How to mock func delegate in .net

I have a Func<> property in a controller, and I am wondering how can I mock it.

Here is my code:

public Func<HttpRequest, string> GetId { get; set; }

Here is the usage in the controller's constructor:

GetId = req => req.HttpContext.Items["random-id"].ToString();

and here is the usage in individual methods and API calls:

var id = GetId (Request);

The above Func<> property is used in all the API calls in the controller and its constructor. I am trying to mock it so I don't have to deal with that while writing unit tests. Any suggestions?