The Language Integrated Query

Introduction to Querying

To support the ability to query a list, you can use the Language Integrated Query, abbreviated LINQ. To use LINQ in your application, you must include the System.Core.dll assembly in your program. If you started your application as an empty project:

• On the main menu, you can click Project -> Add Reference...
• In the Solution Explorer, you can right-click the name of the project and click Add Reference...
• In the Class View, you can right-click the name of the project and click Add Reference...

In the .NET tab of the Add Reference dialog box, you can click System.Core

Then click OK. You must then use the System.Linq namespace in your code or you should include the using System.Linq; line in your list of namespaces.

If you create an application by selecting the Console Application option from the New Project dialog box, the studio would add the necessary assembly to your project and the necessary namespace to your code file.

To query a list, you write a statement using words and operators of the LINQ.

If you have already used another query language, such as SQL, you may be wondering about LINQ's case-sensitivity. Here is the difference. Because the LINQ statements are written directly in the same sections with the langue of your code, LINQ's case-sensitivity depends on that language. For example, if you write your LINQ statement in a C# application as we will do here or in a C++ application where the language is case-sensitive, the LINQ's statement is case-sensitive and must be in lowercase. If you write the statement in a case-insensitive language such as Visual Basic or Pascal (such as CodeGear Delphi), you can write the statement with the case of your choice.

The most fundamental operation you can perform on LINQ consists of creating, also referred to as selecting, a list of values, from an existing list such as an array. The basic formula to use is:

var SubListName = from ValueHolder in List select ValueHolder;

The var keyword, the assignment operator "=", the from keyword, the in keyword, the select keyword, and the semicolon are required.

The SubListName is a name of a new variable that will hold the list of values produced by this operation.

The ValueHolder is the name of a variable that will be used to identify each resulting member of this operation. This variable will be equivalent to getting each member of the array that responds to a condition.

The List factor represents the name of the variable that you would have created already. The List can be an array. Here is an example:

using System;
using System.Linq;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new double[] { 12.44, 525.38, 6.28, 2448.32, 632.04 };

        var Number = from n in Numbers select n;

        foreach(var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

This would produce:

Number: 12.44
Number: 525.38
Number: 6.28
Number: 2448.32
Number: 632.04
Press any key to continue . . .

To make the code easier to read, you can spread the select statement to various lines. Here is an example:

using System;
using System.Linq;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new double[] { 12.44, 525.38, 6.28, 2448.32, 632.04 };

        var Number = from n
                     in Numbers
                     select n;

        foreach(var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

We mentioned that the List factor of our formula could be an array. It can also be a collection-based variable; that is, a variable created from a collection-based class. The collection class you use must implement the IEnumerable generic interface. If you want, you can create your own class that implements this interface but the .NET Framework provides a complete set of classes that should suit every need. One of the built-in generic classes of the .NET Framework is called List and you can easily use it to create a list of values. Here is an example:

using System;
using System.Collections.Generic;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new List<double>();
        
        Numbers.Add(12.44);
        Numbers.Add(525.38);
        Numbers.Add(6.28);
        Numbers.Add(2448.32);
        Numbers.Add(632.04);

        return 0;
    }
}

After creating the collection, you can use the same LINQ formula we saw for an array. Here is an example:

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

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new List<double>();
        
        Numbers.Add(12.44);
        Numbers.Add(525.38);
        Numbers.Add(6.28);
        Numbers.Add(2448.32);
        Numbers.Add(632.04);

        var Number = from n
                     in Numbers
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

This would produce the same result as seen earlier. Notice that, as always, the var keyword does not indicate the type of variable it is dealing with.

As you can see, simply using a select statement as done above produces the same list of values in the array. A criterion is a condition applied to a set of values to find out which one(s) respond(s) to the condition or validate the condition. When applied to an array, a criterion examines each member of the array, finds out what member responds to it, if so, adds that member to the from list.

