How to build a query string for a URL in C

Question

A common task when calling web resources from a code is building a query string to including all the necessary parameters  While by all means no rocket science  there are some nifty details you need to take care of like  appending an  amp  if not the first parameter  encoding the parameters etc   The code to do it is very simple  but a bit tedious   StringBuilder SB   new StringBuilder    if  NeedsToAddParameter A        SB Append  A     SB Append HttpUtility UrlEncode  TheValueOfA         if  NeedsToAddParameter B       if  SB Length gt 0  SB Append   amp        SB Append  B     SB Append HttpUtility UrlEncode  TheValueOfB           This is such a common task one would expect a utility class to exist that makes it more elegant and readable  Scanning MSDN  I failed to find one mdash which brings me to the following question   What is the most elegant clean way you know of doing the above

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EDIT - as pointed out in the comments  this is not the way to go   There is such a class - the URI Class   Provides an object representation of a uniform resource identifier  URI  and easy access to the parts of the URI    Microsoft docs    The following example creates an instance of the Uri class and uses it to create a WebRequest instance   C   example  Uri siteUri   new Uri  http   www contoso com      WebRequest wr   WebRequest Create siteUri    Check it out  there are lots of methods on this class

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While not elegant  I opted for a simpler version that doesn t use NameValueCollecitons - just a builder pattern wrapped around StringBuilder   public class UrlBuilder        region Variables   Properties      private readonly StringBuilder  builder        endregion Variables   Properties       region Constructor      public UrlBuilder string urlBase                 builder   new StringBuilder urlBase               endregion Constructor       region Methods      public UrlBuilder AppendParameter string paramName  string value                if   builder ToString   Contains                    builder Append   amp             else              builder Append                 builder Append HttpUtility UrlEncode paramName             builder Append                builder Append HttpUtility UrlEncode value             return this             public override string ToString                 return  builder ToString                endregion Methods     Per existing answers  I made sure to use HttpUtility UrlEncode calls   It s used like so   string url   new UrlBuilder  http   www somedomain com                  AppendParameter  a    true                 AppendParameter  b    muffin                 AppendParameter  c    muffin button                 ToString       Result  http   www somedomain com a true amp b muffin amp c muffin 20button

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If you look under the hood the QueryString property is a NameValueCollection  When I ve done similar things I ve usually been interested in serialising AND deserialising so my suggestion is to build a NameValueCollection up and then pass to   using System Linq  using System Web  using System Collections Specialized   private string ToQueryString NameValueCollection nvc        var array             from key in nvc AllKeys         from value in nvc GetValues key              select string Format                    0   1                    HttpUtility UrlEncode key                   HttpUtility UrlEncode value             ToArray        return       string Join   amp    array       I imagine there s a super elegant way to do this in LINQ too

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USAGE  TestMethod  public void TestUrlBuilder         Console WriteLine          new UrlBuilder  http   www google com A B                AddPath  SomePathName                AddPath  AnotherPathName                SetQuery  SomeQueryKey    SomeQueryValue                AlterQuery  A   x   gt  x    C         Output      http   www google com SomePathName AnotherPathName A BC amp SomeQueryKey SomeQueryValue   The code  you can all thank me somewhere  somehow  D  using System  using System Collections Generic  using System Linq  using System Web      By Demetris Leptos namespace TheOperator Foundation Web       public class UrlBuilder               public string Scheme   get  set             public string Host   get  set             public int  Port   get  set             public List lt string gt  Paths   get  set             public SortedDictionary lt string  string gt  QueryPairs   get  set             public UrlBuilder string url                        this Paths   new List lt string gt                 this QueryPairs   new SortedDictionary lt string  string gt                  string path   null              string query   null              Uri relativeUri   null              if   Uri TryCreate url  UriKind Relative  out relativeUri                                 var uriBuilder   new UriBuilder url                   this Scheme   uriBuilder Scheme                  this Host   uriBuilder Host                  this Port   uriBuilder Port                  path   uriBuilder Path                  query   uriBuilder Query                            else                               var queryIndex   url IndexOf                       if  queryIndex  gt   0                                        path   url Substring 0  queryIndex                       query   url Substring queryIndex   1                                     else                                       path   url                                              this Paths AddRange path Split new char            StringSplitOptions RemoveEmptyEntries                if  query    null                                var queryKeyValuePairs   HttpUtility ParseQueryString query                   foreach  var queryKey in queryKeyValuePairs AllKeys                                        this QueryPairs queryKey    queryKeyValuePairs queryKey                                                      public UrlBuilder AddPath string value                        this Paths Add value               return this                     public UrlBuilder SetQuery string key  string value                        this QueryPairs key    value              return this                     public UrlBuilder RemoveQuery string key                        this QueryPairs Remove key               return this                     public UrlBuilder AlterQuery string key  Func lt string  string gt  alterMethod  bool removeOnNull   false                        string value              this QueryPairs TryGetValue key  out value               value   alterMethod value               if  removeOnNull  amp  amp  value    null                                return this RemoveQuery key                             else                               return this SetQuery key  value                                    public override string ToString                         var path    string IsNullOrWhiteSpace this Host                    string Join      this Host  string Join      this Paths                     string Join      this Paths               var query   string Join   amp    this QueryPairs Select x   gt  string Concat x Key       HttpUtility UrlEncode x Value                  return string Concat                   string IsNullOrWhiteSpace this Scheme    string Concat this Scheme           null                  path                   string IsNullOrWhiteSpace query    string Concat      query    null

