C# Single() Method

The Single() method in C# returns the only element of a sequence. It throws an exception if the sequence is empty or contains more than one element, making it useful when you expect exactly one element.

The Single() method is available in both LINQ to Objects and LINQ to Entities through the System.Linq namespace.

Syntax

Following is the syntax for the Single() method −

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

Parameters

• source − The IEnumerable<T> to return the single element from.

• predicate − A function to test each element for a condition (optional).

Return Value

Returns the single element of the input sequence, or the single element that satisfies the condition.

Using Single() with One Element

When you have a sequence with exactly one element, Single() returns that element −

using System;
using System.Linq;

public class Demo {
    public static void Main() {
        string[] str = { "one" };
        string res = str.Single();
        Console.WriteLine("Single element: " + res);
        
        int[] numbers = { 42 };
        int number = numbers.Single();
        Console.WriteLine("Single number: " + number);
    }
}

The output of the above code is −

Single element: one
Single number: 42

Using Single() with Predicate

You can use Single() with a predicate to find the single element that matches a condition −

using System;
using System.Linq;

public class Demo {
    public static void Main() {
        int[] numbers = { 1, 2, 3, 4, 5 };
        
        // Find the single even number greater than 3
        int result = numbers.Single(x => x > 3 && x % 2 == 0);
        Console.WriteLine("Single even number > 3: " + result);
        
        string[] names = { "Alice", "Bob", "Charlie" };
        string name = names.Single(n => n.StartsWith("B"));
        Console.WriteLine("Name starting with B: " + name);
    }
}

The output of the above code is −

Single even number > 3: 4
Name starting with B: Bob

Exception Handling with Single()

The Single() method throws exceptions in specific scenarios. Here's how to handle them −

using System;
using System.Linq;

public class Demo {
    public static void Main() {
        try {
            // Empty sequence - throws InvalidOperationException
            int[] empty = { };
            int result1 = empty.Single();
        }
        catch (InvalidOperationException ex) {
            Console.WriteLine("Empty sequence error: " + ex.Message);
        }
        
        try {
            // Multiple elements - throws InvalidOperationException
            int[] multiple = { 1, 2, 3 };
            int result2 = multiple.Single();
        }
        catch (InvalidOperationException ex) {
            Console.WriteLine("Multiple elements error: Sequence contains more than one element");
        }
    }
}

The output of the above code is −

Empty sequence error: Sequence contains no elements
Multiple elements error: Sequence contains more than one element

Single() vs SingleOrDefault() vs First()

Common Use Cases

• Retrieving a unique record from a database query

• Getting configuration settings that should have exactly one value

• Finding a specific item in a collection when you expect exactly one match

• Validating that a filter condition returns exactly one result

Conclusion

The Single() method is ideal when you need exactly one element from a sequence and want to ensure data integrity. It provides built-in validation by throwing exceptions for empty sequences or multiple elements, making your code more robust and predictable.