Lambda expression in 'if' statement condition

Question

I am new to C#, but from my understanding this code should work. Why doesn't it work?

This is an example of my code.

List<Car> cars // This has many cars initialized in it already
if (() => {
   foreach(Car car in cars){
       if (car.door == null) return true;
   }
}){then .......}

Simply put, all I want the code to do is run the if statement if any car does not have a door.

After trying to compile I get this error:

Cannot convert lambda expression to type 'bool' because it is not a delegate type.

You're not even using the correct lambda syntax. It would be () => {..., not ()={... Of course, it wouldn't work if you were using it, but that's another story. — Daniel Mann
– Daniel Mann, Commented Jul 7, 2014 at 17:00

Sergey Berezovskiy · Accepted Answer · 2014-07-07 17:10:12Z

35

If you want to check if any car does not have a door then simply use Enumerable.Any - it determines whether any element of a sequence satisfies a condition:

if (cars.Any(c => c.door == null))
   // then ...

Just for fun: you should execute lambda to get boolean result in if condition (but for this case use Any)

Func<bool> anyCarDoesNotHaveDoor = () => { 
    foreach(var car in cars)
       if (car.door == null)
           return true;
    return false; 
};

if (anyCarDoesNotHaveDoor())
   // then ...

I introduced local variable to make things more clear. But of course you can make this puzzle more complicated

 if (new Func<bool>(() => { 
        foreach(var car in cars)
           if (car.door == null)
               return true;
        return false; })())
    // then ...

edited Jul 7, 2014 at 17:10

answered Jul 7, 2014 at 16:50

Sergey Berezovskiy

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3 Comments

ZNackasha Over a year ago

Just to make sure could you have done this. if (Func<bool>() => {})

BradleyDotNET Over a year ago

@user3813249, You still have to call the delegate. A delegate in and of itself makes no sense to an if statement.

Sergey Berezovskiy Over a year ago

@user3813249 Func<bool> delegate should return boolean value, so you can't just assign empty body to it, like you can do with Action

BradleyDotNET · Accepted Answer · 2014-07-08 16:05:12Z

9

Well, the error says it all. An if statement is expecting a boolean expression which a delegate is not. If you were to call the delegate (assuming it returned a bool), you would be fine. However, if does not know to call it.

The easy way to do this is with the Any LINQ extension method:

if (cars.Any(car => car.door == null))

The Any method knows to actually invoke the lambda expression on each member of the collection, and returns a bool. This makes it a valid boolean expression for the if statement.

edited Jul 8, 2014 at 16:05

answered Jul 7, 2014 at 16:52

BradleyDotNET

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2 Comments

00dani Over a year ago

This seems a misleading explanation. A delegate is an expression, as evidenced by your calling it a lambda expression a moment later; it's just that if demands its condition be a Boolean expression - one of type bool. Lambdas are of type Func<T> for some return type T and hence can't satisfy that requirement. (They can also be expression-tree types, depending on inference from the context of their use, but they can't be bool.)

BradleyDotNET Over a year ago

@00Davo You are correct, that could have been worded better. I'll edit and see if I can clear it up.

Branko Dimitrijevic · Accepted Answer · 2014-07-07 23:24:12Z

2

In case you want to actually do something to cars without doors:

foreach (var car in cars.Where(car => car.door == null)) {
    car.door = <whatever>;
}

answered Jul 7, 2014 at 23:24

Branko Dimitrijevic

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