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Here is an implementation which uses very basic language features   It s part of a class which we have to port and maintain in Objective C so we choose to have more lines of code but easier to port and understand by a programmer that isn t very familiar with C                   lt summary gt              Builds a complete http url with query strings               lt  summary gt               lt param name  pHostname  gt  lt  param gt               lt param name  pPort  gt  lt  param gt               lt param name  pPage  gt ex   index html  or index html lt  param gt               lt param name  pGetParams  gt a Dictionary lt string string gt  collection containing the key value pairs   Pass null if there are none  lt  param gt               lt returns gt a string of the form  http    pHostname   pPort  pPage  key1 val1 amp key2 val2    lt  returns gt     static public string buildURL string pHostname  int pPort  string pPage  Dictionary lt string string gt  pGetParams                        StringBuilder sb   new StringBuilder 200               sb Append  http                   sb Append pHostname               if  pPort    80                     sb Append pPort                                Allows page param to be passed in with or without leading slash              if   pPage StartsWith                          sb Append                                 sb Append pPage                if  pGetParams    null  amp  amp  pGetParams Count  gt  0                                sb Append                       foreach  KeyValuePair lt string  string gt  kvp in pGetParams                                        sb Append kvp Key                       sb Append                           sb Append  System Web HttpUtility UrlEncode kvp Value                         sb Append   amp                                       sb Remove sb Length - 1  1      Remove the final   amp                             return sb ToString

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Just for those that need the VB NET version of the top-answer   Public Function ToQueryString nvc As System Collections Specialized NameValueCollection  As String     Dim array As String     nvc AllKeys SelectMany Function key As String  nvc GetValues key   Function key As String  value As String  String Format   0   1    System Web HttpUtility UrlEncode key   System Web HttpUtility UrlEncode value    ToArray       Return       String Join   amp    array  End Function   And the version without LINQ   Public Function ToQueryString nvc As System Collections Specialized NameValueCollection  As String     Dim lsParams As New List Of String         For Each strKey As String In nvc AllKeys         Dim astrValue As String     nvc GetValues strKey           For Each strValue As String In astrValue             lsParams Add String Format   0   1    System Web HttpUtility UrlEncode strKey   System Web HttpUtility UrlEncode strValue            Next   Next strValue     Next   strKey     Dim astrParams As String     lsParams ToArray       lsParams Clear       lsParams   Nothing      Return       String Join   amp    astrParams  End Function   ToQueryString   And the C  version without LINQ       public static string ToQueryString System Collections Specialized NameValueCollection nvc                List lt string gt  lsParams   new List lt string gt              foreach  string strKey in nvc AllKeys                        string   astrValue   nvc GetValues strKey                foreach  string strValue in astrValue                                lsParams Add string Format   0   1    System Web HttpUtility UrlEncode strKey   System Web HttpUtility UrlEncode strValue                      Next strValue               Next strKey          string   astrParams  lsParams ToArray            lsParams Clear            lsParams   null           return       string Join   amp    astrParams            End Function ToQueryString

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public static string ToQueryString this Dictionary lt string  string gt  source                return String Join   amp    source Select kvp   gt  String Format   0   1    HttpUtility UrlEncode kvp Key   HttpUtility UrlEncode kvp Value    ToArray                public static string ToQueryString this NameValueCollection source                return String Join   amp    source Cast lt string gt    Select key   gt  String Format   0   1    HttpUtility UrlEncode key   HttpUtility UrlEncode source key     ToArray

