changes: 8 additions & 5 deletions src/libraries/System.Collections.Immutable/tests/ImmutableListTestBase.cs
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Expand Up @@ -177,14 +177,17 @@ public void FindLastIndexTest()
ImmutableList<int> singleElementList = ImmutableList.Create(10);
Assert.Equal(0, this.GetListQuery(singleElementList).FindLastIndex(0, 1, n => n == 10));
Assert.Equal(-1, this.GetListQuery(singleElementList).FindLastIndex(0, 1, n => n == 99));
Assert.Throws<ArgumentOutOfRangeException>(() => this.GetListQuery(singleElementList).FindLastIndex(1, n => n == 10));
Assert.Throws<ArgumentOutOfRangeException>(() => this.GetListQuery(singleElementList).FindLastIndex(1, 1, n => n == 10));
AssertExtensions.Throws<ArgumentOutOfRangeException>("startIndex", () => this.GetListQuery(singleElementList).FindLastIndex(1, n => n == 10));
AssertExtensions.Throws<ArgumentOutOfRangeException>("startIndex", () => this.GetListQuery(singleElementList).FindLastIndex(1, 1, n => n == 10));
AssertExtensions.Throws<ArgumentOutOfRangeException>("count", () => this.GetListQuery(singleElementList).FindLastIndex(0, 2, n => n == 10));

ImmutableList<int> multipleElementList = ImmutableList.Create(1, 2, 3, 4);
Assert.Equal(2, this.GetListQuery(multipleElementList).FindLastIndex(3, 4, n => n == 3));
Assert.Throws<ArgumentOutOfRangeException>(() => this.GetListQuery(multipleElementList).FindLastIndex(4, n => n == 1));
Assert.Throws<ArgumentOutOfRangeException>(() => this.GetListQuery(multipleElementList).FindLastIndex(4, 1, n => n == 1));
Assert.Throws<ArgumentOutOfRangeException>(() => this.GetListQuery(multipleElementList).FindLastIndex(4, 4, n => n == 1));
AssertExtensions.Throws<ArgumentOutOfRangeException>("startIndex", () => this.GetListQuery(multipleElementList).FindLastIndex(4, n => n == 1));
AssertExtensions.Throws<ArgumentOutOfRangeException>("startIndex", () => this.GetListQuery(multipleElementList).FindLastIndex(2, 4, n => n == 1));
AssertExtensions.Throws<ArgumentOutOfRangeException>("startIndex", () => this.GetListQuery(multipleElementList).FindLastIndex(4, 1, n => n == 1));
AssertExtensions.Throws<ArgumentOutOfRangeException>("startIndex", () => this.GetListQuery(multipleElementList).FindLastIndex(4, 4, n => n == 1));
AssertExtensions.Throws<ArgumentOutOfRangeException>("count", () => this.GetListQuery(multipleElementList).FindLastIndex(3, 5, n => n == 1));

// 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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33 changes: 18 additions & 15 deletions src/libraries/System.Collections.Immutable/tests/IndexOfTests.cs
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Expand Up @@ -85,26 +85,29 @@ public static void LastIndexOfTest<TCollection>(
Func<TCollection, int, IEqualityComparer<int>, int> lastIndexOfItemEQ,
Func<TCollection, int, int, int> lastIndexOfItemIndex,
Func<TCollection, int, int, int, int> lastIndexOfItemIndexCount,
Func<TCollection, int, int, int, IEqualityComparer<int>, int> lastIndexOfItemIndexCountEQ)
Func<TCollection, int, int, int, IEqualityComparer<int>, int> lastIndexOfItemIndexCountEQ,
string expectedIndexParamName,
string expectedCountParamName)
{
TCollection emptyCollection = factory(new int[0]);
TCollection singleCollection = factory(new[] { 10 });
TCollection collection1256 = factory(new[] { 1, 2, 5, 6 });

Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(emptyCollection, 100, 1, 1, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(emptyCollection, 100, -1, 1, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(singleCollection, 100, 1, 1, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(singleCollection, 100, -1, 1, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(collection1256, 100, 1, 20, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(collection1256, 100, 1, -1, EqualityComparer<int>.Default));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(emptyCollection, 100, 1, 1, new CustomComparer(50)));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(emptyCollection, 100, -1, 1, new CustomComparer(50)));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(singleCollection, 100, 1, 1, new CustomComparer(50)));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(singleCollection, 100, -1, 1, new CustomComparer(50)));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(collection1256, 100, 1, 20, new CustomComparer(1)));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(collection1256, 100, 1, -1, new CustomComparer(1)));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndex(collection1256, 2, 5));
Assert.Throws<ArgumentOutOfRangeException>(() => lastIndexOfItemIndexCountEQ(collection1256, 6, 4, 4, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndexCountEQ(emptyCollection, 100, 1, 1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndexCountEQ(emptyCollection, 100, -1, 1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndexCountEQ(singleCollection, 100, 1, 1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndexCountEQ(singleCollection, 100, -1, 1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedCountParamName, () => lastIndexOfItemIndexCountEQ(collection1256, 100, 1, 20, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedCountParamName, () => lastIndexOfItemIndexCountEQ(collection1256, 100, 1, -1, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndexCountEQ(emptyCollection, 100, 1, 1, new CustomComparer(50)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndexCountEQ(emptyCollection, 100, -1, 1, new CustomComparer(50)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndexCountEQ(singleCollection, 100, 1, 1, new CustomComparer(50)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndexCountEQ(singleCollection, 100, -1, 1, new CustomComparer(50)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedCountParamName, () => lastIndexOfItemIndexCountEQ(collection1256, 100, 1, 20, new CustomComparer(1)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedCountParamName, () => lastIndexOfItemIndexCountEQ(collection1256, 100, 1, -1, new CustomComparer(1)));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndex(collection1256, 2, 5));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedCountParamName, () => lastIndexOfItemIndexCountEQ(collection1256, 6, 2, 4, EqualityComparer<int>.Default));
AssertExtensions.Throws<ArgumentOutOfRangeException>(expectedIndexParamName, () => lastIndexOfItemIndexCountEQ(collection1256, 6, 4, 4, EqualityComparer<int>.Default));

Assert.Equal(-1, lastIndexOfItem(emptyCollection, 5));
Assert.Equal(-1, lastIndexOfItemEQ(emptyCollection, 5, EqualityComparer<int>.Default));
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