Understanding Single, SingleOrDefault, First and FirstOrDefault

LINQ provides element operators which return a single element or a specific element from a collection. The elements operators are Single, SingleOrDefault, First, FirstOrDefault, Last, LastOrDefault.

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.

List<int> data = new List<int> { 10, 20, 30, 40, 50 };

//Try to get element at specified position
Console.WriteLine(data.ElementAt(1)); //result:20 

//Try to get element at specified position if exist, else returns default value
Console.WriteLine(data.ElementAtOrDefault(10)); //result:0, since default value is 0 

Console.WriteLine(data.First()); //result:10 
Console.WriteLine(data.Last()); //result:50

//try to get first element from matching elements collection
Console.WriteLine(data.First(d => d <= 20)); //result:10 

//try to get first element from matching elements collection else returns default value
Console.WriteLine(data.SingleOrDefault(d => d >= 100)); //result:0, since default value is 0 

//Try to get single element 
// data.Single(); //Exception:Sequence contains more than one element 

//Try to get single element if exist otherwise returns default value
// data.SingleOrDefault(); //Exception:Sequence contains more than one element 

//try to get single element 10 if exist
Console.WriteLine(data.Single(d => d == 10)); //result:10 

//try to get single element 100 if exist otherwise returns default value
Console.WriteLine(data.SingleOrDefault(d => d == 100)); //result:0, since default value is 0

First

It returns first specific element from a collection of elements if one or more than one match found for that element. An exception is thrown, if no match is found for that element in the collection.

FirstOrDefault

It returns first specific element from a collection of elements if one or more than one match found for that element. A default value is returned, if no match is found for that element in the collection.

When to use Single, SingleOrDefault, First and FirstOrDefault

You should take care of following points while choosing Single, SingleOrDefault, First and FirstOrDefault:

1. When you want an exception to be thrown if the result set contains many records, use Single or SingleOrDefault.

2. When you want a default value is returned if the result set contains no record, use SingleOrDefault.

3. When you always want one record no matter what the result set contains, use First or FirstOrDefault.

4. When you want a default value if the result set contains no record, use FirstOrDefault.

Perfomance of SingleOrDefault and FirstOrDefault

FirstOrDefault usually perform faster as compared SingleOrDefault, since these iterate the collection until they find the first match. While SingleOrDefault iterate the whole collection to find one single match.

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