AdvancedWebDriverUsageTests.cs

[TestMethod]
public void SetHiddenField()
{
    ////<input type="hidden" name="country" value="Bulgaria"/>
    IWebElement theHiddenElem = driver.FindElement(By.Name("country"));
    string hiddenFieldValue = theHiddenElem.GetAttribute("value");
    Assert.AreEqual("Bulgaria", hiddenFieldValue);
    ((IJavaScriptExecutor)driver).ExecuteScript(
        "arguments[0].value='Germany';",
        theHiddenElem);
    hiddenFieldValue = theHiddenElem.GetAttribute("value");
    Assert.AreEqual("Germany", hiddenFieldValue);
}