value attribute is not present for select control.You can select the option by setting the option index.
In the below code option 4 will be selected.
currentTag.SetAttribute("selectedIndex", "4");
You can also parse through the child option controls and set the attribute selected as "selected"
Please find the code snippet.
HtmlElementCollection tagsCollection = webBrowser1.Document.GetElementsByTagName("select");
foreach (HtmlElement currentTag in tagsCollection)
{
if (currentTag.GetAttribute(attribute).Equals(attName))
{
HtmlElementCollection optionCollection = currentTag.Document.GetElementsByTagName("option");
foreach (HtmlElement optionTag in optionCollection)
{
if (optionTag.GetAttribute("value").Equals("Thevalue you want to check"))
optionTag.SetAttribute("selected", "selected");
}
}
}