for loop 5-6 times slower than foreach loop in HtmlAgility Pack

I test 2 loops with the same values and code: 1) For loop:

for (int i = 0; i < MyGlobals.check1Count; i++)
{
addToMyGlobals(root2.Sel开发者_如何转开发ectNodes(strXPath)[i].OuterHtml.Trim(), XPathArrayIndex);
}

2) Foreach loop:

        foreach (HtmlNode link1 in root2.SelectNodes(strXPath))
        {
          addToMyGlobals(link1.OuterHtml.Trim(), XPathArrayIndex);
        }

And the results is that 'Foreach loop' is 5-6 times faster than the 'For Loop'. Why is that, what is happening, can somebody gives explanation about it?

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In your for loop, you are executing the expensive SelectNodes() method on every iteration of the loop. The foreach loop only does this once.