I have the following function.
public String getSomething(){
TextInputDialog dialog = new TextInputDialog();
dialog.setHeaderText("X");
dialog.setTitle("Y");
Optional<String> result = dialog.showAndWait();
return result;
}
Obviously, the returned value is not of String type. How could I return a String when result's type is Optional<String>?
Try doing result.get() should return a string.
I would do something like this to make sure you are protecting from null
if(a. isPresent()) {a.get()}
I'd use orElse which provides a default value in case the actual value is absent:
public String getSomething(){
TextInputDialog dialog = new TextInputDialog();
dialog.setHeaderText("X");
dialog.setTitle("Y");
return dialog.showAndWait().orElse("n/a");
}
Use String.valueOf(optionalString);
Optional<String> s1 =Optional.of("Hello");
// s1 will print - Optional["Hello"]
String s1 = String.valueOf(optionalObj.get());
// now s1 will print "Hello"
this would extract the exact string literal from the optional object.
I am querying an xml and i am storing the results using singleordefault
var query = from nm in xelement.Descendants("EmployeeFinance")
where (int)nm.Element("EmpPersonal_Id") == empID
select new AllowancePaid
{
gradeTaxId = nm.Element("Allow-GradeTax").Elements("Amount").Attributes("BenListId").Select(a => (int)a).ToList(),
gradeTaxAmt = nm.Element("Allow-GradeTax").Elements("Amount").Select(a => (double)a).ToList()
};
Debug.WriteLine("2");
var resultquery = query.SingleOrDefault();
now this line: var resultquery = query.SingleOrDefault(); works fine if it found in the xml file. However, i have a case where my query will result in a null. If i have no value, it would make an entry in the xml file and my query obviously results in null. My question is how do i cater for this without causing my programe to crash. obviously, singleordefault() doesnt work.
***************** EDITED *************************
I read what everyone said so far and it make sense but i am still having a problem.
if (query.Count() == 0)
{
Debug.WriteLine("NULL");
}
else {
var resultquery = query.SingleOrDefault();
Debug.WriteLine("NOT NULL");
}
OR
if (query == null)
{
Debug.WriteLine("NULL");
}
else {
var resultquery = query.SingleOrDefault();
Debug.WriteLine("NOT NULL");
}
OR
var resultquery = query.SingleOrDefault();
if (resultquery == null)
{
Debug.WriteLine("NULL Result");
}
else
{
Debug.WriteLine("NOT NULL");
}
I am getting a System.NullReferenceException error when the first part of the if statement is true. One user said to do this: var resultquery = query.SingleOrDefault(); then use my if..else statement to do the comparison. However i am getting the error at the point of assign query.singleofdefault() to resultquery. So i am lost.. hope someone can help. thank you
what i am trying to understand is this. the documentation states if the result query is 0 it will give a default value, if it is not, it will be a single value. so why doesnt this give a default value? [taken from the comments]
null is the default value for reference types. Apparently AllowancePaid is a reference type (a custom class).
What is the value you want when the there is no value found.
You could either do:
if (resultquery == null) {
// Logic for No result
} else {
// Logic for result found
}
Or you could force a default value
eg.
var resultquery = query.SingleOrDefault() ?? new AllowancePaid();
UPDATE
From the comments posted it appears that the null reference exception is actually caused within the query itself rather than by the assignment to resultquery and use of later.
This updated query should solve the issue
var query = from nm in xelement.Descendants("EmployeeFinance")
where nm.Element("EmpPersonal_Id") != null
&& (int)nm.Element("EmpPersonal_Id") == empID
&& nm.Element("Allow-GradeTax") != null
&& nm.Element("Allow-GradeTax").Elements("Amount") != null
select new AllowancePaid
{
gradeTaxId = nm.Element("Allow-GradeTax").Elements("Amount").Attributes("BenListId").Select(a => (int)a).ToList(),
gradeTaxAmt = nm.Element("Allow-GradeTax").Elements("Amount").Select(a => (double)a).ToList()
};
var resultquery = query.SingleOrDefault();
if (resultquery == null) {
Debug.WriteLine("NULL Result");
} else {
// Logic here
}
I'm write this function:
public static String QueryString(string queryStringKey)
{
if (HttpContext.Current.Request.QueryString[queryStringKey] != null)
{
if (HttpContext.Current.Request.QueryString[queryStringKey].ToString() != string.Empty)
return HttpContext.Current.Request.QueryString.GetValues(1).ToString();
}
return "NAQ";
}
And i want to get just one value from querystring parameter.
for example i send "page" to my function and url is: "sth.com/?page=1&page=2"
and function return to me: "1,2" ; but i want first value: "1", How?
GetValues returns a string[] if the key exists. An array is zero based, so you get the first element by using array[0], you are using GetValues(1) in your code, i assume that you wanted the first.
