cannot implicitly convert type 'bool?' to 'bool'. An explicit conversion exists (are you missing a cast?)

63
votes

Error : cannot implicitly convert type 'bool?' to 'bool'. An explicit conversion exists (are you missing a cast?)

Code :

Test obj = new Test();
obj.IsDisplay = chkDisplay.IsChecked;

but when I use this method to cast the property into a bool then there is no error.

Test obj = new Test();
obj.IsDisplay = (bool) chkDisplay.IsChecked;

I would like to know why I need to cast this bool to bool?

9
c#.net

9 Answers

43
votes

You've declared IsChecked as a bool? (Nullable<bool>). A nullable boolean can be either true, false or null. Now ask yourself: If IsChecked was null, then what value should be assigned to IsDisplay (which can only take a true or false)? The answer is that there is no correct answer. An implicit cast here could only produce hidden trouble, which is why the designers decided to only allow an explicit conversion and not an implicit one.

59
votes

As the others stated bool? is not equal to bool. bool? can also be null, see Nullable<t> (msdn).

If you know what the null state wants to imply, you easily can use the ?? - null-coalescing operator (msdn) to convert your bool? to bool without any side effects (Exception).

Example:

//Let´s say "chkDisplay.IsChecked = null" has the same meaning as "chkDisplay.IsChecked = false" for you
//Let "check" be the value of "chkDisplay.IsChecked", unless "chkDisplay.IsChecked" is null, in which case "check = false"

bool check = chkDisplay.IsChecked ?? false;
28
votes

I'm facing your question when I'm using the null check operator ?.:

if (!RolesList?.Any()) //Not accepted: cannot convert bool? to bool

So I'm using this instead

if (RolesList?.Any() != true)
  //value is null or false

In your case you should set it like so:

obj.IsVisible = chkDisplayStuff.IsChecked ?? false;
14
votes

bool? is not a bool. It is in reality a Nullable<bool> http://msdn.microsoft.com/en-us/library/b3h38hb0(v=vs.110).aspx

If you need the bool value then you should either cast like you are doing or call the .Value property on the bool?. There is also a .HasValue property you can check to make sure that it is not null.

If IsChecked is null, this line will error. 

obj.IsDisplay = (bool) chkDisplay.IsChecked;
5
votes

bool is not equal to bool?.

bool can take two values, true and false.

bool? can take three, true, false, and null.

That is why they are different.

4
votes

You can use below code

obj.IsDisplay = chkDisplay.IsChecked == true?true:false;
3
votes

chkDisplay.IsChecked is of type bool?. Which means it can hold values true, false and null. However, obj.IsDisplay is of type bool. Which means it can only hold true or false.

Hence you have to explicitly cast it to type bool. However, this will still throw an exception if, the value you are trying to cast to bool is null.

bool? nullableBool = null;
bool notNullableBool = (bool)nullableBool; //This will throw InvalidOperationException
2
votes

Try this 

if (asset.IsUp ?? false)
0
votes

cast your nullable value to value type

[HttpPost]
public ActionResult Index(bool? checkOffAge)
{
    if (checkOffAge != null) {
       model.CheckOffAge =(bool)checkOffAge;
    }
}