ReSharper 2021.1 Help

Code Inspection: Specify string comparison explicitly

Similarly to the Specify a culture in string conversion explicitly, this inspection helps to avoid problems with running your code on machines with different culture settings.

When string results returned by String.ToUpper()/ String.ToLower() are compared using equality operators (==/ != ), comparison results may differ depending on the machine's locale. The canonical example is the Turkish where there is a lowercase dotless "ı" with the corresponding uppercase "I", and a lowercase "i" with an uppercase "İ" with the dot.

So in a case of using String.ToUpper()/ String.ToLower() for a case-insensitive comparison:

public void Foo(string a, string b) { if (a.ToUpper() != b.ToUpper()) { } // or if (a.ToLower() != b.ToLower()) { } }
Public Sub Foo(a As String, b As String) If a.ToUpper() <> b.ToUpper() Then End If ' or If a.ToLower() <> b.ToLower() Then End If End Sub

ReSharper suggests replacing it with an overload of String.Equals():

if (!String.Equals(a, b, StringComparison.CurrentCultureIgnoreCase)) {}
If Not String.Equals(a, b, StringComparison.CurrentCultureIgnoreCase) Then End If

where it is immediately clear that the strings are compared taking the culture settings into consideration.

If the comparison is explicitly made culture-insensitive, then it is also suggested to use String.Equals() with the InvariantCultureIgnoreCase in the comparison rule parameter, for example:

if (a.ToLowerInvariant() == b.ToLowerInvariant()) {}
If a.ToLowerInvariant() = b.ToLowerInvariant() Then End If

could be replaced with

if (String.Equals(a, b, StringComparison.InvariantCultureIgnoreCase)) {}
If [String].Equals(a, b, StringComparison.InvariantCultureIgnoreCase) Then End If
Last modified: 13 April 2021