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Flurl  disclosure  I m the author  supports building query strings via anonymous objects  among other ways   var url    quot http   www some-api com quot  SetQueryParams new       api key   ConfigurationManager AppSettings  quot SomeApiKey quot        max results   20      q    quot Don t worry  I ll get encoded  quot       The optional Flurl Http companion lib allows you to do HTTP calls right off the same fluent call chain  extending it into a full-blown REST client  T result   await  quot https   api mysite com quot       AppendPathSegment  quot person quot        SetQueryParams new   ap key    quot my-key quot          WithOAuthBearerToken  quot MyToken quot        PostJsonAsync new   first name   firstName  last name   lastName         ReceiveJson lt T gt      The full package is available on NuGet  PM gt  Install-Package Flurl Http or just the stand-alone URL builder  PM gt  Install-Package Flurl

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With the inspiration from Roy Tinker s comment  I ended up using a simple extension method on the Uri class that keeps my code concise and clean   using System Web   public static class HttpExtensions       public static Uri AddQuery this Uri uri  string name  string value                var httpValueCollection   HttpUtility ParseQueryString uri Query            httpValueCollection Remove name           httpValueCollection Add name  value            var ub   new UriBuilder uri           ub Query   httpValueCollection ToString             return ub Uri            Usage   Uri url   new Uri  http   localhost rest something browse              AddQuery  page    0              AddQuery  pageSize    200        Edit - Standards compliant variant  As several people pointed out  httpValueCollection ToString   encodes Unicode characters in a non-standards-compliant way  This is a variant of the same extension method that handles such characters by invoking HttpUtility UrlEncode method instead of the deprecated HttpUtility UrlEncodeUnicode method   using System Web   public static Uri AddQuery this Uri uri  string name  string value        var httpValueCollection   HttpUtility ParseQueryString uri Query        httpValueCollection Remove name       httpValueCollection Add name  value        var ub   new UriBuilder uri           this code block is taken from httpValueCollection ToString   method        and modified so it encodes strings with HttpUtility UrlEncode     if  httpValueCollection Count    0          ub Query   String Empty      else               var sb   new StringBuilder             for  int i   0  i  lt  httpValueCollection Count  i                          string text   httpValueCollection GetKey i                                 text   HttpUtility UrlEncode text                    string val    text    null     text          string Empty                  string   vals   httpValueCollection GetValues i                    if  sb Length  gt  0                      sb Append   amp                      if  vals    null    vals Length    0                      sb Append val                   else                                       if  vals Length    1                                                sb Append val                           sb Append HttpUtility UrlEncode vals 0                                               else                                               for  int j   0  j  lt  vals Length  j                                                          if  j  gt  0                                  sb Append   amp                                  sb Append val                               sb Append HttpUtility UrlEncode vals j                                                                                                        ub Query   sb ToString               return ub Uri

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The code below  is taken off the HttpValueCollection implementation of ToString  via ILSpy  which gives you a name value querystring   Unfortunately HttpValueCollection is an internal class which you only ever get back if you use HttpUtility ParseQueryString    I removed all the viewstate parts to it  and it encodes by default   public static class HttpExtensions       public static string ToQueryString this NameValueCollection collection                   This is based off the NameValueCollection ToString   implementation         int count   collection Count          if  count    0              return string Empty           StringBuilder stringBuilder   new StringBuilder             for  int i   0  i  lt  count  i                          string text   collection GetKey i               text   HttpUtility UrlEncodeUnicode text               string value    text    null     text          string Empty              string   values   collection GetValues i               if  stringBuilder Length  gt  0                                stringBuilder Append   amp                               if  values    null    values Length    0                                stringBuilder Append value                             else                               if  values Length    1                                        stringBuilder Append value                       string text2   values 0                       text2   HttpUtility UrlEncodeUnicode text2                       stringBuilder Append text2                                     else                                       for  int j   0  j  lt  values Length  j                                                  if  j  gt  0                                                        stringBuilder Append   amp                                                       stringBuilder Append value                           string text2   values j                           text2   HttpUtility UrlEncodeUnicode text2                           stringBuilder Append text2                                                                            return stringBuilder ToString

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Combined the top answers to create an anonymous object version   var queryString   HttpUtility2 BuildQueryString new       key2    value2       key1    value1         That generates this           key2 value2 amp key1 value1      Here s the code   public static class HttpUtility2       public static string BuildQueryString lt T gt  T obj                var queryString   HttpUtility ParseQueryString string Empty            foreach  var property in TypeDescriptor GetProperties typeof T   Cast lt PropertyDescriptor gt                           var value    property GetValue obj         ToString                queryString Add property Name  value                      return queryString ToString

