LINQ: check for null return from function

One of of my colleagues wrote a LINQ query similar to the following:
var q=from i in keys
select GetTuplesOrPossibleNull(i)
into kvp
orderby kvp.Item2
select kvp;
I’ve noticed, that for some data it throws null exception on kvp.Item2 in orderby statement. It was found that calling function sometimes return null instead of expected key-value pair.

I thought where I should insert check for null and my first idea was to add check in where statement before select function call, something like the following

var q=from i in keys
where GetTuplesOrPossibleNull(i)!=null
select GetTuplesOrPossibleNull(i)
into kvp
orderby kvp.Item2
select kvp;

but it looks ugly, against DRY(don’t repeat yourself) principle and could be double slow, because the same function is called twice.
Fortunately check can be done just before orderby statement
var q=from i in keys
select GetTuplesOrPossibleNull(i)
into kvp
where kvp!=null
orderby kvp.Item2
select kvp;

It looks much nicer and almost twice quicker. I’ve done some benchmark runs using LinqPad.

void Main()
{
TimeSpan ts=Benchmark(TestCheckingForNullInWhere );
ts.Dump();//00:00:00.2343750
ts=Benchmark(TestCheckingForNullFunctionResult);
ts.Dump();//00:00:00.4062500

}

void TestCheckingForNullFunctionResult()
{
int count=20;
//IEnumerable<Tuple<string, string>> tuples =NotCheckingForNull(count);
IEnumerable<Tuple<string, string>> tuples =CheckingForNullFunctionResult(count);
tuples.Dump();

}
void TestCheckingForNullInWhere()
{
int count=20;
//IEnumerable<Tuple<string, string>> tuples =NotCheckingForNull(count);
IEnumerable<Tuple<string, string>> tuples =CheckingForNullInWhere(count);
tuples.Dump();

}

public delegate void TestProcedure();
//from http://stackoverflow.com/questions/626679/datatable-select-vs-datatable-rows-find-vs-foreach-vs-findpredicatet-lambda
public TimeSpan Benchmark(TestProcedure tp)
{
int testBatchSize = 5;
List<TimeSpan> results = new List<TimeSpan>();
for(int i = 0; i<testBatchSize; i++)
{
DateTime start = DateTime.Now;
tp();
results.Add(DateTime.Now - start);
}
return results.Min();
}
// Define other methods and classes here
IEnumerable<Tuple<string, string>> NotCheckingForNull(int count)
{

List<int> keys=FillListOfInts( count);
var q=from i in keys
select GetTuplesOrPossibleNull(i)
into kvp
orderby kvp.Item2
select kvp;
return q;
}
IEnumerable<Tuple<string, string>> CheckingForNullFunctionResult(int count)
{

List<int> keys=FillListOfInts( count);
var q=from i in keys
where GetTuplesOrPossibleNull(i)!=null
select GetTuplesOrPossibleNull(i)
into kvp
orderby kvp.Item2
select kvp;
return q;
}

IEnumerable<Tuple<string, string>> CheckingForNullInWhere(int count)
{

List<int> keys=FillListOfInts( count);
var q=from i in keys
select GetTuplesOrPossibleNull(i)
into kvp
where kvp!=null
orderby kvp.Item2
select kvp;
return q;
}

List<int> FillListOfInts(int count)
{
var keys=new List<int>();
for(int i=0;i<count;i++)
{
keys.Add(i);
}
return keys;
}

Tuple<string, string> GetTuplesOrPossibleNull(int i)
{
int delay =10;
Thread.Sleep(delay);
if (i%4==0)
return null;
else
return new Tuple<string, string>(i.ToString(),delay.ToString() +"mc delayed "+i.ToString());
}

Posted on Sunday, June 12, 2011 7:29 AM LINQ | Back to top