Develop Reference

Exception handling with lambda/linq statements

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.

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.

...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.

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.

Good practice - throw an exception to satisfy a function return

Built a custom IDataReader which goes looking in XML for values that match particular element names and, if found, returns the values. The GetValue function - which has to return an Object, as specified by the interface, looks like this:
public object GetValue(int i)
{
string SearchString = _columns[i];
var searchedAttributeValue = from nm in _el.Attributes(SearchString) select nm;
if (searchedAttributeValue.Count() > 0)
{
return ParseTypes(searchedAttributeValue.First().Value);
}
var searchedElementValue = from nm in _el.Elements(SearchString) select nm;
if (searchedElementValue.Count() > 0)
{
return ParseTypes(searchedElementValue.First().Value);
}
}
This won't build, obviously, because it's possible to reach the end of the function without a valid return.
In this instance, if that happens, it means there's a config error - the user is looking for an element or attribute in the XML that's not there.
I discovered (this was new to me) that you can get round the problem by doing this:
public object GetValue(int i)
{
if (_el == null)
{
_el = XNode.ReadFrom(_reader) as XElement;
}
string SearchString = _columns[i];
var searchedAttributeValue = from nm in _el.Attributes(SearchString) select nm;
if (searchedAttributeValue.Count() > 0)
{
return ParseTypes(searchedAttributeValue.First().Value);
}
var searchedElementValue = from nm in _el.Elements(SearchString) select nm;
if (searchedElementValue.Count() > 0)
{
return ParseTypes(searchedElementValue.First().Value);
}
throw new Exception("oh crap!");
}
So the function doesn't return, it just throws an error.
However, something feels fundamentally wrong with this approach. Is it okay, why is it okay/not okay and is there a better way to handle the situation?

Well unless you really need the catch block, you don't need to introduce try/catch/finally. You can just add a throw statement at the end of your initial method.
Along the way, I'd start using FirstOrDefault(), follow normal C# naming conventions for local variables, stop using query expressions when they're not terribly useful, and throw a more specific exception:
public object GetValue(int i)
{
string searchString = _columns[i];
var searchedAttribute = _el.Attributes(searchString).FirstOrDefault();
if (searchedAttribute != null)
{
return ParseTypes(searchedAttribute.Value);
}
var searchedElement = _el.Elements(searchString).FirstOrDefault();
if (searchedElement != null)
{
return ParseTypes(searchedElement.Value);
}
// Nothing found - oops.
throw new SomeSpecificException("Some message here");
}
If you need the catch block for logging, I'd probably try to catch a specific exception, which probably means moving that into ParseTypes anyway (as you shouldn't get any exceptions from the rest of the calls...) Either way, keep the throw statement at the end.
EDIT: In terms of design, whether you should throw an exception when the value isn't found or not really depends on the expectations. We have no way of knowing whether that indicates something wrong with the system, or just a perfectly normal absence of data. That should determine your choice here. In particular, if every caller is going to try to catch the exception, then that's a design smell and you should either return null or use something like the Dictionary<,>.TryGetValue approach.

I like to make my methods similar to the .NET framework Parse() and TryParse() methods. In your case I would either do:
public object GetValue(int i)
{
// ...
// fail
throw new Exception("Cannot get value");
}
or do:
public bool TryGetValue(int i, out object result)
{
// ...
// fail
result = null;
return false;
}

The try catch is irrelvant. you could simply refactor your code to:
public object GetValue(int i)
{
if (_el == null)
{
_el = XNode.ReadFrom(_reader) as XElement;
}
string SearchString = _columns[i];
var searchedAttributeValue = from nm in _el.Attributes(SearchString) select nm;
if (searchedAttributeValue.Count() > 0)
{
return ParseTypes(searchedAttributeValue.First().Value);
}
var searchedElementValue = from nm in _el.Elements(SearchString) select nm;
if (searchedElementValue.Count() > 0)
{
return ParseTypes(searchedElementValue.First().Value);
}
throw new Exception("oh crap!");
}
this has the same net result.
That said throwing exceptions are expensive (computationaly). If you want this to bomb out (stop processing completly, this is a major problem and it should simply die) then thrown an exception.
If it's simply a way to identify when the if statements are not being met maybe change it to a TryParse type function:
public bool GetValue(int i, out object returnVal)
{
if (_el == null)
{
_el = XNode.ReadFrom(_reader) as XElement;
}
string SearchString = _columns[i];
var searchedAttributeValue = from nm in _el.Attributes(SearchString) select nm;
if (searchedAttributeValue.Count() > 0)
{
returnVal = ParseTypes(searchedAttributeValue.First().Value);
return true;
}
var searchedElementValue = from nm in _el.Elements(SearchString) select nm;
if (searchedElementValue.Count() > 0)
{
returnVal = ParseTypes(searchedElementValue.First().Value);
return true;
}
return false;
}
then
if (GetValue(i, out value))
//success
else
//it's failed.