To apply a criterion, you create Boolean operation between the in statement and the select statement. This criterion is actually formulated using the where operator. The formula to use is:

var SubListName = from ValueHolder in List where Condition select ValueHolder;

In the new section, the where keyword is required. The Condition is formulated using the logical operators we studied in Lesson 9. It can be in the form of:

• ValueHolder == SomeValue: If ValueHolder is the same as SomeValue, the list member that is being examined is valid
• ValueHolder < SomeValue: If ValueHolder is less than SomeValue, the list member that is being examined is valid
• ValueHolder <= SomeValue: If ValueHolder is the same as, or lower than, SomeValue, the list member that is being examined is valid
• ValueHolder > SomeValue: If ValueHolder is greater than SomeValue, the list member that is being examined is valid
• ValueHolder >= SomeValue: If ValueHolder is the same as, or greater than, SomeValue, the list member that is being examined is valid
• ValueHolder != SomeValue: If ValueHolder is different from SomeValue, the list member that is being examined is valid
• !(ValueHolder == SomeValue): If ValueHolder is different from SomeValue, the list member that is being examined is valid

Remember that you must create the criterion as a Boolean operation. After applying the select statement, you can then use it as you see fit. For example, you can display its result(s) to the console. Here is an example:

using System;
using System.Linq;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     where n == 327
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

This would produce:

Number: 327
Press any key to continue . . .

Of course, the purpose of querying a list is to isolate one or more values. As such, you can create an expression that checks a value and applies some condition to it. For example, in this list of numbers, you may want to find out whether it contains one or more numbers that are divisible by 5. This operation can be carried by the % operator as in "number % 5"; but number % 5 is pure algebra, not Boolean. Therefore, you must add a condition to make it a valid Boolean expression. For example, you can find out if the number % 5 operation is equal to 0. Here is an example:

using System;
using System.Linq;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     where n % 5 == 0
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

This would produce:

Number: 525
Press any key to continue . . .

using System;
using System.Linq;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     where (n % 5) == 0
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

using System;
using System.Linq;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     where n % 2 == 0
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

Number: 102
Number: 44
Number: 38
Number: 6
Number: 28
Number: 632
Number: 104
Press any key to continue . . .

In Lesson 10, we saw that you could use logical conjunctions and disjunctions to combine two or more Boolean expressions. These also are available in LINQ. To do this, use the where statement to create the expression as you see fit. Here is an expression that uses a logical disjunction (&&) operation:

using System;
using System.Linq;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     where n % 2 == 0 && n <= 100
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

This would produce:

Number: 44
Number: 38
Number: 6
Number: 28
Press any key to continue . . .

Once again, remember that the use of parentheses makes it easier to read the code and better understand it:

using System;
using System.Linq;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     where (n % 2 == 0) && (n <= 100)
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

Or better yet:

using System;
using System.Linq;

public class Exercise
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     where ((n % 2) == 0) && (n <= 100)
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);

        return 0;
    }
}

When you create a list, you add the items in any order of your choice. For example, if you register the students of a school, you enter their information as it becomes available. In the same way, when you create a select statement, the items are added to its list in the order they appear in the main list. When treating the new list or when displaying it to the user, you may want to arrange it in alphabetical, numerical, or in chronological order. To support this operation, the LINQ provides the orderdy operator. To apply it, write the operator before the select operation followed by the from list. Here is an example:

using System;
using System.Linq;

public class Program
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     orderby n
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);
        return 0;
    }
}

This would produce:

Number: 6
Number: 28
Number: 38
Number: 44
Number: 102
Number: 104
Number: 327
Number: 525
Number: 632
Number: 24481
Press any key to continue . . .

If you apply the orderby operator simply followed by a variable, the list is ordered alphabetically or numerically depending on the types of values in the list. This is referred to as ascending. To re-enforce this, you can follow the variable with the ascending keyword. Here is an example:

using System;
using System.Linq;

public class Program
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     orderby n ascending
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);
        return 0;
    }
}

To arrange the list in reverse order, you can follow the variable name with the descending keyword. Here is an example:

using System;
using System.Linq;

public class Program
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     orderby n descending
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);
        return 0;
    }
}

This would produce:

Number: 24481
Number: 632
Number: 525
Number: 327
Number: 104
Number: 102
Number: 44
Number: 38
Number: 28
Number: 6
Press any key to continue . . .

