Type Cast Operator Precedence

Question

I ran into a scenario today while implementing search functionality in my application that has left me puzzled. Check out this snippet:

public string GetFirstProductName(SortedList<string, object> itemsList) {
    for (int i = 0; i < itemsList.Values.Count; i++) {
        if (itemsList.Values[i] is Product)
           // Doesn't Compile:
           // return (Product)itemsList.Values[i].ProductName;

           // Does compile. Precedence for the "." higher than the cast?
           return ((Product)itemsList.Values[i]).ProductName;
        }
    }
}

So, what is the precedence for the cast? Is a cast an operator? What about the as keyword - is that an operator and what is its precedence?

There's an order of precedence table at https://msdn.microsoft.com/en-us/library/2bxt6kc4.aspx — James Moore, Feb 13 '17 at 19:18

score 14 · Accepted Answer · answered Feb 27 '14 at 21:56

14

Its quite simple really.

When you don't wrap the cast in brackets.. you're casting the entire expression:

return (Product)itemsList.Values[i].ProductName;
//              |______________________________|

You're essentially casting a string to a Product. Whereas:

return ((Product)itemsList.Values[i]).ProductName;
//     |____________________________|

Casts just that part, allowing the . to access the properties of a Product. Hopefully the bars help show you the difference more clearly.

answered Feb 27 '14 at 21:56

Simon Whitehead

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That's a wonderful explanation your made there. +1 for the visual implementation. – hsim Feb 27 '14 at 21:59
2

"When you don't wrap the cast in brackets.. you're casting the entire expression" is true *for this specific example*, because the "." is a Primary operator and has the highest level of precedence while the (T)x operator is Unary and is second-highest. It is not necessarily true for all expressions! – Christopher Jun 22 '17 at 16:47

score 4 · Answer 2 · answered Feb 27 '14 at 21:59

x.y has a higher precedence than the cast:

7.3.1 Operator precedence and associativity

The following table summarizes all operators in order of precedence from highest to lowest:

Primary x.y f(x) a[x] x++ x-- new typeof default checked unchecked delegate

Unary + - ! ~ ++x --x (T)x

score 1 · Answer 3 · edited May 23 '17 at 11:53

It is not about precedence. Always value is converted

// Does compile. Precedence for the "." higher than the cast?
 return ((Product)itemsList.Values[i]).ProductName;

in your case value is being returned by itemsList.Values[i] which is being casted into Product. Then you are trying to access ProductName from it.

CAST is an Operator

Is/as works only on reference types

 return (itemsList.Values[i] as Product).ProductName;

READ MORE to understand difference between CAST and AS

score 1 · Answer 4 · answered Feb 27 '14 at 21:57

1

(Product)itemsList.Values[i].ProductName; means (Product)(itemsList.Values[i].ProductName); whereas second line you explicitly say to cast Values[i] and then do .ProductName;

answered Feb 27 '14 at 21:57

LB2

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Type Cast Operator Precedence

4 Answers4