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Untested  but I think something along these lines would work quite nicely  public class QueryString       private Dictionary lt string string gt   Params   new Dictionary lt string string gt          public overide ToString                 List lt string gt  returnParams   new List lt string gt              foreach  KeyValuePair param in  Params                        returnParams Add String Format   0   1    param Key  param Value                          return String Format    0    String Join   amp    returnParams ToArray                   credit annakata         return       String Join   amp    returnParams ToArray                public void Add string key  string value                 Params Add key  HttpUtility UrlEncode value             QueryString query   new QueryString     query Add  param1    value1    query Add  param2    value2     return query ToString

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I answered a similar question a while ago  Basically  the best way would be to use the class HttpValueCollection  which ASP NET s Request QueryString property actually is  unfortunately it is internal in the  NET framework  You could use Reflector to grab it  and place it into your Utils class   This way you could manipulate the query string like a NameValueCollection  but with all the url encoding decoding issues taken care for you   HttpValueCollection extends NameValueCollection  and has a constructor that takes an encoded query string  ampersands and question marks included   and it overrides a ToString   method to later rebuild the query string from the underlying collection   Example     var coll   new HttpValueCollection       coll  userId      50     coll  paramA      A     coll  paramB      B            string query   coll ToString true      true means use urlencode    Console WriteLine query      prints  userId 50 amp paramA A amp paramB B

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Assuming that you want to reduce dependencies to other assemblies and to keep things simple  you can do   var sb   new System Text StringBuilder     sb Append  a     HttpUtility UrlEncode  TheValueOfA       amp     sb Append  b     HttpUtility UrlEncode  TheValueOfB       amp     sb Append  c     HttpUtility UrlEncode  TheValueOfC       amp     sb Append  d     HttpUtility UrlEncode  TheValueOfD       amp      sb Remove sb Length-1  1      Remove the final   amp    string result   sb ToString      This works well with loops too   The final ampersand removal needs to go outside of the loop   Note that the concatenation operator is used to improve readability   The cost of using it compared to the cost of using a StringBuilder is minimal  I think Jeff Atwood posted something on this topic

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In dotnet core QueryHelpers AddQueryString   will accept an IDictionary lt string string gt  of key-value pairs  To save a few memory allocs and CPU cycles you can use SortedList lt   gt  instead of Dictionary lt   gt   with an appropriate capacity and items added in sort order    var queryParams   new SortedList lt string string gt  2   queryParams Add  quot abc quot    quot val1 quot    queryParams Add  quot def quot    quot val2 quot     string requestUri   QueryHelpers AddQueryString  quot https   localhost api quot   queryParams

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I needed to solve the same problem for a portable class library  PCL  that I m working on   In this case  I don t have access to System Web so I can t use ParseQueryString   Instead I used System Net Http FormUrlEncodedContent like so   var url   new UriBuilder  http   example com     url Query   new FormUrlEncodedContent new Dictionary lt string string gt            param1    val1          param2    val2          param3    val3       ReadAsStringAsync   Result

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public string UrlQueryStr object data        if  data    null          return string Empty       object val      StringBuilder sb   new StringBuilder         foreach  PropertyDescriptor prop in TypeDescriptor GetProperties data                 if   val   prop GetValue data      null                        sb AppendFormat   0  1   2    sb Length    0           amp                    HttpUtility UrlEncode prop Name   HttpUtility UrlEncode val ToString                          return sb ToString

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This is the identical to the accepted answer except slightly more compact   private string ToQueryString NameValueCollection nvc        return       string Join   amp    nvc AllKeys Select k   gt  string Format   0   1             HttpUtility UrlEncode k            HttpUtility UrlEncode nvc k

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The query string can be added to a URL by    create a name value collection object  add the query string items and their values to this object  encode this name value collection object to the url the code is provided in the below link   https   blog codingnovice com blog  public ActionResult Create           declaring name value collection object     NameValueCollection collection   new NameValueCollection           adding new value to the name value collection object     collection Add  Id1    wwe323        collection Add  Id2    454w        collection Add  Id3    tyt5656        collection Add  Id4    343wdsd           generating query string     string url   GenerateQueryString collection        return View       private string GenerateQueryString NameValueCollection collection        var querystring             from key in collection AllKeys         from value in collection GetValues key          select string Format   0   1                HttpUtility UrlEncode key               HttpUtility UrlEncode value         ToArray        return       string Join   amp    querystring

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Works for multiple values per key in NameValueCollection    ex      k1    v1      k1    v1         k1 v1 amp k1 v1       lt summary gt      Get query string for name value collection       lt  summary gt  public static string ToQueryString this NameValueCollection collection      bool prefixQuestionMark   true        collection NullArgumentCheck        if  collection Keys Count    0                return               var buffer   new StringBuilder        if  prefixQuestionMark                buffer Append                 var append   false      for  int i   0  i  lt  collection Keys Count  i                  var key   collection Keys i           var values   collection GetValues key           key NullCheck            values NullCheck            foreach  var value in values                        if  append                                buffer Append   amp                               append   true              buffer AppendFormat   0   1    key UrlEncode    value UrlEncode                         return buffer ToString

