Question

Possible Duplicate:
Casting vs using the ‘as’ keyword in the CLR

Which method is regarded as best practice?

Cast first?

public string Describe(ICola cola)
{
    var coke = cola as CocaCola;
    if (coke != null)
    {
        string result;
        // some unique coca-cola only code here.
        return result;
    }
    var pepsi = cola as Pepsi;
    if (pepsi != null)
    {
        string result;
        // some unique pepsi only code here.
        return result;
    }
}

Or should I check first, cast later?

public string Describe(ICola cola)
{
    if (cola is CocaCola)
    {
        var coke = (CocaCola) cola;
        string result;
        // some unique coca-cola only code here.
        return result;
    }
    if (cola is Pepsi)
    {
        var pepsi = (Pepsi) cola;
        string result;
        // some unique pepsi only code here.
        return result;
    }
}

Can you see any other way to do this?

Answer 1

If the object may or may not be of the type you want then the as operator (your first method) is better in two ways:

• Readability and ease of maintenance: you are only specifying the type once
• Performance: you are only performing the cast once, instead of twice. (Trivia: when you use the is keyword, the C# compiler internally translates it to as, ie. coke is Cola is equivalent to (coke as Cola) != null)

If the object should always be of the requested type then just do (Coke)cola and let it throw an exception if that’s not the case.