Taking a look at the DynamicDictionary - NancyFx/Nancy GitHub Wiki

The DynamicDictionary is a dynamic type with dictionary-like behaviors, but there is a lot more to it under the covers and it's a big part of what helps Nancy provide a terse syntax in many places. The most obvious place is in the lambda expression that forms the route action. Any captured values from the request will be put into a DynamicDictionary and passed into the action for the route.

With the DynamicDictionary you are able to access the values using a standard indexer approach or by using the name of the captured value just as a normal property. Both produce the same result; it all comes down to personal preference.

Get["/hello/{name}"] = parameters => {
    return "Hello " + parameters.name;
};

Get["/goodbye/{name}"] = parameters => {
    return "Goodbye " + parameters["name"];
};

It is worth noting that if you prefer accessing captured properties via a normal property you must assign the capture group a valid C# identifier (i.e.: one that starts with an alphabetic character). There's no restriction on the name of the DynamicDictionary instance specified in your handler signature, but Nancy borrows a convention from functional programming whereby the underscore (_) indicates the value is not used:

// Underscore indicates that our method doesn't use the parameters object. 
Get["/hello"] = _ => {
    return "Hello, World!";
};

// whereas in this method we're using the DynamicDictionary so we give it a proper name.
Get["/hello/{name}"] = parameters => {
    return "Hello, " + parameters.name;
};

The stored values can explicitly or implicitly be cast to most of the primitive types and a special property, called HasValue can be checked to determine if a value has been assigned or not. The values also implement the IEquatable<> and IConvertible interfaces.