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I wrote a helper for my razor project using some of the hints from other answers   The ParseQueryString business is necessary because we are not allowed to tamper with the QueryString object of the current request    helper GetQueryStringWithValue string key  string value        var queryString   System Web HttpUtility ParseQueryString HttpContext Current Request QueryString ToString         queryString key    value       Html Raw queryString ToString        I use it like this   location search      Helpers GetQueryStringWithValue  var-name    var-value       If you want it to take more than one value  just change the parameters to a Dictionary and add the pairs to the query string

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Here s my late entry   I didn t like any of the others for various reasons  so I wrote my own   This version features    Use of StringBuilder only   No ToArray   calls or other extension methods   It doesn t look as pretty as some of the other responses  but I consider this a core function so efficiency is more important than having  fluent    one-liner  code which hide inefficiencies  Handles multiple values per key   Didn t need it myself but just to silence Mauricio     public string ToQueryString NameValueCollection nvc        StringBuilder sb   new StringBuilder            bool first   true       foreach  string key in nvc AllKeys                foreach  string value in nvc GetValues key                         if   first                                sb Append   amp                                sb AppendFormat   0   1    Uri EscapeDataString key   Uri EscapeDataString value                 first   false                       return sb ToString         Example Usage          var queryParams   new NameValueCollection                            x    1                    y    2                    foo    bar                    foo    baz                    special chars         amp                          string url    http   example com stuff    ToQueryString queryParams            Console WriteLine url     Output  http   example com stuff x 1 amp y 2 amp foo bar amp foo baz amp special 20chars  3F 20 3D 20 26

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Chain-able wrapper class for HttpValueCollection   namespace System Web Mvc       public class QueryStringBuilder           private NameValueCollection collection          public QueryStringBuilder                 collection   System Web HttpUtility ParseQueryString string Empty                     public QueryStringBuilder Add string key  string value                collection Add key  value               return this                    public QueryStringBuilder Remove string key                collection Remove key               return this                    public string this string key                get   return collection key                 set   collection key    value                      public string ToString                 return collection ToString                        Example usage   QueryStringBuilder parameters   new QueryStringBuilder        Add  view   ViewBag PageView       Add  page   ViewBag PageNumber       Add  size   ViewBag PageSize   string queryString   parameters ToString

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Just wanted to throw in my 2 cents   public static class HttpClientExt       public static Uri AddQueryParams this Uri uri  string query                var ub   new UriBuilder uri           ub Query   string IsNullOrEmpty uri Query    query   string Join   amp    uri Query Substring 1   query           return ub Uri             public static Uri AddQueryParams this Uri uri  IEnumerable lt string gt  query                return uri AddQueryParams string Join   amp    query                public static Uri AddQueryParams this Uri uri  string key  string value                return uri AddQueryParams string Join      HttpUtility UrlEncode key   HttpUtility UrlEncode value                public static Uri AddQueryParams this Uri uri  params KeyValuePair lt string string gt    kvps                return uri AddQueryParams kvps Select kvp   gt  string Join      HttpUtility UrlEncode kvp Key   HttpUtility UrlEncode kvp Value                 public static Uri AddQueryParams this Uri uri  IDictionary lt string  string gt  kvps                return uri AddQueryParams kvps Select kvp   gt  string Join      HttpUtility UrlEncode kvp Key   HttpUtility UrlEncode kvp Value                 public static Uri AddQueryParams this Uri uri  NameValueCollection nvc                return uri AddQueryParams nvc AllKeys SelectMany nvc GetValues   key  value    gt  string Join      HttpUtility UrlEncode key   HttpUtility UrlEncode value                The docs say that uri Query will start with a   if it s non-empty and you should trim it off if you re going to modify it   Note that HttpUtility UrlEncode is found in System Web   Usage   var uri   new Uri  https   api del icio us v1 posts suggest   AddQueryParam  url   http   stackoverflow com

