Introduction
Sometimes we pass values from one page to other page in URL. QueryString is main object which helps to do the same.
Using code
QueryString - As per W3(WWW), a query string is the part of a uniform resource locator (URL)
containing data. The QueryString collection is used to retrieve the variable values in the HTTP query string. It is also considered as Client-Side state management technique in ASP.NET. As a security, always encode header data or user input before using it. A method for encoding data is Server.HTMLEncode() and for decode use Server.HTMLDecode().
The QueryString is specified by the values following the question mark (?), like this:
http://localhost:1997/WebForm1.aspx?id=10&name=manas
How to get all Keys and Values
Option 1: Using AllKeys properties
  1. foreach (var cookieKey in Request.QueryString.AllKeys)
  2. {
  3. Response.Write("Key: " + cookieKey + ", Value:" + Request.QueryString[cookieKey]);
  4. }
Option 2: Using no of QueryString Count
  1. for (int idx = 0; idx < Request.QueryString.Count; idx++)
  2. {
  3. Response.Write("Value:" + Request.QueryString[idx]);
  4. }
Option 3: Using Keys Count
  1. for (int idx = 0; idx < Request.QueryString.Keys.Count; idx++)
  2. {
  3. Response.Write("Key: " + Request.QueryString.Keys[idx] +
  4. ", Value:" + Request.QueryString[Request.QueryString.Keys[idx]]);
  5. }
If you want to get querystring value as string:
  1. string temp = Request.QueryString.ToString(); // id=10&name=manas
How to check a key is present:
Sometimes we check whether a key is present in QueryString. Below are two ways to check key is available in QueryString:
Option 1: Using AllKeys properties
  1. Request.QueryString.AllKeys.Contains("name"); // Return true
  2. Request.QueryString.AllKeys.Contains("name1"); // Return false
  3. Request.QueryString.AllKeys.Contains("Name"); // Return false due to Case-sensitive
Option 2 : Using NULL checking
  1. if(Request.QueryString["name"] != null)
  2. {
  3. //your code that depends on requirement
  4. }
We have HasKeys properties of QueryString which provides whether any keys is present. If there is no QueryString value then it returns false.
  1. // http://localhost:1997/WebForm1.aspx?id=10&name=manas
  2. Request.QueryString.HasKeys(); // true
  4. // http://localhost:1997/WebForm1.aspx
  5. Request.QueryString.HasKeys(); // false
Conclusion
We discussed how to get all cookies are available in a page.
Hope this helps.