gistfile1.cs

var querystrings = new System.Collections.Generic.Dictionary<string, string>();
querystrings.Add("where", "ContactNumber = \"123\"");
querystrings.Add("order", "Name DESC");

IConsumerRequest filteredGetContactRequest = consumerSession
    .Request()
    .ForMethod("GET")
    .WithQueryParameters((IDictionary)querystrings)
    .ForUri(new Uri("https://api.xero.com/api.xro/2.0/Contacts"))
    .SignWithToken(accessToken);