Collection size or array length compared against an impossible bound

ID

vbnet.correctness.meaningless_size_comparison

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Control Flow

Language

VB.NET

Description

Flags a comparison between a Count, LongCount, Length or LongLength member and 0 or -1 whose outcome is fixed: < 0 and ⇐ -1 can never be True, while >= 0 and > -1 are always True. Both operand orders are matched, so 0 > slots.Length is reported as well as slots.Length < 0. The rule keys on the member name of a qualified access, so it sees Lines.Count < 0 and grid.LongLength < 0. Meaningful tests are untouched: = 0 and < 1 for empty, > 0 for non-empty, and comparisons against a genuine bound such as > 100.

Rationale

A collection count and an array length are non-negative by definition, so the comparison cannot be expressing the check the author intended. An impossible test leaves its body permanently dead: a validation that never rejects anything, an error path that never runs, so the malformed input it was written to catch passes straight through. An always-True test is the mirror image — the guard admits everything, and the code behind it runs against the empty collection it was supposed to be protected from, usually failing later on an index or a first element, well away from the guard that let it through. In both cases the code reads as though the case were handled, which is exactly why the mistake survives review.

The following code illustrates the pattern detected by this rule:

Public Function IsCorrupt() As Boolean
    ' FLAGGED: Collection size or array length compared against an impossible bound
    If Lines.Count < 0 Then
        Return True
    End If
    Return False
End Function

Remediation

Test emptiness with = 0 and non-emptiness with > 0, or compare the size against a real bound.

' Before: a count is never negative, so the body is dead code
If Lines.Count < 0 Then
    Reject()
End If

' After
If Lines.Count = 0 Then
    Reject()
End If

Configuration

This detector does not need any configuration.

References