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How about creating extension methods that allow you to add the parameters in a fluent style like this   string a    http   www somedomain com somepage html       AddQueryParam  A    TheValueOfA        AddQueryParam  B    TheValueOfB        AddQueryParam  Z    TheValueOfZ     string b   new StringBuilder  http   www somedomain com anotherpage html        AddQueryParam  A    TheValueOfA        AddQueryParam  B    TheValueOfB        AddQueryParam  Z    TheValueOfZ        ToString       Here s the overload that uses a string   public static string AddQueryParam      this string source  string key  string value        string delim      if   source    null      source Contains                     delim                  else if  source EndsWith         source EndsWith   amp                   delim   string Empty            else               delim     amp               return source   delim   HttpUtility UrlEncode key                  HttpUtility UrlEncode value       And here s the overload that uses a StringBuilder   public static StringBuilder AddQueryParam      this StringBuilder source  string key  string value        bool hasQuery   false      for  int i   0  i  lt  source Length  i                  if  source i                                hasQuery   true              break                       string delim      if   hasQuery                delim                  else if   source source Length - 1                      source source Length - 1       amp                   delim   string Empty            else               delim     amp               return source Append delim  Append HttpUtility UrlEncode key            Append      Append HttpUtility UrlEncode value

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A quick extension method based version   class Program       static void Main string   args                var parameters   new List lt KeyValuePair lt string  string gt  gt                                                                  new KeyValuePair lt string  string gt   A    AValue                                     new KeyValuePair lt string  string gt   B    BValue                                            string output         string Join   amp    parameters ConvertAll param   gt  param ToQueryString    ToArray              public static class KeyValueExtensions       public static string ToQueryString this KeyValuePair lt string  string gt  obj                return obj Key         HttpUtility UrlEncode obj Value             You could use a where clause to select which parameters get added to the string

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Another approach is to create an extension of the class NameValueCollection that returns the complete Url  public static class CustomMethods       public static string ToUrl this System Collections Specialized NameValueCollection collection                if  collection Count    0  return  quot  quot            string completeUrl    quot   quot           for  int i   0  i  lt  collection Count  i                          completeUrl    new Page   Server UrlEncode collection GetKey i      quot   quot    new Page   Server UrlEncode collection Get i                if   i   1   lt  collection Count  completeUrl     quot  amp  quot                      return completeUrl           Then  you can use your new method  for example  System Collections Specialized NameValueCollection qString   new System Collections Specialized NameValueCollection    qString Add  quot name quot    quot MyName quot    qString Add  quot email quot    quot myemail test com quot    qString ToUrl      Result   quot  name MyName amp email myemail 40test com quot

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My offering   public static Uri AddQuery this Uri uri  string name  string value           this actually returns HttpValueCollection   NameValueCollection        which uses unicode compliant encoding on ToString       var query   HttpUtility ParseQueryString uri Query        query Add name  value        var uriBuilder   new UriBuilder uri                Query   query ToString               return uriBuilder Uri      Usage   var uri   new Uri  http   stackoverflow com   AddQuery  such    method                                                 AddQuery  wow    soFluent        http   stackoverflow com such method amp wow soFluent

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I added the following method to my PageBase class   protected void Redirect string url                Response Redirect url         protected void Redirect string url  NameValueCollection querystrings                StringBuilder redirectUrl   new StringBuilder url            if  querystrings    null                        for  int index   0  index  lt  querystrings Count  index                                  if  index    0                                        redirectUrl Append                                          redirectUrl Append querystrings Keys index                    redirectUrl Append                       redirectUrl Append HttpUtility UrlEncode querystrings index                      if  index  lt  querystrings Count - 1                                        redirectUrl Append   amp                                                        this Redirect redirectUrl ToString             To call   NameValueCollection querystrings   new NameValueCollection        querystrings Add  language    en    querystrings Add  id    134    this Redirect  http   www mypage com   querystrings

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You can create a new writeable instance of HttpValueCollection by calling System Web HttpUtility ParseQueryString string Empty   and then use it as any NameValueCollection  Once you have added the values you want  you can call ToString on the collection to get a query string  as follows   NameValueCollection queryString   System Web HttpUtility ParseQueryString string Empty    queryString Add  key1    value1    queryString Add  key2    value2     return queryString ToString       Returns  key1 value1 amp key2 value2   all URL-encoded   The HttpValueCollection is internal and so you cannot directly construct an instance  However  once you obtain an instance you can use it like any other NameValueCollection  Since the actual object you are working with is an HttpValueCollection  calling ToString method will call the overridden method on HttpValueCollection  which formats the collection as a URL-encoded query string   After searching SO and the web for an answer to a similar issue  this is the most simple solution I could find    NET Core  If you re working in  NET Core  you can use the Microsoft AspNetCore WebUtilities QueryHelpers class  which simplifies this greatly   https   docs microsoft com en-us dotnet api microsoft aspnetcore webutilities queryhelpers  Sample Code   const string url    https   customer-information azure-api net customers search taxnbr   var param   new Dictionary lt string  string gt         CIKey    123456789        var newUrl   new Uri QueryHelpers AddQueryString url  param

