LINQ Single

Summary: in this tutorial, you’ll learn how to use the LINQ Single() method to return a single element of a sequence that satisfies a condition.

Introduction to the LINQ Single() method

The LINQ Single() method returns one and only one element of a sequence that satisfies a specified condition.

Here’s the syntax of the Single() method:

public static TSource Single<TSource> (
    IEnumerable<TSource> source, 
    Func<TSource,bool> predicate
);Code language: C# (cs)

In this syntax:

• TSource is the type of elements of the source sequence.
• source is an IEnumerable<T> that contains elements.
• predicate is a function that tests an element for a condition.

The Single() method returns a single element with the type TSource of the source sequence.

If two or more such elements or no element exists, the Single() method throws an exception. If either source or predicate is null, the Single() method throws an ArgumentNullException.

The Single() method has an overload that doesn’t have any argument:

public static TSource Single<TSource> (
    IEnumerable<TSource> source
);Code language: C# (cs)

In this case, the Single() method returns a single element of the source sequence. If the source sequence has more than one element, the Single() method will throw an InvalidOperationException.

LINQ Single() example

The following program demonstrates how to use the Single() method to select a single employee from an employee list:

using static System.Console;

class Employee
{
    public int? Id { get; set; }
    public string? Name { get; set; }
    public int Salary { get; set; }
}

class Program
{
    public static void Main()
    {
        var employees = new List<Employee>()
        {
            new Employee { Id = 1, Name = "John", Salary = 50000 },
            new Employee { Id = 2, Name = "Jane", Salary = 60000 },
            new Employee { Id = 3, Name = "Bob",  Salary = 45000 },
            new Employee { Id = 4, Name = "Sara", Salary = 55000 },
            new Employee { Id = 5, Name = "Tom",  Salary = 65000 }
        };

        var employee = employees.Single(e => e.Id == 1);

        WriteLine($"{employee.Id} - {employee.Name}");

    }
}Code language: C# (cs)

Output:

1 - JohnCode language: C# (cs)

How it works.

First, define the Employee class that has three properties Id, Name, and Salary:

class Employee
{
    public int? Id { get; set; }
    public string? Name { get; set; }
    public int Salary { get; set; }
}Code language: C# (cs)

Second, create a new list of employees:

var employees = new List<Employee>()
{
    new Employee { Id = 1, Name = "John", Salary = 50000 },
    new Employee { Id = 2, Name = "Jane", Salary = 60000 },
    new Employee { Id = 3, Name = "Bob",  Salary = 45000 },
    new Employee { Id = 4, Name = "Sara", Salary = 55000 },
    new Employee { Id = 5, Name = "Tom",  Salary = 65000 }
};Code language: C# (cs)

Third, select a single employee with id 1 by using the Single() method. The Single() method returns a single Employee object:

var employee = employees.Single(e => e.Id == 1);Code language: C# (cs)

Finally, write the Employee object to the console:

WriteLine($"{employee.Id} - {employee.Name}");Code language: C# (cs)

SingleOrDefault method

If you want to return a default value if no element in a sequence satisfies a condition, you can use the SingleOrDefault() method:

public static TSource? SingleOrDefault<TSource> (
   this IEnumerable<TSource> source, 
   Func<TSource,bool> predicate,
   TSource defaultValue
);Code language: C# (cs)

For example:

using Models;
using static System.Console;

class Employee
{
    public int? Id { get; set; }
    public string? Name { get; set; }
    public int Salary { get; set; }
}

class Program
{
    public static void Main()
    {
        var employees = new List<Employee>()
        {
            new Employee { Id = 1, Name = "John", Salary = 50000 },
            new Employee { Id = 2, Name = "Jane", Salary = 60000 },
            new Employee { Id = 3, Name = "Bob",  Salary = 45000 },
            new Employee { Id = 4, Name = "Sara", Salary = 55000 },
            new Employee { Id = 5, Name = "Tom",  Salary = 65000 }
        };
        var employee = employees.SingleOrDefault(e => e.Id == 10, null);
        if (employee != null)
        {
            WriteLine($"{employee.Id} - {employee.Name}");
        }
        else
        {
            WriteLine("The element was not found.");
        }
    }

}Code language: C# (cs)

Output:

The element was not found.Code language: C# (cs)

In this example, the employees list has no Employee object with id 10. Therefore, the SingleOrDefault() method returns null as the default value.

Summary

• Use LINQ Single() method to return a single element of a sequence that satisfies a condition.