How to terminate a while loop when an Xpath query returns a null reference html agility pack

Question

I'm trying to loop through every row of a variable length table on the a webpage (http://www.oddschecker.com/golf/the-masters/winner) and extract some data

The problem is I can't seem to catch the null reference and terminate the loop without it throwing an exception!

int i = 1;
bool test = string.IsNullOrEmpty(doc.DocumentNode.SelectNodes(String.Format("//*[@id='t1']/tr[{0}]/td[3]/a[2]", i))[0].InnerText);

while (test != true)
{
    string name = doc.DocumentNode.SelectNodes(String.Format("//*[@id='t1']/tr[{0}]/td[3]/a[2]", i))[0].InnerText;
    //extract data
    i++;
}

try-catch statements don't catch it either:

bool test = false;
try
{
     string golfersName = doc.DocumentNode.SelectNodes(String.Format("//*[@id='t1']/tr[{0}]/td[3]/a[2]", i))[0].InnerText;
 }
 catch
 {
      test = true;
 }

 while (test != true)
 {
...

Answer 1

The code logic is a bit off. With the original code, if test evaluated true the loop will never terminates. It seems that you want to do checking in every loop iteration instead of only once at the beginning.

Anyway, there is a better way around. You can select all relevant nodes without specifying each <tr> indices, and use foreach to loop through the node set :

var nodes = doc.DocumentNode.SelectNodes("//*[@id='t1']/tr/td[3]/a[2]");
foreach(HtmlNode node in nodes)
{
    string name = node.InnerText;
    //extract data
}

or using for loop instead of foreach, if index of each node is necessary for the "extract data" process :

for(i=1; i<=nodes.Count; i++)
{
    //array index starts from 0, unlike XPath element index
    string name = nodes[i-1].InnerText;
    //extract data
}

Side note : To query single element you can use SelectSingleNode("...") instead of SelectNodes("...")[0]. Both methods return null if no nodes match XPath criteria, so you can do checking against the original value returned instead of against InnerText property to avoid exception :

var node = doc.DocumentNode.SelectSingleNode("...");
if(node != null)
{
    //do something
}