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Same as accepted solution  but transfred to  dot  LINQ syntax     private string ToQueryString NameValueCollection nvc        if  nvc    null  return String Empty      var queryParams              string Join   amp    nvc AllKeys Select key   gt                 string Join   amp    nvc GetValues key  Select v   gt  string Format   0   1    HttpUtility UrlEncode key   HttpUtility UrlEncode v            return       queryParams

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Add this class to your project  using System  using System Collections Generic  using System Linq  using System Web   public class QueryStringBuilder       private readonly List lt KeyValuePair lt string  object gt  gt   list       public QueryStringBuilder                  list   new List lt KeyValuePair lt string  object gt  gt                public void Add string name  object value                 list Add new KeyValuePair lt string  object gt  name  value               public override string ToString                 return String Join   amp     list Select kvp   gt  String Concat Uri EscapeDataString kvp Key        Uri EscapeDataString kvp Value ToString                  And use it like this   var actual   new QueryStringBuilder         foo   123         bar    val31          bar    val32       actual Add  a b    c d     actual ToString        foo 123 amp bar val31 amp bar val32 amp a 2bb c 2bd

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Here s a fluent lambda-ish way as an extension method  combining concepts in previous posts  that supports multiple values for the same key   My personal preference is extensions over wrappers for discover-ability by other team members for stuff like this   Note that there s controversy around encoding methods  plenty of posts about it on Stack Overflow  one such post  and MSDN bloggers  like this one    public static string ToQueryString this NameValueCollection source        return String Join   amp    source AllKeys          SelectMany key   gt  source GetValues key               Select value   gt  String Format   0   1    HttpUtility UrlEncode key   HttpUtility UrlEncode value              ToArray         edit  with null support  though you ll probably need to adapt it for your particular situation  public static string ToQueryString this NameValueCollection source  bool removeEmptyEntries        return source    null   String Join   amp    source AllKeys          Where key   gt   removeEmptyEntries    source GetValues key               Where value   gt   String IsNullOrEmpty value                Any             SelectMany key   gt  source GetValues key               Where value   gt   removeEmptyEntries     String IsNullOrEmpty value                Select value   gt  String Format   0   1    HttpUtility UrlEncode key   value    null   HttpUtility UrlEncode value    string Empty             ToArray              string Empty

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I went with the solution proposed by DSO  answered on Aug 2  11 at 7 29   his solution does not require using HttpUtility  However  as per an article posted in Dotnetpearls  using a Dictionary is faster  in performance  than using NameValueCollection  Here is DSO s solution modified to use Dictionary in place of NameValueCollection       public static Dictionary lt string  string gt  QueryParametersDictionary                 var dictionary   new Dictionary lt string  string gt             dictionary Add  name    John Doe            dictionary Add  address city    Seattle            dictionary Add  address state code    WA            dictionary Add  api key    5352345263456345635             return dictionary             public static string ToQueryString Dictionary lt string  string gt  nvc                StringBuilder sb   new StringBuilder             bool first   true           foreach  KeyValuePair lt string  string gt  pair in nvc                            if   first                                        sb Append   amp                                        sb AppendFormat   0   1    Uri EscapeDataString pair Key   Uri EscapeDataString pair Value                     first   false                     return sb ToString

