How to check a var for null value?

Question

I am using PetaPoco Micro-ORM with C# 4.0.

The code below retrieves a single row from the database:

var result = db.SingleOrDefault<TdUsers>(getUserQuery);

I would like to check whether or not the result contains any rows, and whether is null. What is the best way to do this?

score 33 · Accepted Answer · edited Sep 06 '18 at 05:42

33

if (result == null || result.Count() == 0) {
    // Checks whether the entire result is null OR
    // contains no resulting records.
}

I think the problem is not in your check for null, because linq is lazy loading. Your error is in using the expression db.SingleOrDefault<TdUsers>(getUserQuery);.

.Single<T>(expression) does not return null - it errors if the result returns no values. .SingleOrDefault<T>(expression), however, returns a null value if the expression results in no values - and therefore is best combined with an if (result == null) type check, as you're using here.

edited Sep 06 '18 at 05:42

AutomatedChaos

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answered May 25 '12 at 11:34

Parv Sharma

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Tried result != null, but it displays "Object reference not set to an instance of an object". – RKh May 25 '12 at 11:35
4

@RPK - can you post your code in the question then. This should work. – ChrisF May 25 '12 at 11:36
or `!result.Any()` :) – nawfal Jan 16 '14 at 11:04

score 5 · Answer 2 · answered May 25 '12 at 11:48

var result = db.SingleOrDefault<TdUsers>(getUserQuery);

In above code SingleOrDefault will return null vale or the specified generic type(it's known on runtime).

Inorder to check whether the returned values is null or not you can simply use

if(result!=null)
{
//do your code stuff 
}
else
{
//stuff do be done in case where result==null
}

Darren · Answer 3 · 2012-05-25T12:03:48.193

4

You could do:

result.ToList() // Convert result to a list

if (result.Any()) {
   // result is not null
}

edited May 25 '12 at 12:03

answered May 25 '12 at 11:40

Darren

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NO result==null wont be false always.. what do you think is the default value of any reference type? – Parv Sharma May 25 '12 at 11:49
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This approach won't work in every case, because `result` could be null, at which point you will get a `NullReferenceException` thrown when `.ToList()` or `.Any()` are called. – Troy Alford Aug 08 '12 at 22:03

Romil Kumar Jain · Answer 4 · 2020-09-18T14:21:22.367

2

 var v = result.ToList();

now check

if (v is not null)
{

}
else if (v.Count()>0)
{


}

edited Sep 18 '20 at 14:21

answered May 25 '12 at 11:37

Romil Kumar Jain

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See my comment to the answer above. – Troy Alford Aug 08 '12 at 22:03

How to check a var for null value?

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