In Selenium for C#, sending Keys.Control
simply toggles the Control key's state: if it's up, then it becomes down; if it's down, then it becomes up. So to simulate pressing Control+A, send Keys.Control
twice, once before sending "a" and then after.
For example, if we is an input IWebElement, the following statement will select all of its contents:
we.SendKeys(Keys.Control + "a" + Keys.Control);