Repeated condition in an If / ElseIf chain

ID

vbnet.correctness.duplicate_conditions

Severity

high

Remediation Complexity

medium

Remediation Risk

medium

Remediation Effort

low

Resource

Duplicate Logic

Language

VB.NET

Description

Flags an If / ElseIf chain in which two branches test the same condition, whether the repeat is the very next branch or sits two or three branches further down the chain. The same condition appearing in two separate, consecutive If statements is not reported: each statement is evaluated on its own, so both bodies can run.

Rationale

Only the first matching branch of a chain runs, so a repeated condition makes the later branch unreachable — its body is dead code no input can ever execute, however correct it looks. The usual cause is a branch copied and pasted with the body edited but the condition left alone, which means the case the new branch was meant to handle silently falls through to whatever comes next: a trailing Else, or nothing at all. A shipping table that skips a weight band or a lookup that returns the wrong zone is the kind of defect this leaves behind, and reading the chain top to bottom rarely makes it obvious, because each branch looks reasonable in isolation.

The following code illustrates the pattern detected by this rule:

Public Function RateFor(ByVal weight As Integer) As Decimal
    ' FLAGGED: Repeated condition in an If / ElseIf chain
    If weight > 20 Then
        Return 12.5D
    ElseIf weight > 20 Then
        Return 8D
    End If
    Return 5D
End Function

Remediation

Change the repeated condition to the case it was meant to test, or delete the branch when it is genuinely redundant.

' Before: the second branch can never run
If weight > 20 Then
    Return 12.5D
ElseIf weight > 20 Then
    Return 8D
End If

' After
If weight > 20 Then
    Return 12.5D
ElseIf weight > 5 Then
    Return 8D
End If

Configuration

This detector does not need any configuration.

References