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I have an extension method for Uri that    Accepts anonymous objects  uri WithQuery new   name    value     Accepts collections of string string pairs  e g  Dictionary 2   Accepts collections of string object pairs  e g  RouteValueDictionary   Accepts NameValueCollections  Sorts the query values by key so the same values produce equal URIs  Supports multiple values per key  preserving their original order      The documented version can be found here   The extension   public static Uri WithQuery this Uri uri  object values        if  uri    null          throw new ArgumentNullException nameof uri         if  values    null                var query   string Join                amp    from p in ParseQueryValues values                   where  string IsNullOrWhiteSpace p Key                   let k   HttpUtility UrlEncode p Key Trim                     let v   HttpUtility UrlEncode p Value                   orderby k                  select string IsNullOrEmpty v    k      k   v              if  query Length    0    uri Query Length    0              uri   new UriBuilder uri    Query   query   Uri             return uri      The query parser   private static IEnumerable lt KeyValuePair lt string  string gt  gt  ParseQueryValues object values           Check if a name value collection      var nvc   values as NameValueCollection      if  nvc    null          return from key in nvc AllKeys                from val in nvc GetValues key                 select new KeyValuePair lt string  string gt  key  val           Check if a string string dictionary      var ssd   values as IEnumerable lt KeyValuePair lt string  string gt  gt       if  ssd    null          return ssd          Check if a string object dictionary      var sod   values as IEnumerable lt KeyValuePair lt string  object gt  gt       if  sod    null                   Check if a non-generic dictionary          var ngd   values as IDictionary          if  ngd    null              sod   ngd Cast lt dynamic gt    ToDictionary lt dynamic  string  object gt                   p   gt  p Key ToString    p   gt  p Value as object               Convert object properties to dictionary          if  sod    null              sod   new RouteValueDictionary values                 Normalize and return the values      return from pair in sod            from val in pair Value as IEnumerable lt string gt                 new     pair Value  ToString                select new KeyValuePair lt string  string gt  pair Key  val         Here are the tests   var uri   new Uri  https   stackoverflow com yo oldKey oldValue        Test with a string string dictionary  var q   uri WithQuery new Dictionary lt string  string gt          k1     string Empty        k2     null        k3      v3       Debug Assert q    new Uri       https   stackoverflow com yo k1 amp k2 amp k3 v3         Test with a string object dictionary  q   uri WithQuery new Dictionary lt string  object gt          k1      v1         k2     new      v2a    v2b           k3     null      Debug Assert q    new Uri       https   stackoverflow com yo k1 v1 amp k2 v2a amp k2 v2b amp k3         Test with a name value collection  var nvc   new NameValueCollection           k1     string Empty        k2      v2a      nvc Add  k2    v2b     q   uri WithQuery nvc   Debug Assert q    new Uri       https   stackoverflow com yo k1 amp k2 v2a amp k2 v2b         Test with any dictionary  q   uri WithQuery new Dictionary lt int  HashSet lt string gt  gt         1    new HashSet lt string gt     v1          2    new HashSet lt string gt     v2a    v2b          3    null      Debug Assert q    new Uri       https   stackoverflow com yo 1 v1 amp 2 v2a amp 2 v2b amp 3         Test with an anonymous object  q   uri WithQuery new       k1    v1       k2   new      v2a    v2b         k3   new List lt string gt     v3         k4   true      k5   null as Queue lt string gt       Debug Assert q    new Uri       https   stackoverflow com yo k1 v1 amp k2 v2a amp k2 v2b amp k3 v3 amp k4 True amp k5         Keep existing query using a name value collection  nvc   HttpUtility ParseQueryString uri Query   nvc Add  newKey    newValue     q   uri WithQuery nvc   Debug Assert q    new Uri       https   stackoverflow com yo newKey newValue amp oldKey oldValue         Merge two query objects using the RouteValueDictionary  var an1   new   k1    v1     var an2   new   k2    v2      q   uri WithQuery      new RouteValueDictionary an1  Concat          new RouteValueDictionary an2      Debug Assert q    new Uri       https   stackoverflow com yo k1 v1 amp k2 v2

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I wrote some extension methods that I have found very useful when working with QueryStrings   Often I want to start with the current QueryString and modify before using it   Something like   var res   Request QueryString Duplicate      ChangeField  field1    somevalue      ChangeField  field2    only if following is true   true     ChangeField  id   id  id gt 0     WriteLocalPathWithQuery Request Url      Uses context to write the path   For more and the source  http   www charlesrcook com archive 2008 07 23 c-extension-methods-for-asp net-query-string-operations aspx  It s basic  but I like the style

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This is another  maybe redundant  -   way for do that   The conceptuals are the same of the Vedran answer in this page  take a look here    But this class is more efficient  because it iterate through all Keys only one time  when ToString is invoked   The formatting code is also semplified and improved   Hope that could be helpful   public sealed class QueryStringBuilder       public QueryStringBuilder                 this inner   HttpUtility ParseQueryString string Empty              public QueryStringBuilder string queryString                this inner   HttpUtility ParseQueryString queryString              public QueryStringBuilder string queryString  Encoding encoding                this inner   HttpUtility ParseQueryString queryString  encoding              private readonly NameValueCollection inner       public QueryStringBuilder AddKey string key  string value                this inner Add key  value           return this             public QueryStringBuilder RemoveKey string key                this inner Remove key           return this             public QueryStringBuilder Clear                 this inner Clear            return this             public override String ToString                 if  this inner Count    0              return string Empty           var builder   new StringBuilder             for  int i   0  i  lt  this inner Count  i                          if  builder Length  gt  0                  builder Append   amp                  var key   this inner GetKey i               var values   this inner GetValues i                if  key    null    values    null    values Length    0                  continue               for  int j   0  j  lt  values Length  j                                  if  j  gt  0                      builder Append   amp                      builder Append HttpUtility UrlEncode key                    builder Append                       builder Append HttpUtility UrlEncode values j                                      return builder ToString

[c#] How to build a query string for a URL in C#?

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