How to access all querystring parameters as a dictionary

You can use the GetQueryNameValuePairs extension method on the HttpRequestMessage to get the parsed query string as a collection of key-value pairs.

public IHttpActionResult Get()
{
    var queryString = this.Request.GetQueryNameValuePairs();
}

And you can create some further extension methods to make it eaiser to work with as described here: WebAPI: Getting Headers, QueryString and Cookie Values

/// <summary>
/// Extends the HttpRequestMessage collection
/// </summary>
public static class HttpRequestMessageExtensions
{
    /// <summary>
    /// Returns a dictionary of QueryStrings that's easier to work with 
    /// than GetQueryNameValuePairs KevValuePairs collection.
    /// 
    /// If you need to pull a few single values use GetQueryString instead.
    /// </summary>
    /// <param name="request"></param>
    /// <returns></returns>
    public static Dictionary<string, string> GetQueryStrings(
        this HttpRequestMessage request)
    {
         return request.GetQueryNameValuePairs()
                       .ToDictionary(kv => kv.Key, kv=> kv.Value, 
                            StringComparer.OrdinalIgnoreCase);
    }
}