XPath: How to select a node by its attribute?

Question

I have an XML that goes like this:

<?xml version="1.0" encoding="utf-8" ?> <colors> <color index = "0">#FF0000</color> <color index = "1">#FF0200</color> <color index = "2">#FF0300</color> <color index = "3">#FF0500</color> [..]

I'm trying to select a node by its index:

XmlDocument ColorTable = new XmlDocument(); ColorTable.Load(HttpContext.Current.Server.MapPath("~/App_Data/ColorTable.xml")); int percentage = 2; string xpath = string.Format(@"//color[index={0}]", percentage.ToString()); //string xpath = string.Format(@"//color[index=""{0}""]", percentage.ToString()); //string xpath = string.Format(@"//color[index='{0}']", percentage.ToString()); var r = ColorTable.SelectSingleNode(xpath).Value;

I tried also the commented versions, but it does not return any result. Any suggestion?

Jon Skeet · Accepted Answer · 2009-07-01 09:36:15Z

Use //color[@index='{0}'] instead. The @ sign means "attribute".

I note that you're using a verbatim string literal by the way - the @ sign at the start of the string. There's no need in this case - you don't have any backslashes in the string, and it's not multi-line. You also don't need to explicitly call ToString on percentage - it will be converted automatically.

string xpath = string.Format("//color[@index='{0}']", percentage);

Why does the 0 have to be enclosed in curly braces? I use XPATH inside Novell IDM, and @index=0 would have been sufficient there. What do the curly braces indicate?
its not part of the xpath, it's part of the String.Format, it will be replaces with the 1st param (ie percentage)

Shay Erlichmen · Accepted Answer · 2011-06-24 19:53:12Z

BTW, for those of us who doesn't speak native XPath, there are many online XPath "playgrounds" that allow you to write XML and XPath expression and see the results online.

Whenever I found myself in a "XPath hell" I usually go to those playgrounds and try various combination till I get my (needed) results, for some reason it works faster than writing C#/Python test program or even running those bloated so called XML editors.

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