Variable assigned to itself

ID

vbnet.correctness.self_assignment

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Redundant Operation

Language

VB.NET

Description

Flags an assignment whose target and source are the same expression: a local assigned to itself (subtotal = subtotal), a field assigned to itself (Me.Total = Me.Total), or an indexed element assigned to itself (values(0) = values(0)). Only assignment statements are matched, so an equality test that happens to compare a value with itself is not reported here, and neither is a compound update such as Total = Total + delta, which reads and writes the same variable but is not a no-op.

Rationale

The statement compiles and runs, and changes nothing — dead code that every later reader has to stop and reason about. More importantly, it is usually the visible symptom of a value that never got stored. The classic VB.NET case is a member and a parameter sharing a name: inside the method the unqualified name resolves to the parameter, so Discount = Discount copies the parameter over itself and the field keeps its previous value. The object then quietly carries stale state and the defect surfaces far from the assignment, in a total that never changes or a setting that never takes effect, which makes it expensive to track down.

The following code illustrates the pattern detected by this rule:

Public Sub Recalculate(ByVal lineCount As Integer)
    Dim subtotal As Decimal = 0D
    ' FLAGGED: Variable assigned to itself
    subtotal = subtotal
    Total = subtotal
End Sub

Remediation

Remove the statement when it is simply dead, or fix the operand that was meant to differ. When a parameter shadows a member of the same name, qualify the assignment target with Me..

' Before: the parameter is assigned to itself and the field is never updated
Public Sub SetDiscount(ByVal Discount As Decimal)
    Discount = Discount
End Sub

' After
Public Sub SetDiscount(ByVal Discount As Decimal)
    Me.Discount = Discount
End Sub

Configuration

This detector does not need any configuration.

References