什么HttpContext.Current.Request [“ CarName”]！= null，它在做什么？

Question

I am modifying a subject line of code but did not get it. I was thinking HttpContext.Current.Request is read only and it return the url of request. Can we set value for HttpContext.Current.Request["CarName"] just like a variable or a session variable?

Please guide me what this line is doing.

Edit:

if (HttpContext.Current.Request["CarName"] != null){
}

Answer 1

The indexer method on the Request object looks up the KeyValuePair 's within Request 's members. It is important to note that Request is an entire object.. not just a URL.

ILSpy shows this as the implementation of the indexer method:

public string this[string key]
{
    get
    {
        string text = this.QueryString[key];
        if (text != null)
        {
            return text;
        }
        text = this.Form[key];
        if (text != null)
        {
            return text;
        }
        HttpCookie httpCookie = this.Cookies[key];
        if (httpCookie != null)
        {
            return httpCookie.Value;
        }
        text = this.ServerVariables[key];
        if (text != null)
        {
            return text;
        }
        return null;
    }
}

So, your line of code is checking to see if "CarName" is a key contained in any of the above KeyValuePair members of the Request object.

Answer 2

See HttpContext.Request.Item (When looking for x[] syntax, lookup x.Item ):

Gets the specified object from the QueryString , Form , Cookies , or ServerVariables collections.

If a value with the given key is not found from one of the sources then null is returned. In this case, "CarName" is presumably supplied in a valid request (eg ../search?CarName=Rusty ) so the conditional might check for "no search conditions".

This is not the same as using sessions! Sessions provide a generic mechanism to associated data (but can be backed by direct cookies) "per session". In any case, never directly trust user data obtained from the client as it can be spoofed.