ToString() of copied NameValueCollection doesn't output desired results

The problem is there are two actual types in your code. The fist one is System.Web.HttpValueCollection which has it's ToString method overriden to get the result you expect and the second one is System.Collection.Specialized.NameValueCollection which does not override ToString. What you can do, if you really need to use System.Collection.Specialized.NameValueCollection is to create an extension method.

 public static string ToQueryString(this NameValueCollection collection)
 {
        var array = (from key in collection.AllKeys
                     from value in collection.GetValues(key)
                     select string.Format("{0}={1}", HttpUtility.UrlEncode(key), HttpUtility.UrlEncode(value))).ToArray();
        return "?" + string.Join("&", array);
    }

and use it:

var newUrl = String.Concat(_rootPath,nameValues.ToQueryString());

It is not NameValueCollection that provides the string formatting. That functionality is in an internal class System.Web.HttpValueCollection that is returned by HttpUtility.ParseQueryString.

So you will not be able to achieve this behavior by using built in functionality. Your best bet would be to create an extension method that formats the values in a URL format.

Here is the method from HttpValueCollection class - you might be able to use it with some modifications.

// System.Web.HttpValueCollection
internal virtual string ToString(bool urlencoded, IDictionary excludeKeys)
{
    int count = this.Count;
    if (count == 0)
    {
        return string.Empty;
    }
    StringBuilder stringBuilder = new StringBuilder();
    bool flag = excludeKeys != null && excludeKeys["__VIEWSTATE"] != null;
    for (int i = 0; i < count; i++)
    {
        string text = this.GetKey(i);
        if ((!flag || text == null || !text.StartsWith("__VIEWSTATE", StringComparison.Ordinal)) && (excludeKeys == null || text == null || excludeKeys[text] == null))
        {
            if (urlencoded)
            {
                text = HttpValueCollection.UrlEncodeForToString(text);
            }
            string value = (text != null) ? (text + "=") : string.Empty;
            string[] values = this.GetValues(i);
            if (stringBuilder.Length > 0)
            {
                stringBuilder.Append('&');
            }
            if (values == null || values.Length == 0)
            {
                stringBuilder.Append(value);
            }
            else
            {
                if (values.Length == 1)
                {
                    stringBuilder.Append(value);
                    string text2 = values[0];
                    if (urlencoded)
                    {
                        text2 = HttpValueCollection.UrlEncodeForToString(text2);
                    }
                    stringBuilder.Append(text2);
                }
                else
                {
                    for (int j = 0; j < values.Length; j++)
                    {
                        if (j > 0)
                        {
                            stringBuilder.Append('&');
                        }
                        stringBuilder.Append(value);
                        string text2 = values[j];
                        if (urlencoded)
                        {
                            text2 = HttpValueCollection.UrlEncodeForToString(text2);
                        }
                        stringBuilder.Append(text2);
                    }
                }
            }
        }
    }
    return stringBuilder.ToString();
}

internal static string UrlEncodeForToString(string input)
{
    return HttpUtility.UrlEncodeUnicode(input);
}