Empty-string test written as a comparison against "" or String.Empty

ID

vbnet.performance.test_for_empty_strings_using_length

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

String Handling

Language

VB.NET

Description

Reports a string tested for emptiness by comparing it with "" or String.Empty using = or <>, instead of inspecting its length or calling String.IsNullOrEmpty.

Rationale

Visual Basic does not compile s = "" into a reference or ordinal check. It emits a call to CompilerServices.Operators.CompareString, a full string comparison that honours Option Compare - so with Option Compare Text the test becomes a culture-aware comparison. Inspecting Length reads a field that the runtime already stores and returns as soon as it is non-zero, which is why it is the documented way to test for an empty string. The comparison also hides which of two different intents is meant: s = "" is True when s is Nothing, because CompareString treats Nothing as an empty string, so a genuine "no value at all" is silently folded into "empty".

The following code illustrates the pattern detected by this rule:

Public Function IsIncomplete(ByVal name As String, ByVal email As String) As Boolean
    ' FLAGGED: Empty-string test written as a comparison against "" or String.Empty
    If name = "" AndAlso email = "" Then
        Return True
    End If
    ' FLAGGED: Empty-string test written as a comparison against "" or String.Empty
    Return name = ""
End Function

Remediation

Use String.IsNullOrEmpty(s) when Nothing and "" should be treated alike - it is the direct equivalent of the comparison and states the intent. When the string is known not to be Nothing, test s.Length = 0 (or s.Length > 0 for the <> form). Use String.IsNullOrWhiteSpace if blank input should count as empty too.

' Before: a full CompareString call, and Nothing silently counts as empty
If name = "" Then Return True
If comment <> String.Empty Then Process(comment)

' After
If String.IsNullOrEmpty(name) Then Return True
If comment.Length > 0 Then Process(comment)

Configuration

This detector does not need any configuration.

References