Reverse case of all alphabetic characters in C# string

Question

What is the simplest way to reverse the case of all alphabetic characters in a C# string? For example "aBc1$;" should become "AbC1$;" I could easily write a method that does this, but I am hoping there is a library call that I don't know about that would make this easier. I would also like to avoid having a list of all known alphabetic characters and comparing each character to what is in the list. Maybe this can be done with regular expressions, but I don't know them very well. Thanks.

Thanks for the help. I created a string extension method for this that is mostly inspired by Anthony Pegram's solution, but without the LINQ. I think this strikes a good balance between readability and performance. Here is what I came up with.

public static string SwapCase(this string source) {
    char[] caseSwappedChars = new char[source.Length];
    for(int i = 0; i < caseSwappedChars.Length; i++) {
        char c = source[i];
        if(char.IsLetter(c)) {
            caseSwappedChars[i] =
                char.IsUpper(c) ? char.ToLower(c) : char.ToUpper(c);
        } else {
            caseSwappedChars[i] = c;
        }
    }
    return new string(caseSwappedChars);
}

Answer 1

You could do it in a line with LINQ. One method:

string input = "aBc1$";
string reversedCase = new string(
    input.Select(c => char.IsLetter(c) ? (char.IsUpper(c) ?
                      char.ToLower(c) : char.ToUpper(c)) : c).ToArray());

Answer 2

If you don't care about internationalization:

string input = "aBc1$@[\\]^_{|{~";
Encoding enc = new System.Text.ASCIIEncoding();
byte[] b = enc.GetBytes(input);
for (int i = input.Length - 1; i >= 0; i -= 1) {
   if ((b[i] & 0xdf) >= 65 && (b[i] & 0xdf) <= 90) { //check if alpha
      b[i] ^= 0x20; // then XOR the correct bit to change case
   }
}
Console.WriteLine(input);
Console.WriteLine(enc.GetString(b));

If, on the other hand, you DO care about internationalization, you'll want to pass in CultureInfo.InvariantCulture to your ToUpper() and ToLower() functions...