C# - How To Lock A Private Static Field Of A Class In One Static Method

I have a private static field in my Controller class in an MVC web application.

I have a static method in that controller that assigns some value to that static field, I want to apply lock on that static field until some other instance method in the controller uses the value stored in the static field and then releases it.

DETAILS:

I have a controller named BaseController having a static ClientId field as follows and two methods as follows:-

public static string ClientId = "";
static void OnClientConnected(string clientId, ref Dictionary<string, object> list)
{
list.Add("a", "b");
// I want the ClientId to be locked here, so that it can not be accessed by other requests coming to the server and wait for ClientId to be released:-
BaseController.clientId = clientId;
}
public ActionResult Handler()
{
if (something)
{
// use the static ClientId here
}
// Release the ClientId here, so it can now be used by other web requests coming to the server.
return View();
}