[LINQ Operators] Single

The “Single” operator returns the single element of a sequence, or throws an exception if no element is found. You can provide the predicate delegate instead of using the “Where” operator.

 

1. Sample Data Objects

To test examples, you need to include the <data classes> in your project.

 

2. “Single” operator

  • NonDeferred operator
  • Purpose: Element

 

3. Prototypes

public static T Single<T>(
  this IEnumerable<T> source)

public static T Single<T>(
  this IEnumerable<T> source,
  Func<T, bool> predicate)

The “Single()” operator is a weird but really handy one. It returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence. The “SingleOrDefault()” returns a default value when the sequence is empty but still throws an exception if there are more than one elements in a sequence.

The “Single()” operator is used when you expect only one element as a result, and is especially useful with DB queries.

 

4. Example 1 (Standard Operator)

IEnumerable<Employee> emps = Employee.GetEmployees();

Employee emp =
  emps.Where(e => e.Name.Contains("King")).Single();
Console.WriteLine($"Matching Employee: {emp.Name}"); // John King

try
{
  Employee emp1 =
    emps.Where(e => e.Name.Contains("Smith")).Single(); // 2
}
catch
{
  Console.WriteLine("Not a single employee");
}

 

5. Example 2 (Query Expression)

There’s no corresponding query expression for the “Single()” operator.

 

6. Example 3 (Standard Operator) – with a condition

IEnumerable<Employee> emps = Employee.GetEmployees();

Employee emp =
  emps.Single(e => e.Name.Contains("King")); // John King

Console.WriteLine($"Matching Employee: {emp.Name}");

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