What is the fastest way to join Dictionary<string,string> into querystring?

Question

I have following data:

Dictionary<string,string> dctParameters = new Dictionary(){
 {"a",var1},{"b",var2},{"c",var3},....
}

I want to join the "dctParameters" into a querystring.

What's the fastest / best among the following ways? Can you think you of a better way to do this?

1st method:

StringBuilder data = new StringBuilder();
string result = dctParameters.Aggregate(data, (x, pair) => data.Append(pair.Key).Append("=").Append(pair.Value).Append("&")).ToString();

2nd method:

StringBuilder data = new StringBuilder();
foreach (var item in dctParameters)
{
   data.Append(string.Format("{0}={1}&",item.Key, item.Value));
}
string result = data.ToString();

3rd method:

StringBuilder data = new StringBuilder();
foreach (var item in dctParameters)
{
     data.Append(item.Key).Append("=").Append(item.Value).Append("&");
}
string result = data.ToString();

Answer 1

How about an extension method as a variation of Eric's suggestion:

public static string ToQueryString(this Dictionary<string, string> dict)
{
    return '?' + string.Join("&", dict.Select(p => p.Key + '=' + p.Value).ToArray());
}

eg:

dctParameters.ToQueryString();

The performance cost of any of your implementations is really not worth losing sleep over considering that this kind of process is not likely to bog down a server. The issue is (as you say) better not faster and what you seem to be after is an elegant approach rather than the least CPU cycles.