You can apply a where condition to the statement to select just a few values. For example, to get a list of odd numbers arranged in numerical order, you would write the statement as follows:

using System;
using System.Linq;

public class Program
{
    static int Main(string[] args)
    {
        var Numbers = new[] { 102, 44, 525, 38, 6, 28, 24481, 327, 632, 104 };

        var Number = from n
                     in Numbers
                     where n % 2 != 0
                     orderby n ascending
                     select n;

        foreach (var member in Number)
            Console.WriteLine("Number: {0}", member);
        return 0;
    }
}

From what we have learned so far, the type of list produced by a LINQ statement is primarily based on one of the data types we reviewed in Lesson 2 and in Lesson 3 (integers, double-precision numbers, strings, and date/time values). In Lesson 17, we saw that the integer and numeric types are in fact structures of the .NET Framework. As such, they are equipped with properties and methods. In your LINQ statement, you can explore the characteristics of those data types as you see fit.

Using the String Class

If you create a string-based collection, you can use the properties and methods of the String class to refine the list produced by your LINQ statement. To do this, in the list produced by the where statement, you can access any of the properties or you can call any desired method of the String class using the period operator. For example, to get a list of names that start with a certain letter, you can call the StartsWith() method as follows:

using System;
using System.Linq;

public class Program
{
    static int Main(string[] args)
    {
        var Name = new[]
        {
            "Hermine", "Patrick", "Hank", "Bertine",
            "Julius", "Thomas", "Jeannette", "Patricia",
            "Henriette", "Raul", "David", "Paulette" };

        var Names = from n
                    in Name
                    where n.StartsWith("P")
                    // orderby n ascending
                    select n;

        foreach (var member in Names)
            Console.WriteLine("Name: {0}", member);
        return 0;
    }
}

Or to get a list of names that end with a certain sub-string, you would call the String.EndsWith() method. Here is an example:

using System;
using System.Linq;

public class Program
{
    static int Main(string[] args)
    {
        var Name = new[]
        {
            "Hermine", "Patrick", "Hank", "Bertine",
            "Julius", "Thomas", "Jeannette", "Patricia",
            "Henriette", "Raul", "David", "Paulette" };

        var Names = from n
                    in Name
                    where n.EndsWith("ette")
                    // orderby n ascending
                    select n;

        foreach (var member in Names)
            Console.WriteLine("Name: {0}", member);
        return 0;
    }
}

In the same ways, to create a list of names that contain a certain sub-string, you can call the String.Contains() method as follows:

using System;
using System.Linq;

public class Program
{
    static int Main(string[] args)
    {
        var Name = new[]
        {
            "Hermine", "Patrick", "Hank", "Bertine",
            "Julius", "Thomas", "Jeannette", "Patricia",
            "Henriette", "Raul", "David", "Paulette" };

        var Names = from n
                    in Name
                    where n.Contains("au")
                    // orderby n ascending
                    select n;

        foreach (var member in Names)
            Console.WriteLine("Name: {0}", member);
        return 0;
    }
}

To create more effective LINQ statements, you can use many of the features of the C# language we have studied in previous lessons. For example, instead of using one of the primitive types to create your list as we have done so far, you may want to use your own class. You can start by creating the class normally as you see fit. Here is an example:

public class Person
{
    public string FirstName;
    public string LastName;
    public char Gender;

    public Person(string First, string Last, char Sex)
    {
        FirstName = First;
        LastName = Last;
        Gender = Sex;
    }
}

Obviously before considering the class in your LINQ statement, you should first create a collection variable that would hold its values. You can create the list as an array. Once the list is ready, when formulating your LINQ statement, use the from variable to access a member of the class using the period operator. Here is an example:

using System;
using System.Linq;

public class Person
{
    public string FirstName;
    public string LastName;
    public char Gender;

    public Person(string First, string Last, char Sex)
    {
        FirstName = First;
        LastName = Last;
        Gender = Sex;
    }
}

public class Program
{
    static int Main(string[] args)
    {
        var People = new Person[]
        {
            new Person("Paulette", "Cranston", 'F'),
            new Person("Harry", "Kumar", 'M'),
            new Person("Jules", "Davidson", 'M'),
            new Person("Leslie", "Harrington", 'U'),
            new Person("Ernest", "Colson", 'M'),
            new Person("Patricia", "Katts", 'F'),
            new Person("Patrice", "Abanda", 'U'),
            new Person("Frank", "Thomasson", 'M')
        };

        var Persons = from Pers
                      in People
                      select Pers.LastName;

        foreach (var Individual in Persons)
            Console.WriteLine("Person Name: {0}", Individual);
        return 0;
    }
}

This would produce:

Person Name: Cranston
Person Name: Kumar
Person Name: Davidson
Person Name: Harrington
Person Name: Colson
Person Name: Katts
Person Name: Abanda
Person Name: Thomasson
Press any key to continue . . .

