(bug) fix issues where asspan() and indexer might return out-of-range data

CHANGELOG.md

@@ -8,6 +8,8 @@ All notable changes to **ValueStringBuilder** will be documented in this file. T
 
 ### Fixed
 
+- `AsSpan(int startIndex, int length)` and `ToString(int startIndex, int length)` throw an `ArgumentOutOfRangeException` when the requested range ends after `Length`, instead of returning characters from the unused part of the buffer.
+- The indexer throws an `IndexOutOfRangeException` for an index at or after `Length`, instead of giving access to the unused part of the buffer.
 - `Append`/`AppendLine` with an interpolated string could return the rented array to the `ArrayPool` twice when an interpolation hole threw after the buffer grew. Reported by [@dhhoang](https://github.com/dhhoang) in #324
 
 ## [3.7.1] - 2026-09-25

docs/site/articles/exceptions_and_edge_cases.md

@@ -12,7 +12,7 @@ uid: exceptions_and_edge_cases
 | `Insert(int, ...)` overloads | `ArgumentOutOfRangeException` | `index` is negative, or greater than `Length`. |
 | `Remove(int startIndex, int length)` | `ArgumentOutOfRangeException` | `length` is negative, `startIndex` is negative, or `startIndex + length` is greater than `Length`. |
 | `Append(char value, int repeatCount)` | `ArgumentOutOfRangeException` | `repeatCount` is negative. |
-| `AsSpan(int startIndex, int length)` / `ToString(int startIndex, int length)` | `ArgumentOutOfRangeException` | `length` is greater than `Length`. |
+| `AsSpan(int startIndex, int length)` / `ToString(int startIndex, int length)` | `ArgumentOutOfRangeException` | `startIndex` or `length` is negative, or `startIndex + length` is greater than `Length`. |
 | `AsSpan(Range)` / `ToString(Range)` | `ArgumentOutOfRangeException` | The resolved range falls outside `0..Length`. |
 | `AppendFormat` | `FormatException` | A placeholder's argument index is not a valid non-negative integer within range of the supplied arguments, or a custom format specifier (e.g. `{0:00}`) is used - see [`AppendFormat` is intentionally limited](xref:best_practices#appendformat-is-intentionally-limited). |
 | `Insert<T>` (the `ISpanFormattable` overload) | `InvalidOperationException` | `value.TryFormat` does not fit into the temporary buffer sized by `bufferSize` (default 36 characters) - increase `bufferSize` for large custom `ISpanFormattable` types. |

src/LinkDotNet.StringBuilder/ValueStringBuilder.cs

@@ -117,7 +117,7 @@ public readonly bool IsEmpty
     public readonly ref char this[int index]
     {
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        get => ref buffer[index];
+        get => ref buffer[..bufferPosition][index];
     }
 
     /// <summary>
@@ -192,12 +192,7 @@ public readonly ref char this[int index]
     /// <param name="length">The length of the substring.</param>
     /// <returns>The filled array as <see cref="ReadOnlySpan{T}"/>.</returns>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public readonly ReadOnlySpan<char> AsSpan(int startIndex, int length)
-    {
-        ArgumentOutOfRangeException.ThrowIfGreaterThan(length, bufferPosition);
-
-        return buffer.Slice(startIndex, length);
-    }
+    public readonly ReadOnlySpan<char> AsSpan(int startIndex, int length) => buffer[..bufferPosition].Slice(startIndex, length);
 
     /// <summary>
     /// Returns the string as an <see cref="ReadOnlySpan{T}"/>.

tests/LinkDotNet.StringBuilder.UnitTests/ValueStringBuilderTests.cs

@@ -21,6 +21,25 @@ public void ShouldThrowIndexOutOfRangeWhenStringShorterThanIndex()
         Assert.False(true);
     }
 
+    [Fact]
+    public void ShouldThrowIndexOutOfRangeWhenIndexIsBetweenLengthAndCapacity()
+    {
+        using var stringBuilder = new ValueStringBuilder();
+        stringBuilder.Append("Hello");
+
+        try
+        {
+            _ = stringBuilder[5];
+        }
+        catch (IndexOutOfRangeException)
+        {
+            Assert.True(true);
+            return;
+        }
+
+        Assert.Fail();
+    }
+
     [Fact]
     public void ShouldTryToCopySpan()
     {
@@ -418,6 +437,25 @@ public void ReturnSubstring()
         result.ShouldBe("ell");
     }
 
+    [Fact]
+    public void ShouldThrowWhenSubstringEndsAfterLength()
+    {
+        using var stringBuilder = new ValueStringBuilder();
+        stringBuilder.Append("Hello");
+
+        try
+        {
+            stringBuilder.ToString(3, 5);
+        }
+        catch (ArgumentOutOfRangeException)
+        {
+            Assert.True(true);
+            return;
+        }
+
+        Assert.Fail();
+    }
+
     [Fact]
     public void ImplicitCastFromStringToValueStringBuilder()
     {