How to safely access actionContext.Request.Headers.GetValues if the key is not found?

class MyFilter : System.Web.Http.Filters.ActionFilterAttribute
{
    public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext)
    {
        IEnumerable<string> values;
        if (actionContext.Request.Headers.TryGetValues("some_key", out values) && values.First() == "hello")
        {

        }
    }
}

Since the release of C# 6.0, here is an alternative syntax to Yuriy Faktorovich's answer where you will get the header value or null as a result in a single line.

actionContext.Request.Headers.SingleOrDefault(x => x.Key == "some_key").Value?.First()