In the same way, you can use any of the regular C# operators to produce any result of your choice. For example, when studying strings, we saw that you could use the + operator to combine strings. Here is an example:

using System;
using System.Linq;

public class Person
{
    public string FirstName;
    public string LastName;
    public char Gender;

    public Person(string First, string Last, char Sex)
    {
        FirstName = First;
        LastName = Last;
        Gender = Sex;
    }
}

public class Program
{
    static int Main(string[] args)
    {
        var People = new Person[]
        {
            new Person("Paulette", "Cranston", 'F'),
            new Person("Harry", "Kumar", 'M'),
            new Person("Jules", "Davidson", 'M'),
            new Person("Leslie", "Harrington", 'U'),
            new Person("Ernest", "Colson", 'M'),
            new Person("Patricia", "Katts", 'F'),
            new Person("Patrice", "Abanda", 'U'),
            new Person("Frank", "Thomasson", 'M')
        };

        var Persons = from Pers
                      in People
                      select Pers.LastName + ", " + Pers.FirstName;

        foreach (var Individual in Persons)
            Console.WriteLine("Person Name: {0}", Individual);
        return 0;
    }
}

This would produce:

Person Name: Cranston, Paulette
Person Name: Kumar, Harry
Person Name: Davidson, Jules
Person Name: Harrington, Leslie
Person Name: Colson, Ernest
Person Name: Katts, Patricia
Person Name: Abanda, Patrice
Person Name: Thomasson, Frank
Press any key to continue . . .

Remember that the essence of querying a list is to create a list of particular data based on a criterion, which is done by applying a where condition. Here is an example:

public class Program
{
    static int Main(string[] args)
    {
        var People = new Person[]
        {
            new Person("Paulette", "Cranston", 'F'),
            new Person("Harry", "Kumar", 'M'),
            new Person("Jules", "Davidson", 'M'),
            new Person("Leslie", "Harrington", 'U'),
            new Person("Ernest", "Colson", 'M'),
            new Person("Patricia", "Katts", 'F'),
            new Person("Patrice", "Abanda", 'U'),
            new Person("Frank", "Thomasson", 'M')
        };

        var Persons = from Pers
                      in People
                      where Pers.Gender == 'F'
                      select Pers.LastName + ", " + Pers.FirstName;

        foreach (var Individual in Persons)
            Console.WriteLine("Person Name: {0}", Individual);
        return 0;
    }
}

This would produce:

Person Name: Cranston, Paulette
Person Name: Katts, Patricia
Press any key to continue . . .

To perform a more particular operation on a class, you can create a method in it and then call that method in your LINQ statement. This means that, just as you can access a field or a property of the class, you can access any of its internal or public methods. Here is an example:

using System;
using System.Linq;

public class Person
{
    public string FirstName;
    public string LastName;
    public char Gender;

    public Person(string First, string Last, char Sex)
    {
        FirstName = First;
        LastName = Last;
        Gender = Sex;
    }

    internal string GetFullName()
    {
        return LastName + ", " + FirstName;
    }
}

public class Program
{
    static int Main(string[] args)
    {
        var People = new Person[]
        {
            new Person("Paulette", "Cranston", 'F'),
            new Person("Harry", "Kumar", 'M'),
            new Person("Jules", "Davidson", 'M'),
            new Person("Leslie", "Harrington", 'U'),
            new Person("Ernest", "Colson", 'M'),
            new Person("Patricia", "Katts", 'F'),
            new Person("Patrice", "Abanda", 'U'),
            new Person("Frank", "Thomasson", 'M')
        };

        var Persons = from Pers
                      in People
                      where Pers.Gender == 'M'
                      select Pers.GetFullName();

        foreach (var Individual in Persons)
            Console.WriteLine("Person Name: {0}", Individual);
        return 0;
    }
}

This would produce:

Person Name: Kumar, Harry
Person Name: Davidson, Jules
Person Name: Colson, Ernest
Person Name: Thomasson, Frank
Press any key to continue . . .

There are two types of built-in classes you can use in your application when it comes to LINQ. You can use any of the non-generic collection classes to create a list of values. The other category is the generic collection classes.

