For C# NUnit, you need to convert WebDriver to JSExecuter and then execute the script to check if document.ready state is complete or not. Check below code for reference:
public static void WaitForLoad(IWebDriver driver)
{
IJavaScriptExecutor js = (IJavaScriptExecutor)driver;
int timeoutSec = 15;
WebDriverWait wait = new WebDriverWait(driver, new TimeSpan(0, 0, timeoutSec));
wait.Until(wd => js.ExecuteScript("return document.readyState").ToString() == "complete");
}
This will wait until the condition is satisfied or timeout.