.NET/C# LINQ gem: How to check if a char in a list of characters (via Stack Overflow)
Posted by jpluimers on 2012/11/21
Often when comparing characters with a list of characters, that list does not consist of consts.
It if were, you could make a switch statement:
switch (item)
{
case '/':
case '\\':
case ';':
addSeparator(separatorsUsed, item);
break;
}
But reality is that you cannot do things like this:
switch (item)
{
case Path.AltDirectorySeparatorChar: // Error: A constant value is expected
case Path.DirectorySeparatorChar:
case Path.PathSeparator:
addSeparator(separatorsUsed, item);
break;
}
However, you can perform a small trick and use LINQ to write some pretty elegant code based on Contains.
char[] pathSeparators = { Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar, Path.PathSeparator };
// LINQ: http://stackoverflow.com/questions/1818611/how-to-check-if-a-char-in-a-char-array/1818635#1818635
if (pathSeparators.Contains(item))
addSeparator(separatorsUsed, item);
The LINQ logic has the logic backwards (you can think of it like “item in pathSeparators”, but it is far easier to read than this:
if ((item == Path.AltDirectorySeparatorChar) || (item == Path.DirectorySeparatorChar) || (item == Path.PathSeparator))
addSeparator(separatoseparatorsUsedrsInUse, item);
Full source of a demo application:
using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Threading.Tasks;
namespace CharIsInSetConsoleApplication
{
class Program
{
/*
* call with arguments like this:
* ftp://besharp.com;C:\Windows\system32\cmd.exe
*/
static void Main(string[] args)
{
List<char> separatorsUsed = getSeparatorsUsed(args);
Console.WriteLine("Separators used in args:");
// regular foreach is more readable, but you can use LINQ here too:
// http://stackoverflow.com/questions/823532/apply-function-to-all-elements-of-collection-through-linq/823563#823563
separatorsUsed.ForEach(item =>
Console.WriteLine(item)
);
Console.Write("Press <Enter>");
Console.ReadLine();
}
private static List<char> getSeparatorsUsed(string[] args)
{
char[] pathSeparators = { Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar, Path.PathSeparator };
List<char> separatorsUsed = new List<char>();
foreach (string arg in args)
{
foreach (char item in arg)
{
switch (item)
{
case '/':
case '\\':
case ';':
addSeparator(separatorsUsed, item);
break;
}
//switch (item)
//{
// case Path.AltDirectorySeparatorChar: // Error: A constant value is expected
// case Path.DirectorySeparatorChar:
// case Path.PathSeparator:
// addSeparator(separatorsUsed, item);
// break;
//}
if ((item == Path.AltDirectorySeparatorChar) || (item == Path.DirectorySeparatorChar) || (item == Path.PathSeparator))
addSeparator(separatorsUsed, item);
// LINQ: http://stackoverflow.com/questions/1818611/how-to-check-if-a-char-in-a-char-array/1818635#1818635
if (pathSeparators.Contains(item))
addSeparator(separatorsUsed, item);
}
}
return separatorsUsed;
}
private static void addSeparator(List<Char> separatorsUsed, char item)
{
if (!separatorsUsed.Contains(item))
separatorsUsed.Add(item);
}
}
}
–jeroen
via: c# – How to check if a char in a char array – Stack Overflow.
The Wiert Corner - irregular stream of stuff 
Paul Hilt said
Would a regular expression not be more appropriate? At least it is as elegant to read :-)
jpluimers said
Depends on the characters, especially if there is escaping involved.
Maybe I’ll find some time to compare the two and do some speed comparisons.
–jeroen