We saw that you can get the result of a LINQ statement from the select section. In reality, the select statement simply indicates that the result is ready and it hands it to the other parts of the program. Instead of getting the result directly from the select statement, you can first store it in a local LINQ variable. This allows you to treat the result as a variable that you can then manipulate before getting the final result.

To create a local variable in the LINQ statement, you can use the let operator. You must use it before the select statement to hold the result. Here is an example:

public class Program
{
    static int Main(string[] args)
    {
        var Persons = from Pers
                      in People
                      let FullName = Pers.LastName + ", " + Pers.FirstName
                      select FullName;

        foreach (var Individual in Persons)
            Console.WriteLine("Person Name: {0}", Individual);
        return 0;
    }
}

If you need a where condition but your let variable would be used only to hold the final result, you can create that variable after the where statement. Here is an example:

public class Program
{
    static int Main(string[] args)
    {
        var Persons = from Pers
                      in People
                      where Pers.Gender == 'U'
                      let FullName = Pers.LastName + ", " + Pers.FirstName
                      select FullName;

        foreach (var Individual in Persons)
            Console.WriteLine("Person Name: {0}", Individual);
        return 0;
    }
}

In this case, you can create the let variable before the where statement and you would get the same result:

using System;
using System.Linq;

public class Person
{
    public string FirstName;
    public string LastName;
    public char Gender;

    public Person(string First, string Last, char Sex)
    {
        FirstName = First;
        LastName = Last;
        Gender = Sex;
    }
}

public class Program
{
    static int Main(string[] args)
    {
        var People = new Person[]
        {
            new Person("Paulette", "Cranston", 'F'),
            new Person("Harry", "Kumar", 'M'),
            new Person("Jules", "Davidson", 'M'),
            new Person("Leslie", "Harrington", 'U'),
            new Person("Ernest", "Colson", 'M'),
            new Person("Patricia", "Katts", 'F'),
            new Person("Patrice", "Abanda", 'U'),
            new Person("Frank", "Thomasson", 'M')
        };

        var Persons = from Pers
                      in People
                      let FullName = Pers.LastName + ", " + Pers.FirstName
                      where Pers.Gender == 'U'
                      select FullName;

        foreach (var Individual in Persons)
            Console.WriteLine("Person Name: {0}", Individual);
        return 0;
    }
}

This would produce:

Person Name: Harrington, Leslie
Person Name: Abanda, Patrice
Press any key to continue . . .

To get the final result of a query, you may want to combine a few fields or properties of each member of the result. For example, as we have seen so far, you may want to combine the last and the first name of each result to create a full name. Besides, or instead of, the let operator, you can use the new operator to create such a combination.

To use the new operator, after the select keyword, type new followed by an opening and a closing curly brackets. Inside the brackets, create an expression as you see fit and assign it to a local variable in the curly brackets. When accessing the result in your foreach loop, apply the period operator on the foreach variable to access the new local variable. Here is an example:

public class Person
{
    public string FirstName;
    public string LastName;
    public char Gender;

    public Person(string First, string Last, char Sex)
    {
        FirstName = First;
        LastName = Last;
        Gender = Sex;
    }
}

public class Program
{
    static int Main(string[] args)
    {
        var People = new Person[]
        {
            new Person("Paulette", "Cranston", 'F'),
            new Person("Harry", "Kumar", 'M'),
            new Person("Jules", "Davidson", 'M'),
            new Person("Leslie", "Harrington", 'U'),
            new Person("Ernest", "Colson", 'M'),
            new Person("Patricia", "Katts", 'F'),
            new Person("Patrice", "Abanda", 'U'),
            new Person("Frank", "Thomasson", 'M')
        };

        var Persons = from Pers
                      in People
                      select new {FullName=Pers.LastName+", "+Pers.FirstName };

        foreach (var Individual in Persons)
            Console.WriteLine("Person Name: {0}", Individual.FullName);
        return 0;
    }
}

To make the statement easier to read, you can span it on various lines:

public class Program
{
    static int Main(string[] args)
    {
        var Persons = from Pers
                      in People
                      select new
                      {
                          FullName = Pers.LastName +
                                     ", " +
                                     Pers.FirstName
                      };

        foreach (var Individual in Persons)
            Console.WriteLine("Person Name: {0}", Individual.FullName);
        return 0;
    }
}