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: 21 July 2022