If it is genuinely an exceptional circumstance that the search won't return anything because the configuration is wrong, then by all means, throw an exception. The problem with this is continuation. Can client code continue if an item isn't found? If it can, client code will need to closely examine any exception thrown from your code to determine if it was thrown because an item didn't exist, or because something else went wrong that can't be continued from.
The other option is to return something that indicates that the value wasn't found, and allow calling code to deal with it. You could return null to indicate to calling code that no item was found, and that might be fine in this instance. A better solution might be to create a simple Optional<T> class, that contains an indication of whether the object existed (perhaps HasValue), and the object itself if it did. Calling code can easily and more concretely check whether an object was returned and deal with a situation where it isn't, and exceptions don't require additional scrutiny.
EDIT: A better alternative altogether might be the Parse and TryParse paradigm suggested by others, but I think this answer might have some use, so I'll leave it here :)

There are quite a few bad practices in this approach: catch (Exception) should always be avoided. Only catch the exceptions you expect there. Also, throw Exception() is bad style. Always throw something more specific. But for what you're looking for: You don't need a finally after all, just place a throw new ArgumentException() or something as the last line of the function. That is good practice, if it is really a programming error if the code ever runs there.

Check null result before accessing property or calling method

I would like to know if there is a better way or general rule to check null result before accessing property or calling method in C# Linq or lambda expressions. Thanks in advance.
At my understanding, to avoid exception like "Object reference not set to an instance of an object" or "Check null from Lambda result before accessing CSharp property or calling method", I should use "if" statement, or "try/catch" block like:
var product1 = _myProductRepository.FindOne(p => p.Id == -20301);
//1. use "if" statement to let code flow continue
if(product1 != null) // to prevent exception "Object reference not set to an instance of an object."
{
int id1 = sh1.Id; // note: accessing property "Id" is safe here
Console.WriteLine("Product Id: {0}", id1);
}
//2. use "try/catch" block to let code flow continue
var product2 = _myProductRepository.FindOne(123);
var possibleItems = product2.Orders.Where(x => x.Id == -1);
List<Order> myOrderList = null;
try
{
myOrderList = possibleItems.ToList(); // note: ToList() method call is safe here
}
catch
{
myOrderList = new List<Order>();
}

I should use "if" statement, or "try/catch" block like:
In general, I would prefer an explicit null check if the null return is something that is expected as part of the normal operation of your application.
In your case, this is likely true, as finding a product seems like an operation that very well could fail, as a product may not exist.
That being said, if the ID is a known element and really should always be in the DB, then null would not be an expected result. In this case, the FindOne method really shouldn't return a null instance, and that is truly an exceptional case, then exception handling may be preferred.

Sharepoint list item throws null error

I have am iterating through a list of SharePoint list items .
A couple of items do not have data so throw the null exception.
I used
if (!string.IsNullOrEmpty(xt["ows_LinkTitle"].ToString()))
{
Entity.DefectType = xt["ows_LinkTitle"].ToString();
}
but an error still occurs.
I also tried
if(xt["ows_LinkTitle"].ToString()!= null)
{
Entity.DefectType = xt["ows_LinkTitle"].ToString();
}
I could put a try catch block around it but I don't want to do it for each and every line.
Is there any way to check the sharepoint item value without throwing an error.

Try using explicit cast instead:
Entity.DefectType = (string)xt["ows_LinkTitle"];
If you want to check for a null before assigning the value try
if(xt["ows_LinkTitle"] != null)
As .ToString() on a null is the cause of exception.

what to check for when a datareader might not have a particular column?

I am doing a null check but I am getting a index of out range exception when doing:
if(myReader["column1"] != null)
{
}
Under certain conditions that column will not be present, why isn't a null check working?

Check to see if the column exists first
for(int i = 0; i < myReader.FieldCount; i++)
if (myReader.GetName(i) == "column1")
return true;

You want something like:
if (myReader.GetSchemaTable().Columns.Contains("ColumnName"))
{
}

Why is your query changing the columns that it is returning? Referencing the column using the name of the column throws the exception because the column index that corresponds to the name of the column is not found. If I were you I'd capture the exception and handle the error accordingly.

you might try GetSchemaTable()

If the same query returns different columns depending on stuff, doesn't look like a very good design to me. Anyway, you can use myReader.GetName(i) on the column that might be different to check whether you got the right column or not.

Well, the DataReader will expose a GetSchemaTable() which will give you the schema/columns within the DataReader.
Another alternative is to just attempt to grab the value from a column and catch the resulting exception, should that column not exist:
try
{
object value = dataReader["ColumnName"];
// Do something with the value...
}
catch(IndexOutOfRangeException e)
{
// Column not there!
}
However, I wouldn't advise this as throwing/catching exception is expensive business and should be truly reserved for something exceptional happening within your code.
I think the best thing is to use the GetSchemaTable() function but "roll-your-own" DoesColumnExist style function, something like:
public bool DoesColumnExist(IDataReader reader, string columnName)
{
reader.GetSchemaTable().DefaultView.RowFilter = "ColumnName= '" + columnName + "'";
return (reader.GetSchemaTable().DefaultView.Count > 0);
}