You could also use the Enumerable.First extension method:
Request.QueryString.GetValues("page").First();
Since GetValues returns not an empty array but null if the key was not present you need to check that explicitely (FirstOrDefault doesn't work):
public static String QueryString(string queryStringKey)
{
if (HttpContext.Current != null && HttpContext.Current.Request != null)
{
string[] values = HttpContext.Current.Request.QueryString.GetValues("queryStringKey");
if (values != null) return values.First();
}
return "NAQ";
}
A better approach would be -
public static String QueryString(string queryStringKey)
{
if (HttpContext.Current!=null && HttpContext.Current.Request!=null && !string.IsNullOrEmpty(HttpContext.Current.Request.QueryString[queryStringKey])
{
return HttpContext.Current.Request.QueryString.GetValues(queryStringKey).First();
}
return "NAQ";
}
I know i can do this
var nv = HttpUtility.ParseQueryString(req.RawUrl);
But is there a way to convert this back to a url?
var newUrl = HttpUtility.Something("/page", nv);
Simply calling ToString() on the NameValueCollection will return the name value pairs in a name1=value1&name2=value2 querystring ready format. Note that NameValueCollection types don't actually support this and it's misleading to suggest this, but the behavior works here due to the internal type that's actually returned, as explained below.
Thanks to #mjwills for pointing out that the HttpUtility.ParseQueryString method actually returns an internal HttpValueCollection object rather than a regular NameValueCollection (despite the documentation specifying NameValueCollection). The HttpValueCollection automatically encodes the querystring when using ToString(), so there's no need to write a routine that loops through the collection and uses the UrlEncode method. The desired result is already returned.
With the result in hand, you can then append it to the URL and redirect:
var nameValues = HttpUtility.ParseQueryString(Request.QueryString.ToString());
string url = Request.Url.AbsolutePath + "?" + nameValues.ToString();
Response.Redirect(url);
Currently the only way to use a HttpValueCollection is by using the ParseQueryString method shown above (other than reflection, of course). It looks like this won't change since the Connect issue requesting this class be made public has been closed with a status of "won't fix."
As an aside, you can call the Add, Set, and Remove methods on nameValues to modify any of the querystring items before appending it. If you're interested in that see my response to another question.
string q = String.Join("&",
nvc.AllKeys.Select(a => a + "=" + HttpUtility.UrlEncode(nvc[a])));
Make an extension method that uses a couple of loops. I prefer this solution because it's readable (no linq), doesn't require System.Web.HttpUtility, and it supports duplicate keys.
public static string ToQueryString(this NameValueCollection nvc)
{
if (nvc == null) return string.Empty;
StringBuilder sb = new StringBuilder();
foreach (string key in nvc.Keys)
{
if (string.IsNullOrWhiteSpace(key)) continue;
string[] values = nvc.GetValues(key);
if (values == null) continue;
foreach (string value in values)
{
sb.Append(sb.Length == 0 ? "?" : "&");
sb.AppendFormat("{0}={1}", Uri.EscapeDataString(key), Uri.EscapeDataString(value));
}
}
return sb.ToString();
}
Example
var queryParams = new NameValueCollection()
{
{ "order_id", "0000" },
{ "item_id", "1111" },
{ "item_id", "2222" },
{ null, "skip entry with null key" },
{ "needs escaping", "special chars ? = &" },
{ "skip entry with null value", null }
};
Console.WriteLine(queryParams.ToQueryString());
Output
?order_id=0000&item_id=1111&item_id=2222&needs%20escaping=special%20chars%20%3F%20%3D%20%26
This should work without too much code:
NameValueCollection nameValues = HttpUtility.ParseQueryString(String.Empty);
nameValues.Add(Request.QueryString);
// modify nameValues if desired
var newUrl = "/page?" + nameValues;
The idea is to use HttpUtility.ParseQueryString to generate an empty collection of type HttpValueCollection. This class is a subclass of NameValueCollection that is marked as internal so that your code cannot easily create an instance of it.
The nice thing about HttpValueCollection is that the ToString method takes care of the encoding for you. By leveraging the NameValueCollection.Add(NameValueCollection) method, you can add the existing query string parameters to your newly created object without having to first convert the Request.QueryString collection into a url-encoded string, then parsing it back into a collection.
This technique can be exposed as an extension method as well:
public static string ToQueryString(this NameValueCollection nameValueCollection)
{
NameValueCollection httpValueCollection = HttpUtility.ParseQueryString(String.Empty);
httpValueCollection.Add(nameValueCollection);
return httpValueCollection.ToString();
}
Actually, you should encode the key too, not just value.
string q = String.Join("&",
nvc.AllKeys.Select(a => $"{HttpUtility.UrlEncode(a)}={HttpUtility.UrlEncode(nvc[a])}"));
Because a NameValueCollection can have multiple values for the same key, if you are concerned with the format of the querystring (since it will be returned as comma-separated values rather than "array notation") you may consider the following.
