Pass IEnumerable KeyValuePair into String function

I am constructing a REST Client that needs to authenticate a string on every post as a signature. Within that signature I have to Canonicalize the Headers and string variables that are being sent, sort them, then out put them in a format to then do a SHA256 hash on the value.

Ok, I have the String function that Canolicalizes the headers and does the sort with a bit of Linq, I also have the SHA covered.
In my String function shown below, I am expecting to pass into it an IEnumberable key value pair, as defined by an inheritance. So my question is, how can I pass an IEnumberable collection into the String Function when calling it from another class?

The reason I have to have it as a function of its own, because as it is REST, depending on the endpoint and resource I am calling for, I can have different values in the collection. Second snippet of code is an example of that collection, which yes I can create the collection - but do not know if I am constructing it or passing it into my CanonicalizedHeaders() string function correctly.

public interface CanoHeaders
        {
            // Structure for CanonicalizedHeaders List
            string pname { get; set; }
            string pvalue { get; set; }
        }


public string CanonicalizedHeaders(IEnumerable<CanoHeaders> CanonicalHeader)

            CanonicalHeader.OrderBy(x => x.pname);
            var listHeaders = from CanoHeaders in CanonicalHeader
                              select new { param = CanoHeaders.pname, value = CanoHeaders.pvalue };

           string returnheaders = String.Empty;

            foreach (var row in listHeaders)
            {
                returnheaders += row.param.ToString() + ":" + row.value.ToString() + "\n";
            }

            return returnheaders.ToLower().ToString();
        {

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This is the piece that is likely incorrect at the string cano_str (being called by a seperate class)

ICollection<KeyValuePair<String, String>> vmCollec = new Dictionary<String, String>();
                vmCollec.Add(new KeyValuePair<String, String>("offset", str_offset));
                vmCollec.Add(new KeyValuePair<String, String>("limit", str_limit));

            string cano_str = CanonicalizedHeaders(vmCollec.AsEnumerable);

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