What does "Replace with single call to single" mean?

Question

When using LINQ with Single() I always get my line of code underlined in green with the suggestion "Replace with single call to single." What does this mean? Here's an example of a line of code that results in that suggestion:

var user = db.Users.Where(u => u.UserID == userID).Single();

As you can see I'm only using Single() once. So... what's the deal?

I wonder if that's something that should be fixed, since it's not more efficient: http://stackoverflow.com/questions/8663897/why-is-linq-wherepredicate-first-faster-than-firstpredicate — Garrison Neely, Jun 10 '13 at 21:49

score 50 · Accepted Answer · answered Jun 10 '13 at 20:38

50

I assume it means, use the overload of Single which takes a predicate instead of using Where and Single together:

var user = db.Users.Single(u => u.UserID == userID);

answered Jun 10 '13 at 20:38

Lee

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Nice! This also helped me when ReSharper gave a similar suggestion regarding FirstOrDefault(). – midoriha_senpai Oct 27 '17 at 18:45

score 9 · Answer 2 · answered Jun 10 '13 at 20:38

9

var user = db.Users.Single(u => u.UserID == userID)

Syntactic sugar

answered Jun 10 '13 at 20:38

AD.Net

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Craig Gjerdingen · Answer 3 · 2015-04-13T21:06:59.020

A bunch of the Linq expressions, Enumerable Methods, work this way, like the method Single, which takes a predicate and returns a true only if the condition is satisfied (test passed), otherwise a false.

These should be used instead of a Where() and Single() together:

var user = db.Users.Single(u => u.UserID == userID); 
// Checks for if there is one and, only one element that satisfy the condition.

and

var user = db.Users.Any(u => u.UserID == userID);  
// Checks for if there are any elements that satisfy the condition.

and

var user = db.Users.All(u => u.UserID == userID);  
// Checks if all elements satisfy the condition.

score 0 · Answer 4 · answered Nov 24 '16 at 23:58

Select * from where field in (list, of,field) can be implemented in Linq Lambda using two approaches

Using "Contains"
Using "Join"

Code sample here in DotNetFiddle

using System;
 using System.Linq;
using System.Collections;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {

    //Initializing a collection of 4 Employees fromthree Different departments <Name of Employee, Dept ID>
    List<KeyValuePair<string, int>> EmployeeDept = new List<KeyValuePair<string, int>> {
        new KeyValuePair<string, int> ("Gates",2),
        new KeyValuePair<string, int> ("Nadella",2),
            new KeyValuePair<string, int> ("Mark",3),
        new KeyValuePair<string, int> ("Pichai",4)
    };

    //Filter the Employees belongs to these department
    int[] deptToFilter ={3,4};

    //Approach 1 - Using COntains Method
    Console.WriteLine ("Approach 1 - Using Contains");
    Console.WriteLine ("==========================================");   
    var filterdByContains = EmployeeDept.Where(emp => deptToFilter.Contains(emp.Value));
    foreach (var dept3and4employees in filterdByContains)
    {
        Console.WriteLine(dept3and4employees.Key+" - "+dept3and4employees.Value);
    }

    //Approach 2 Using Join
    Console.WriteLine ("\n\nApproach 2 - Using Join");
    Console.WriteLine ("==========================================");   
    var filteredByJoin = EmployeeDept.Join(
        deptToFilter,
        empdept => empdept.Value, 
        filterdept => filterdept, 
        (empdep,filddep) => new KeyValuePair<string, int>(empdep.Key, empdep.Value)
    );

    foreach (var dept3and4employees in filteredByJoin)
    {
        Console.WriteLine(dept3and4employees.Key+" - "+dept3and4employees.Value);
    }

}
}

What does "Replace with single call to single" mean?

4 Answers4

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