Cannot implicitly convert type 'long' to “

问题:

It a very simple question - convert a type long variable to a Nullable int type (int?). (see example below- not the actual code)

int? y = 100;

long x = long.MaxValue;

y = x;

I am getting compile time error.

"Cannot implicitly convert type 'long' to 'int?'. An explicit conversion exists (are you missing a cast?).

I do have the fix ( see below the solution section), My curiosity in posting the question is of the 3 solutions which one is recommended?

Solution

  y = Convert.ToInt32(x);
  y = (int)x;
  y = unchecked((int) x);  

Thanks in advance for your suggestions

回答1:

The reason you require an explicit conversion is that not all values of long can be represented as an int. The compiler is more or less saying 'you can do this, but you have to tell me you know what you're doing'.

Per the docs, Convert.ToInt32 will check for and throw an OverflowException if the long cannot be represented as an int. You can see the implementation in the reference source - it's just this check and then a cast.

The second two options are (usually) the same and allow the cast despite an overflow as unchecked is the compiler default. If you change the compiler default to checked using the /checked switch then you'll get an OverflowException if not in an unchecked block.

As to which is 'best', it depends what your requirement is.

回答2:

int is 32-bit integral while long is 64-bit integral.

long can store/represent all int values but all long values cannot be represented by int so long is bigger type than int that's why int can be implicitly converted to long by compiler but not vice versa.

So that's the reason we need to explicitly cast/convert long to int which may result in information loss if that long is not representable asint