Вот 3 способа сделать это специально с объектом FormCollection.
public ActionResult SomeActionMethod(FormCollection formCollection)
{
foreach (var key in formCollection.AllKeys)
{
var value = formCollection[key];
}
foreach (var key in formCollection.Keys)
{
var value = formCollection[key.ToString()];
}
// Using the ValueProvider
var valueProvider = formCollection.ToValueProvider();
foreach (var key in valueProvider.Keys)
{
var value = valueProvider[key];
}
}