Selenium testing using C#: assertions. Why am I getting the following error?

using System; using System.Text; using System.Collections.Generic; using System.Linq; using Microsoft.VisualStudio.TestTools.UnitTesting; using System.Text.RegularExpressions; using System.Threading; using Selenium;

namespace Search1
{
  [TestClass]
  public class SearchTest1
  {
    public SearchTest1()
    {

    }

    private TestContext testContextInstance;

    /// <summary>
    ///Gets or sets the test context which provides
    ///information about and functionality for the current test run.
    ///</summary>
    public TestContext TestContext
    {
        get
        {
            return testContextInstance;
        }
        set
        {
            testContextInstance = value;
        }
    }

    private ISelenium selenium;

    [TestMethod]
    public void SearchMethod1()
    {

selenium = new DefaultSelenium("localhost", 4444, "*iexplore", "http://localhost/crm.aspx");
        selenium.Start();
        selenium.Open("/crm/SearchPage.aspx?function=3");
        selenium.WaitForPageToLoad("30000");
        Assert.IsTrue(selenium.IsTextPresent("开发者_Go百科Select All  |  Clear All"));
        try
        {
            Assert.IsTrue(selenium.IsTextPresent("Select All  |  Clear All"));
        }
        catch (Exception)
        {

        }
 selenium.Click("//span[@onclick=\"fnCheckGroupWithMessage('You have selected all items.', 'cbxRepeater_');\"]");
        Assert.AreEqual("'You have selected all items.", selenium.GetAlert());

        decimal totalCheckboxes = selenium.GetXpathCount("//input[@type='checkbox']");

        for (int i = 1; i < totalCheckboxes + 1; i++) 
        { 
            Assert.IsTrue(selenium.IsChecked("//input[@type='checkbox'][" + i + "]")); 
        }

    }
 }
}

Test method Search1.SearchTest1.SearchMethod1 threw exception: Selenium.SeleniumException: ERROR: Element //input[@type='checkbox'][2] not found

Try using this instead of selenium.IsChecked("//input[@type='checkbox'][" + i + "]"):

selenium.IsChecked("/descendant-or-self::input[@type='checkbox'][" + i + "]")