使用lambda / linq语句进行异常处理

Question

I have the following statement that I am trying to use a try-catch block with

var val= db.shops.where(x=>×.id==ID).Select (a=>a.address).First();

I tried following but has many issues won't even compile.

var val=db.shops.Where(x=> 
  {
    try 
    {
        (x.Id==ID).Select(a=>a.address).First();
    }
    catch(ex)
    {
      return ex;
    }
  }

Please let me know how can I handle the exception in this statement Thanks. Note: writing this question from mobile phone can't format code. Apologize for it.

Answer 1

Everything inside brackets ( { } ) need to have 'regular' block syntax, so return is required here:

...
.Where
(x=> 
  {
    try 
    {
        return (x.Id == ID);
    }
    catch(ex)
    {
      throw;
    }
  }
)
.Select(a=>a.address)
.First(); // Or FirstOrDefault if you expect this statement to yield no result.

As you see, the Where is more like a regular statement now. The Select is moved to outside the Where . If you need exception handling in there, you have to do the same as in the Where block now. Last, return ex is probably meant to be throw ex , which should be throw in this case to preserve call stack.

Answer 2

...that I am trying to use try catch block with

I am guessing you probably meant to write FirstOrDefault based on where you want the try/catch . In the sample code you have provided I see no reason to try to make a catch block into one of the lambda statements. The best thing to do would be to simply use FirstOrDefault as that is the reason why you might get an exception in the code shown.

var address = db.shops.FirstOrDefault(x => ×.id == ID)?.Address;
if(address == null)
  // nothing was found or address is null, do something else

Or similar code closer to what you had without my "optimization"

var shop = db.shops.FirstOrDefault(x => ×.id == ID);
if(shop == null) {
  // nothing was found, do something else
}
var address = shop.address;

The other reason not to use try/catch is that it can't be translated into SQL and the variable name db which is the container for the shops collection which leads me to believe you are using EF or some other ORM. So even if you were to fix the syntax and your code with added try/catch block compiles you will get a run time error later when you execute the lambda.

Answer 3

You should place the try around your original statement:

try
{
   var val= db.shops.where(x=>×.id==ID).Select (a=>a.address).First();
}
catch (Exception ex)
{
   return ex; //I assume the return type here is wrong.  Maybe return null?
}

However, there's nothing about that line that should require a try/catch . Try this:

var val= db.shops.where(x=>×.id==ID).Select(a=>a.address).FirstOrDefault();

The FirstOrDefault will return null if there are no results.