What is the proper way to check for null values?

Question

I love the null-coalescing operator because it makes it easy to assign a default value for nullable types.

 int y = x ?? -1;

That\'s great,

Answer 1

If you're frequently doing this specifically with ToString() then you could write an extension method:

public static string NullPreservingToString(this object input)
{
    return input == null ? null : input.ToString();
}

...

string y = Session["key"].NullPreservingToString() ?? "none";

Or a method taking a default, of course:

public static string ToStringOrDefault(this object input, string defaultValue)
{
    return input == null ? defaultValue : input.ToString();
}

...

string y = Session["key"].ToStringOrDefault("none");

Answer 2

My preference, for a one off, would be to use a safe cast to string in case the object stored with the key isn't one. Using ToString() may not have the results you want.

var y = Session["key"] as string ?? "none";

As @Jon Skeet says, if you find yourself doing this a lot an extension method or, better, yet maybe an extension method in conjunction with a strongly typed SessionWrapper class. Even without the extension method, the strongly typed wrapper might be a good idea.

public class SessionWrapper
{
    private HttpSessionBase Session { get; set; }

    public SessionWrapper( HttpSessionBase session )
    {
        Session = session;
    }

    public SessionWrapper() : this( HttpContext.Current.Session ) { }

    public string Key
    {
         get { return Session["key"] as string ?? "none";
    }

    public int MaxAllowed
    {
         get { return Session["maxAllowed"] as int? ?? 10 }
    }
}

Used as

 var session = new SessionWrapper(Session);

 string key = session.Key;
 int maxAllowed = session.maxAllowed;

Answer 3

What about

string y = (Session["key"] ?? "none").ToString();

Answer 4

You could also use as, which yields null if the conversion fails:

Session["key"] as string ?? "none"

This would return "none" even if someone stuffed an int in Session["key"].

Answer 5

If it will always be a string, you can cast:

string y = (string)Session["key"] ?? "none";

This has the advantage of complaining instead of hiding the mistake if someone stuffs an int or something in Session["key"]. ;)

Answer 6

create an auxiliary function

public static String GetValue( string key, string default )
{
    if ( Session[ key ] == null ) { return default; }
    return Session[ key ].toString();
}


string y = GetValue( 'key', 'none' );