CODEWITHSUNDEEP
c#
TheTalentedDrAl
TheTalentedDrAl

Reputation: 85

C# '(class) var' - what IS this construction

I'm trying to learn some C#, one of the code snippets I found used:

(CheckBox)c

(Where c was the result of foreach)

I get that it's doing this so that the compiler knows that c will behave as a checkbox, my question is: what is this type of construction called? I'd like to be able to google to understand it better.

Thanks!

Upvotes: 3

Views: 138

Answers (6)

Maximc
Maximc

Reputation: 1762

It casts the c variable to type CheckBox. I think in your example it is a object the variable c, and if you then convert it to CheckBox, you get access to all the properties like "IsChecked" "Name" etc

In other words.

before the cast:

bool isItChecked = c.IsChecked; // this would not compile because object does not have a property "IsChecked"

So to get this working we must tell the compiler that the variable c is of type CheckBox like this:

var checkBoxC = (CheckBox)c; // The Cast
bool isItChecked = checkBoxC.IsChecked; // access the casted items properties.

Here is a good beginners tutorial on typecasting, you should check "Explicit casts" because that is what you are doing.

PS: This is needed because c# is strongly typed and not weakly typed.

Upvotes: 2

Nilay Vishwakarma
Nilay Vishwakarma

Reputation: 3363

It is called "Casting" or "UnBoxing"

static private void TestBoxingAndUnboxing()
{
 int i = 123;
 object o = i; // Implicit boxing
 i = 456; // Change the contents of i
 int j = (int)o; // Unboxing (may throw an exception if the types are incompatible)
}

//this function is about 2*Log(N) faster
static private void TestNoBoxingAndUnboxing()
{
 int i = 123;
 i = 456; // Change the contents of i
 int j = i; // Compatible types
}

Understand the difference between two, although they are doing the same thing.

Upvotes: 1

Moeri
Moeri

Reputation: 9294

This is called casting. You do this when you know that the type of 'c' will be Checkbox when the code executes. However, it is possible that c is not a Checkbox, in which case you will get an InvalidCastException.

In truth, C# provides many constructs to avoid casting, like Generics and Generic Type Constraints

As a rule of thumb: only use casting when you really cannot avoid it.

Upvotes: 1

Pellared
Pellared

Reputation: 1292

It is called "Casting". Check out for more information:

Casting and Type Conversions (C# Programming Guide)

Upvotes: 1

Loetn
Loetn

Reputation: 4030

That we call casting. See MSDN. Some tutorials: one and two

You also have something related that is called boxing and unboxing. MSDN

What your example does is letting you know that the object c is actually a checkbox. That way you can access all the properties of a checkbox.

Upvotes: 1

Matthias Meid
Matthias Meid

Reputation: 12513

It's called a cast. As you say, it's used here to tell the compiler that c is in fact a checkbox. There is also a longer article on casting on MSDN. In case of foreach with a non-generic IEnumerable, which I think is what you're doing, there are also two ways to avoid this cast in the loop body:

for (CheckBox c in xs) {
}

for (var c in xs.Cast<CheckBox>()) {
}

Both pieces of code expose c as a CheckBox now, eliminating the need for a cast. I'd probably use the first one, as it's the most idiomatic (as well as the approach familiar from .NET 1.1 and Java).

Upvotes: 4

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