UrlHelperExtensions.cs

using System;
using System.Linq;
using System.Web;
using System.Web.Mvc;

public static class UrlHelperExtensions
{
    /// <summary>
    /// Modified QueryString with key to value
    /// </summary>
    /// <param name="helper"></param>
    /// <param name="name"></param>
    /// <param name="value"></param>
    /// <returns>?key=value</returns>
    public static string ModifyQueryString<T>(this UrlHelper helper, string key, T value)
    {
        var request = helper.RequestContext.HttpContext.Request;
        var nameValues = HttpUtility.ParseQueryString(request.QueryString.ToString());
        nameValues.Set(key, value.ToString());
        return "?" + nameValues.ToString();
    }

    /// <summary>
    /// Removes QueryString with key
    /// </summary>
    /// <param name="helper"></param>
    /// <param name="name"></param>
    /// <param name="value"></param>
    /// <returns>?key=value</returns>
    public static string RemoveQueryString(this UrlHelper helper, params string[] keys)
    {
        var request = helper.RequestContext.HttpContext.Request;
        var nameValues = HttpUtility.ParseQueryString(request.QueryString.ToString());
        foreach (var key in keys)
            nameValues.Remove(key);

        return "?" + nameValues.ToString();
    }

    /// <summary>
    /// Has QueryString with key
    /// </summary>
    /// <param name="helper"></param>
    /// <param name="name"></param>
    /// <param name="value"></param>
    /// <returns>?key=value</returns>
    public static bool HasQueryString(this UrlHelper helper, string key)
    {
        var request = helper.RequestContext.HttpContext.Request;
        var nameValues = HttpUtility.ParseQueryString(request.QueryString.ToString());
        var value = nameValues.Get(key);
        return !string.IsNullOrEmpty(value);
    }

    /// <summary>
    /// Gets the querystring key value with fallback.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="helper"></param>
    /// <param name="key">The key of the QueryString</param>
    /// <param name="fallback">If empty or null</param>
    /// <returns></returns>
    public static T GetQueryString<T>(this UrlHelper helper, string key, T fallback)
    {
        var request = helper.RequestContext.HttpContext.Request;
        var nameValues = HttpUtility.ParseQueryString(request.QueryString.ToString());
        if (nameValues.AllKeys.Contains(key))
        {
            try
            {
                return (T)Convert.ChangeType(nameValues.Get(key), typeof(T));
            }
            catch (Exception) { }
        }
        return fallback;
    }
}