Example
var nvc = new NameValueCollection();
nvc.Add("key1", "val1");
nvc.Add("key2", "val2");
nvc.Add("empty", null);
nvc.Add("key2", "val2b");
Turn into: key1=val1&key2[]=val2&empty&key2[]=val2b rather than key1=val1&key2=val2,val2b&empty.
Code
string qs = string.Join("&",
// "loop" the keys
nvc.AllKeys.SelectMany(k => {
// "loop" the values
var values = nvc.GetValues(k);
if(values == null) return new[]{ k };
return nvc.GetValues(k).Select( (v,i) =>
// 'gracefully' handle formatting
// when there's 1 or more values
string.Format(
values.Length > 1
// pick your array format: k[i]=v or k[]=v, etc
? "{0}[]={1}"
: "{0}={1}"
, k, HttpUtility.UrlEncode(v), i)
);
})
);
or if you don't like Linq so much...
string qs = nvc.ToQueryString(); // using...
public static class UrlExtensions {
public static string ToQueryString(this NameValueCollection nvc) {
return string.Join("&", nvc.GetUrlList());
}
public static IEnumerable<string> GetUrlList(this NameValueCollection nvc) {
foreach(var k in nvc.AllKeys) {
var values = nvc.GetValues(k);
if(values == null) { yield return k; continue; }
for(int i = 0; i < values.Length; i++) {
yield return
// 'gracefully' handle formatting
// when there's 1 or more values
string.Format(
values.Length > 1
// pick your array format: k[i]=v or k[]=v, etc
? "{0}[]={1}"
: "{0}={1}"
, k, HttpUtility.UrlEncode(values[i]), i);
}
}
}
}
As has been pointed out in comments already, with the exception of this answer most of the other answers address the scenario (Request.QueryString is an HttpValueCollection, "not" a NameValueCollection) rather than the literal question.
Update: addressed null value issue from comment.
The short answer is to use .ToString() on the NameValueCollection and combine it with the original url.
However, I'd like to point out a few things:
You cant use HttpUtility.ParseQueryString on Request.RawUrl. The ParseQueryString() method is looking for a value like this: ?var=value&var2=value2.
If you want to get a NameValueCollection of the QueryString parameters just use Request.QueryString().
var nv = Request.QueryString;
To rebuild the URL just use nv.ToString().
string url = String.Format("{0}?{1}", Request.Path, nv.ToString());
If you are trying to parse a url string instead of using the Request object use Uri and the HttpUtility.ParseQueryString method.
Uri uri = new Uri("<THE URL>");
var nv = HttpUtility.ParseQueryString(uri.Query);
string url = String.Format("{0}?{1}", uri.AbsolutePath, nv.ToString());
I always use UriBuilder to convert an url with a querystring back to a valid and properly encoded url.
var url = "http://my-link.com?foo=bar";
var uriBuilder = new UriBuilder(url);
var query = HttpUtility.ParseQueryString(uriBuilder.Query);
query.Add("yep", "foo&bar");
uriBuilder.Query = query.ToString();
var result = uriBuilder.ToString();
// http://my-link.com:80/?foo=bar&yep=foo%26bar
In AspNet Core 2.0 you can use QueryHelpers AddQueryString method.
As #Atchitutchuk suggested, you can use QueryHelpers.AddQueryString in ASP.NET Core:
public string FormatParameters(NameValueCollection parameters)
{
var queryString = "";
foreach (var key in parameters.AllKeys)
{
foreach (var value in parameters.GetValues(key))
{
queryString = QueryHelpers.AddQueryString(queryString, key, value);
}
};
return queryString.TrimStart('?');
}
This did the trick for me:
public ActionResult SetLanguage(string language = "fr_FR")
{
Request.UrlReferrer.TryReadQueryAs(out RouteValueDictionary parameters);
parameters["language"] = language;
return RedirectToAction("Index", parameters);
}
You can use.
var ur = new Uri("/page",UriKind.Relative);
if this nv is of type string you can append to the uri first parameter.
Like
var ur2 = new Uri("/page?"+nv.ToString(),UriKind.Relative);
Is there any different between stringvariable != NullValue.String and !string.IsNullOrEmpty(stringvariable) in asp.net ? then which is best ?
The first tests that the string isn't "".
As strings can be null (because they are actually references) this test could fail.
By using IsNullOrEmpty you are wrapping:
if (string != null && string.Length > 0)
in one test.
IsNullOrEmpty is implemented like:
public static bool IsNullOrEmpty(string value)
{
if (value != null)
{
return (value.Length == 0);
}
return true;
}
So it checks both an empty string, and a null string. (Where is NullValue.String defined, I cannot seem to find a reference to it in any docs, but I assume it's eiter String.Empty or "null", so your first check only checks for one of these conditions.)
.Net4 has a new function called IsNullOrWhiteSpace(string value) which also returns true if the string contains only white space.