Member Avatar for shazzy99

I've been trying to use substring to get the last three digits as part of the extension but wasn't successful.

Can anyone tell me how I can extract the extension from the file name?

Rgs

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  • Latest Post Latest Post by kia_mls

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Generated

A few clarifications and a small, safe pattern to use.

You were right to try a substring, but manual index math is brittle — 's example illustrates that off-by-one mistakes happen and will break for short names. Also remember 's warning: extensions are not always three characters, and correctly calls out multiple dots inside names. The simplest, most reliable route is to use the framework helpers mentioned by and when you can, but if you must roll your own make it handle these edge cases.

Robust extraction rules to follow:

  • Work on the filename portion only (strip any path or query string first).
  • Find the last dot. If there is no dot, the dot is the first character (hidden Unix file), or the dot is the last character, treat it as “no extension.”
  • Return the substring from that last dot (include the dot if you prefer) and normalize case for comparisons.
  • Decide up front whether you want only the final extension (.gz from archive.tar.gz) or multi-part extensions (.tar.gz) and handle that explicitly.

A compact, safe C# implementation:

private static string GetExtensionSafe(string pathOrName)
{
    if (string.IsNullOrEmpty(pathOrName)) return string.Empty;
    var name = System.IO.Path.GetFileName(pathOrName);
    if (string.IsNullOrEmpty(name)) return string.Empty;
    int i = name.LastIndexOf('.');
    if (i <= 0 || i == name.Length - 1) return string.Empty;
    return name.Substring(i).ToLowerInvariant();
}

Extra tips: trim input, strip URL query fragments before parsing, and never rely solely on the extension for security — validate file type by MIME/“magic bytes” when accepting uploads. 's manual loop works too, but the LastIndexOf approach above is clearer and easier to test.

Recommended Answers

All 6 Replies

Member Avatar for ddanbe

Say L is the last character in your filename.(Same as Fname.Length-1)
Then Fname.Substring(L-3,L); should do the trick.

Member Avatar for rapture

You don't mention if you have to use substring to get the extension or if that is just the method to grab it you decided to use so I'll ask another question . . .

Is there any reason you are not using path.getextension() ?

extension = Path.GetExtension(fileName);

You can get this info from a google search on C# path.getextension or msdn is here: http://msdn.microsoft.com/en-us/library/system.io.path.getextension.aspx

(wondering because .aspx is four while .doc is three etc.)

commented: Nice! +4
Member Avatar for Ramy Mahrous

After the last "." get the string is after
you may have file, its name readme.program.user.txt!!
It's the time to play with string class, you can also use regular expression and you can use FileInfo.

Member Avatar for LizR

*NEVER* make the assumption that a files extension is only 3 characters.

Member Avatar for lhavana

try this simple procedure :

private string Getextension(string fileName)
        {
            string ext = string.Empty;
            Boolean hita = false;
            int i = fileName.Length - 1;
            char[] arr = fileName.ToCharArray();
            while(i>0 & !hita)
            {
                if (arr[i] == '.') hita = true;
                else ext = arr[i] + ext;
                i = i - 1;
            }

            return ext;
        }
Edited by mike_2000_17 because: Fixed formatting
Member Avatar for kia_mls

System.IO.FileInfo fi = new System.IO.FileInfo(FileName);
string ext = fi.Extension.ToString();

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