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string.AsSpan(int, int) throws a misleading exception #115239

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MihaZupan opened this issue May 2, 2025 · 3 comments · May be fixed by #115275
Open

string.AsSpan(int, int) throws a misleading exception #115239

MihaZupan opened this issue May 2, 2025 · 3 comments · May be fixed by #115275
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area-System.Memory

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@MihaZupan
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"foo".AsSpan(0, largeNumber) will throw Specified argument was out of the range of valid values. (Parameter 'start').
Note the indicated parameter name. The issue here is the combination of start/length, and there's nothing you can change start to to make this valid as the issue happens to be in the length argument.

We could consider removing the ExceptionArgument.start, or use a different message that doesn't blame start specifically.

runtime/src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs

Lines 191 to 207 in 27604b5

public static ReadOnlySpan<char> AsSpan(this string? text, int start, int length)
{
if (text == null)
{
if (start != 0 || length != 0)
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
return default;
}
#if TARGET_64BIT
// See comment in Span<T>.Slice for how this works.
if ((ulong)(uint)start + (ulong)(uint)length > (ulong)(uint)text.Length)
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
#else
if ((uint)start > (uint)text.Length || (uint)length > (uint)(text.Length - start))
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
#endif

We don't report the argument in Slice(int, int) for example

runtime/src/libraries/System.Private.CoreLib/src/System/Span.cs

Lines 398 to 412 in 27604b5

public Span<T> Slice(int start, int length)
{
#if TARGET_64BIT
// Since start and length are both 32-bit, their sum can be computed across a 64-bit domain
// without loss of fidelity. The cast to uint before the cast to ulong ensures that the
// extension from 32- to 64-bit is zero-extending rather than sign-extending. The end result
// of this is that if either input is negative or if the input sum overflows past Int32.MaxValue,
// that information is captured correctly in the comparison against the backing _length field.
// We don't use this same mechanism in a 32-bit process due to the overhead of 64-bit arithmetic.
if ((ulong)(uint)start + (ulong)(uint)length > (ulong)(uint)_length)
ThrowHelper.ThrowArgumentOutOfRangeException();
#else
if ((uint)start > (uint)_length || (uint)length > (uint)(_length - start))
ThrowHelper.ThrowArgumentOutOfRangeException();
#endif

@dotnet-issue-labeler dotnet-issue-labeler bot added the needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners label May 2, 2025
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@MihaZupan MihaZupan added area-System.Memory and removed untriaged New issue has not been triaged by the area owner needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners labels May 2, 2025
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@danmoseley
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We could consider removing the ExceptionArgument.start, or use a different message that doesn't blame start specifically.

Or in the exception helper figure out which of the parameters is to blame.

@jkotas
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jkotas commented May 2, 2025

Similar issues #53622 and #90939

the exception helper figure out which of the parameters is to blame.

I expect it to be ambiguous most of the time. I would just delete the argument name from the message.

@MihaZupan MihaZupan linked a pull request May 3, 2025 that will close this issue
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Unify Span/Memory slicing exceptions MihaZupan/runtime
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@danmoseley @jkotas @MihaZupan