Why is StringValues used for Request.Query values?

As already mentioned by others, the type is a StringValues object because technically, multiple values are allowed. While the common practice is to just set a single value, the URI specification does not disallow setting values multiple times. And it’s up to the application to decide how to handle that.

That being said, StringValues has an implicit conversion to string, so you don’t actually need to call ToString() on it, you can just use it as if it was a string. So doing things like Request.Query["color"] == "red", or passing it to a method that expects a string will just work.

And worse, checking for a value to see if a query param is specified can no longer be done like so Request.Query["color"] == null but instead must be checked like so Request.Query["color"].Count == 0

That’s only half true. Yes, in order to check whether a StringValues object is empty, you can check its Count property. You can also check against StringValues.Empty:

Request.Query["color"] == StringValues.Empty

However, the initial “issue” is that Request.Query[x] will always return a non-null StringValues object (so it’s safe to check for any value). If you want to check whether a key exists in the query arguments, you should use ContainsKey:

if (Request.Query.ContainsKey("color"))
    // only now actually retrieve the value
    string colorValue = Request.Query["color"];

Or alternatively, use TryGetValue:

if (Request.Query.TryGetValue("color", out var colorValue))

That all being said, accessing Request.Query is not really necessary most of the times. You should just use make use of model binding instead which will automatically give you the query arguments you need by just having them in the action’s signature:

public ActionResult MyAction(string color)

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