FirstOrDefault and SingleOrDefault in ASP.NET

This is a small, simple and very basic Tip. But for beginners like me can still make silly mistake while using these two gems during LINQ to SQL. The exceptions generated by these really frustrates beginners if they are not concerned about the results they expect from their queries.

Here is the basic difference that might help us during keying our queries.

SingleOrDefault	FirstOrDefault
As the name suggests, returns a SINGLE the FIRST element from the list. or specific element. Else a default value if element the specified element is not found.	This returns or default value if no such specified found.
Here, Exception is thrown when more than element one element is present source is NULL.	Here, when there is no in the result set and or the we expect a single element.
It should be used when 0/1 elements are than one element is expected from our queries. first element.	It should be used when more expected and we want only the

Examples are the best way to express. Lets take one.

Suppose we have a Books table for querying, with BookId as PrimaryKey || BookName as Required and AuthorName as Nullable.

BookId	BookName	AuthorName
1	Asp.Net MV	Adam Freeman
2	C# 2008	Andrew Troelsen
3	SqlServer 2012	Ross Mistry
4	HTML5/CSS3	Andrew Troelsen
5	KnockOut	NULL

SingleOrDefault():

" Books.SingleOrDefault(b=> b.BookId== 2) "----This block of Code returns the C# 2008 as the BookId is unique and single.

"Books.SingleOrDefault(b=> b.AuthorName== "Andrew Troesen")" ----This block of Code returns "InvalidOperationException: Sequence contains more than one element".


FirstOrDefault():

"Books.FirstOrDefault(b=> b.AuthorName== "Andrew Troesen")"----Though there are two elements with author Andrew Troesen,it returns the oldest one or the BookName:-C# 2008.

So, to avoid exception and handling them, It's better to know how we want our queries to behave and what results we expect from our queries.

This small thing has helped me a lot. Hope this helps beginners.

Happy coding.