C# - Substring: index and length must refer to a l

问题:

I have a string that looks like

string url = "www.example.com/aaa/bbb.jpg";

"www.example.com/" is 18 fixed in length. I want to get the "aaa/bbb" part from this string (The actual url is not example nor aaa/bbb though, the length may vary)

so here's what I did:

string newString = url.Substring(18, url.Length - 4);

Then I got the exception: index and length must refer to a location within the string. What's wrong with my code and how to fix it? Thanks in advance.

回答1:

The second parameter in Substring is the length of the substring, not the end index.

You should probably include handling to check that it does indeed start with what you expect, end with what you expect, and is at least as long as you expect. And then if it doesn't match, you can either do something else or throw a meaningful error.

Here's some example code that validates that url contains your strings, that also is refactored a bit to make it easier to change the prefix/suffix to strip:

var prefix = "www.example.com/";
var suffix = ".jpg";
string url = "www.example.com/aaa/bbb.jpg";

if (url.StartsWith(prefix) && url.EndsWith(suffix) && url.Length >= (prefix.Length + suffix.Length))
{
    string newString = url.Substring(prefix.Length, url.Length - prefix.Length - suffix.Length);
    Console.WriteLine(newString);
}
else
    //handle invalid state

回答2:

Your mistake is the parameters to Substring. The first parameter should be the start index and the second should be the length or offset from the startindex.

string newString = url.Substring(18, 7);

If the length of the substring can vary you need to calculate the length.

Something in the direction of (url.Length - 18) - 4 (or url.Length - 22)

In the end it will look something like this

string newString = url.Substring(18, url.Length - 22);

回答3:

How about something like this :

string url = "http://www.example.com/aaa/bbb.jpg";
Uri uri = new Uri(url);
string path_Query = uri.PathAndQuery;
string extension =  Path.GetExtension(path_Query);

path_Query = path_Query.Replace(extension, string.Empty);// This will remove extension

回答4:

You need to find the position of the first /, and then calculate the portion you want:

string url = "www.example.com/aaa/bbb.jpg";
int Idx = url.IndexOf("/");
string yourValue = url.Substring(Idx + 1, url.Length - Idx - 4);

回答5:

Try This:

 int positionOfJPG=url.IndexOf(".jpg");
 string newString = url.Substring(18, url.Length - positionOfJPG);

回答6:

string newString = url.Substring(18, (url.LastIndexOf(".") - 18))

回答7:

Here is another suggestion. If you can prepend http:// to your url string you can do this

  string path = "http://www.example.com/aaa/bbb.jpg";
  Uri uri = new Uri(path);            
  string expectedString = 
      uri.PathAndQuery.Remove(uri.PathAndQuery.LastIndexOf("."));

回答8:

You can use padding right.

string url = "www.example.com/aaa/bbb.jpg";
string newString=url.PadRight(4);
string newString = (url.Substring(18, url.Length - 4)).Trim();

Happy Coding!