asp.net webapi: how to pass optional parameters?

You can make a parameter optional by using a nullable type:

public class OptionalParamsController : ApiController
{
    // GET /api/optionalparams?id=5&optionalDateTime=2012-05-31
    public string Get(int id, DateTime? optionalDateTime)
    {
        return optionalDateTime.HasValue ? optionalDateTime.Value.ToLongDateString() : "No dateTime provided";
    }
}

In addition to the previous answer provided by Ian, which is correct, you can also provide default values which I feel is a cleaner option which avoids having to check whether something was passed or not. Just another option.

public class OptionalParamsController : ApiController
{
    // GET /api/optionalparams?id=5&optionalDateTime=2012-05-31
    public string Get(int id, DateTime optionalDateTime = DateTime.UtcNow.Date)
    {...}
}