Good way to parse query string

You can use this custom function... quick test show that with the string you originally gave in the question it returns the desired output which is two items, one with key sub and value fs&&&FASh*5 and another with key workarea and value London+&Home+Counties+Ltd.

public Dictionary<string, string> ParseQueryString(string qs)
{
    Dictionary<string, string> items = new Dictionary<string, string>();
    string valueBuffer = string.Empty;
    string keyBuffer = string.Empty;
    bool lookingForValue = true;
    for (int i = qs.Length - 1; i >= 0; i--)
    {
        char curChar = qs[i];
        if (curChar.Equals('='))
        {
            lookingForValue = false;
            keyBuffer = string.Empty;
        }
        else if (curChar.Equals('&') && !lookingForValue)
        {
            items.Add(keyBuffer, valueBuffer);
            valueBuffer = string.Empty;
            lookingForValue = true;
        }
        else if (curChar.Equals('?'))
        {
            if (keyBuffer.Length > 0)
                items.Add(keyBuffer, valueBuffer);
            break;
        }
        else
        {
            if (lookingForValue)
                valueBuffer = curChar + valueBuffer;
            else
                keyBuffer = curChar + keyBuffer;
        }
    }

    return items;
}

As I told in the comment, if used for real querystring use Server.URLEncode to encode each value.

Quick explanation: instead of parsing the query string according to & which can't be done, it's looking for = characters, when found start looking for & that appear before.

This can be a good use of a regular expression. Of course, this depends on your exact definition, but here's what I took:

• A key cannot contain & or = - otherwise you have ambiguity.
• Values cannot contain = - we may be able to work around that one, but it's much simpler this way.
• A value will be matched until the end of the string, or until there's one more ampersand. If you have multiple ampersands on the end of the value, this should work well: a=12&&&&b=3 -> a=12&&&, b=3

Here's the code:

MatchCollection parameters = Regex.Matches(query, "([^?&=]+)=([^=]+)(?=&|$)");

Now you have a collection of key and values, where on each Match the second Group is the key (index 1), and the third is the value (index 2).
For example, you may convert the matches to a dictionary (if you don't have any duplicated keys):

var values = parameters.Cast<Match>()
                .ToDictionary(m => m.Groups[1].Value,
                              m => HttpUtility.UrlDecode(m.Groups[2].Value),
                              StringComparer.OrdinalIgnoreCase);
string workAread = values["workarea"];

I happened to write a blog post about this recently that also contains a small utility class for manipulating query string params. Here's some example usage cases (taken from my post):

QueryStringHelper qsh = new QueryStringHelper("?page=5&username=dan&year=2010&enabled=true&[email protected]");

string username = qsh.GetByName("username"); // username = "dan"

qsh.Add("category", "products"); // adds a new key called "category" with the value "products"

qsh.AddOrReplace("year", "1999"); // changes the year value from "2010" to "1999"

int year = qsh.GetByName<int>("year"); // year = 1999

qsh.AddOrReplace("page", 6); // changes the value of "page" to "6"

bool enabled = qsh.GetByName<bool>("enabled"); // enabled = true

qsh.RemoveByName("email"); // removes the "email" key

string qs = qsh.GetQueryString(); // qs = "page=6&username=dan&year=1999&enabled=true&category=products";

int count = qsh.Count(); // count = 5

I hope you find it of use.