What is the logical difference between using Single and SingleOrDefault?

Question

I am not sure where shall I use Single and where shall I use SingleOrDefault. I understood the definition for both.

Single:

1. It will always return one row.

2. If no row found I will get an exception.

3. If multiple rows found I will get an exception.

SingleOrDefault:

1. It will always return one row, if not found then default value is returned (What is the meaning of this DEFAULT VALUE").

2. If multiple rows then exception.

Question 1: What is the meaning of "It will return default value"

Question 2: When to use Single and when to use SingleOrDefault.

I have a delete function where I have below code:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Delete(int resumeId)
{
    var r = _context
        .Resumes
        .Where(c => c.ResumeId == resumeId).SingleOrDefault();
    _context.Resumes.Remove(r);
    _context.SaveChanges();

    return RedirectToAction("ResumeCenter");
}

I just blindly put SingleOrDefault everywhere (where I ATMOST expect one value), Please tell me a real world scenario when I should use "Single" and where I should use "SingleOrDefault".

Answer 1

As you said - Single() will raise an exception if no row found. SingleOrDefault() will instead return the default value (null for reference types, 0 for ints, \0 for chars etc`).

Use Single() in one of two cases:

1. When not getting a row is something that should not happen, and if it does, it means something is wrong.
2. When you are dealing with value types (ints, chars, etc'), and the default value might be a valid value in your values system (for instance, if you are getting 0 for an int but can't tell if it's because you got no row back or it's a valid value.

Use SingleOrDefault() for any other case. If the row is not found, you'll need to compare the value you get to it's default value.

For further reading, here is a link to Microsoft page on the default value expression.

Answer 2

It's almost (but not quite) a question of style. Which do you prefer?

Single:

try
{
    var x = someEnumerable.Single();
}
catch (InvalidOperationException)
{
    //Did not contain exactly one record
}

SingleOrDefault:

var y = someEnumerable.SingleOrDefault();
if (y == null)
{
    //Did not contain exactly one record
}

The exception-based approach is a little more coding, but it is nice because exceptions can be allowed to bubble up. That being said, I usually use the XXXOrDefault variants to avoid stack unwind, which can be poor for performance. As a rule you should avoid throwing exceptions when behaviors are expected.

P.S. The "default" for any type can be obtained by using the keyword default(type). For integers it is zero, for dates it is MinValue, for reference types it is null, etc.

P.P.S. In your example I would use Single and allow an exception to be thrown if the row is missing or there is more than one. Otherwise you may end up passing null to the subsequent Remove method. While that would fail too, it'd fail a line later than it ought to, resulting in a deceptive stack trace and making troubleshooting a little trickier.

OTOH, with small changes, you could make either one work:

public ActionResult Delete(int resumeId)
{
    var r = _context
        .Resumes
        .Where(c => c.ResumeId == resumeId).SingleOrDefault();
    if (r == null)
    {
        return RedirectToAction("ErrorPage");
    }
    else
    {
        _context.Resumes.Remove(r);
        _context.SaveChanges();
        return RedirectToAction("ResumeCenter");
    }
}

Or

public ActionResult Delete(int resumeId)
{
    var r = _context
        .Resumes
        .Where(c => c.ResumeId == resumeId).Single();  //Error will bounce up to Application_Error or other global handler
    _context.Resumes.Remove(r);
    _context.SaveChanges();

    return RedirectToAction("ResumeCenter");
}

Answer 3

When you know that there has to be a value for some thing in the database, use Single() because you would want to get an exception if the "very expected value" in the database does not exist. Data like EmployeeName for a particular employee code.

Use SingleOrDefault() when you know employee is a temporary employee and his department may not show up in the main database. But if value for department exists, it should be only one value. He will not be working for multiple departments at once.

http://www.c-sharpcorner.com/blogs/singleordefault-vs-firstordefault-in-linq-query1

Single() - Atleast one value is expected in data store, and you would want it. You want to know if there is no value or multiple values. In that case, you get an exception and check.

SingleOrDefault() - Give one record with the default value if it does not exist. It does not matter whether on not anything is available for the record. But it should not be multiple records.

Answer 4

as the function name suggest you when you say

list.single();

then that means

"I want the single record from the list" and hence you get the single record (the 1st one)"

and when yo say

list.SingleOrDefault();

then that means

"I wan the single record and if the record dosen't exist the i would like to have the default value of the object"

its your approch completely you can also go for

FirstOrDefault(); 

just like single or default

Answer 5

Single : It returns a single specific element from a collection of elements if element match found. An exception is thrown, if none or more than one match found for that element in the collection.

SingleOrDefault: It returns a single specific element from a collection of elements if element match found. An exception is thrown, if more than one match found for that element in the collection. A default value is returned, if no match is found for that element in the collection.