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C # Linq Query group if result is null

If I have a class like this

public class Test
{
   public string Id {get; set;}
   public string Name {get; set;}
   public int Value {get; set;}
}

and then:

List<Test> values = new List<Test>(); // this contains let say 10 items

and I like this:

var grouped = values.Where(x=>x.Value > 10).GroupBy(x=>x.Name);

My question is how can I check if there is grouped == null

? Or how can I check if there are any groups that meet these criteria?

I ask because if I do this:

if (grouped == null) // this is false although the linq yielded no results
{

}
+3


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5 answers


You can use the method Any()

:

var anyItems = grouped.Any();


You don't need to check for null because the grouping will return an empty collection instead of null

+6


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You can check if there are any groups like below:

var anyGroups = grouped.Any();

If there is at least one group, the extension method called Any

will return true

. Otherwise, it will return false

.

According to MSDN , this is the method signature GroupBy

:



public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector)

From the above, it is clear that this method returns a sequence of elements that implement the interface IGrouping

. ( IGrouping

Represents a collection of objects that share a common key.) An easy way to find out if a sequence contains elements is using an enumerated extension method called Any

.

In addition, it is an operation O(1)

that uses a method Any

and does not pass any predicate in it. When using an enumerated extension method called Count

, in some cases this is an operation O(n)

.

+5


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GroupBy

never returns null

. If there are no records in the result, you get an empty object IEnumerable<IGrouping<TKey,TSource>>

.

GroupBy

Not necessary in your case : you can replace it with

var anyGreaterThanTen = values.Any(x=>x.Value > 10);
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The usage Any()

is good enough, but not the most optimal solution.

Obviously, you need to iterate through the result after you find there are some results, but calling Any()

before iterating over the result calls the query twice. It doesn't really matter for a simple example like you are here, but if it is a database query (LINQ to SQL or Entity Framework), then you had two calls to the database. Once to check if there are any results and next to the results.

To avoid this, you can write:

bool hasResult = false;
// iterating over the result, and performing your task
foreach(var group in grouped)
{
    hasResult = true;

    // process data
    // ...
}

if(!hasResult)
{
    // ...
}
0


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I have the same problem when checking if the result is null. So I debugged this. I found that when the result has no group, it sometimes has no item or 1 item, but that item is null. Therefore, to test where the result is non-zero, we have to combine 2 conditions as shown below:

if(grouped.Count() > 0 && grouped.ElementAt(0) != null)
    {
       //TODO:....enter code here
    }
0


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