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Home Interviews Explain the HttpUtility Class in C#? – MindStick
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ICSM Computer
ICSM Computer 05 May 2025
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The HttpUtility class in C# (in ASP.NET) provides helper methods for encoding and decoding URLs, HTML, and query strings, as well as parsing request data. It's especially useful for ensuring your data is safe for web transport (e.g., avoiding XSS or malformed URLs).

Namespace

using System.Web;

Available in System.Web.dll, so it's fully supported in ASP.NET Web Forms and MVC (not in ASP.NET Core).

Purpose of HttpUtility

Category Purpose
HTML Encoding Prevents injection/XSS by escaping HTML tags
URL Encoding Encodes unsafe characters for URLs
Query String Parsing Converts a query string into a key-value collection

Common Methods

Method Description
HtmlEncode(string) Encodes HTML (e.g., < becomes <)
HtmlDecode(string) Decodes encoded HTML
UrlEncode(string) Encodes URLs (e.g., ? becomes %3F)
UrlDecode(string) Decodes encoded URLs
ParseQueryString(string) Parses a query string into a collection
JavaScriptStringEncode(string) Encodes for safe embedding into JavaScript (to avoid breakage or injection)

Common Examples

1. HTML Encoding

string raw = "<script>alert('XSS');</script>";
string safe = HttpUtility.HtmlEncode(raw); 
// Output: <script>alert('XSS');</script>

2. HTML Decoding

string encoded = "<b>Hello</b>";
string decoded = HttpUtility.HtmlDecode(encoded);  // <b>Hello</b>

3. URL Encoding/Decoding

string url = "https://site.com/search?q=hello world";
string encoded = HttpUtility.UrlEncode(url); 
// https%3a%2f%2fsite.com%2fsearch%3fq%3dhello+world

string decoded = HttpUtility.UrlDecode(encoded);

4. Parse Query String

string query = "name=John&age=30";
var parsed = HttpUtility.ParseQueryString(query);

string name = parsed["name"];  // John
int age = int.Parse(parsed["age"]); // 30

5. Encode JavaScript Strings

string unsafeJs = "alert('dangerous');";
string safeJs = HttpUtility.JavaScriptStringEncode(unsafeJs);
// Output: alert(\u0027dangerous\u0027);

When to Use HttpUtility

Situation Use
Outputting user-generated content to HTML HtmlEncode()
Creating safe URLs or redirect strings UrlEncode()
Reading/parsing raw query string manually ParseQueryString()
Embedding user content inside <script> tags JavaScriptStringEncode()

Summary Table

Function Code
HTML Encode HttpUtility.HtmlEncode("<tag>")
URL Encode HttpUtility.UrlEncode("a b&c")
Parse Query HttpUtility.ParseQueryString("a=1&b=2")
JavaScript Encode HttpUtility.JavaScriptStringEncode("alert('x')")
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