QueryParameters.cs

Usage:
var queryParams = new QueryParameters();
queryParams.Add("public_id", messageId);
queryParams.Add("username", this.operatorConfig.Username);
queryParams.Add("checksum", this.GetChecksum(messageId));
var builder = new UriBuilder(this.serviceUri)
{
    Query = queryParams.ToString()
};

public class QueryParameters
{
    private readonly Dictionary<string, string> @params;

    public QueryParameters()
    {
        this.@params = new Dictionary<string, string>();
    }

    public QueryParameters(string queryString)
    {
        var queryStringCollection = HttpUtility.ParseQueryString(queryString);
        this.@params = queryStringCollection.Keys
            .Cast<string>()
            .ToDictionary(k => k, v => queryStringCollection[v]);
    }

    public void Add(string key, string val)
    {
        if (this.@params.ContainsKey(key))
        {
            throw new InvalidOperationException(string.Format("The key {0} already exists.", key));
        }

        this.@params.Add(key, val);
    }

    public void Add<T>(string key, T val)
    {
        this.Add(key, val.ToString());
    }

    public string this[string key]
    {
        get
        {
            if (this.@params.ContainsKey(key))
            {
                return this.@params[key];
            }
            else
            {
                return null;
            }
        }

        set
        {
            this.Add(key, value);
        }
    }

    public override string ToString()
    {
        var sb = new StringBuilder();
        foreach (var kvp in this.@params)
        {
            if (sb.Length > 0)
            {
                sb.Append("&");
            }

            sb.AppendFormat(
                "{0}={1}",
                HttpUtility.UrlEncode(kvp.Key),
                HttpUtility.UrlEncode(kvp.Value));
        }

        return sb.ToString();
    }

    public bool Contains(string key)
    {
        return this.@params.ContainsKey(key);
    }
}