LINQ: When to use SingleOrDefault vs. FirstOrDefault() with filtering criteria

Question

Consider the IEnumerable extension methods SingleOrDefault() and FirstOrDefault()

MSDN documents that SingleOrDefault:

Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.

whereas FirstOrDefault from MSDN (presumably when using an OrderBy() or OrderByDescending() or none at all),

Returns the first element of a sequence

Consider a handful of example queries, it's not always clear when to use these two methods:

var someCust = db.Customers .SingleOrDefault(c=>c.ID == 5); //unlikely(?) to be more than one, but technically COULD BE  var bobbyCust = db.Customers .FirstOrDefault(c=>c.FirstName == "Bobby"); //clearly could be one or many, so use First?  var latestCust = db.Customers .OrderByDescending(x=> x.CreatedOn) .FirstOrDefault();//Single or First, or does it matter? 

Question

What conventions do you follow or suggest when deciding to use SingleOrDefault() and FirstOrDefault() in your LINQ queries?

Answer 1

If your result set returns 0 records:

• SingleOrDefault returns the default value for the type (e.g. default for int is 0)
• FirstOrDefault returns the default value for the type

If you result set returns 1 record:

• SingleOrDefault returns that record
• FirstOrDefault returns that record

If your result set returns many records:

• SingleOrDefault throws an exception
• FirstOrDefault returns the first record

Conclusion:

If you want an exception to be thrown if the result set contains many records, use SingleOrDefault.

If you always want 1 record no matter what the result set contains, use FirstOrDefault