Aggregation Operators tutorial with examples

Aggregation Operator: Average

Average extension method calculates the average of the numeric items in the collection. Average method returns nullable or non-nullable decimal, double or float value.

The following example demonstrate Agerage method that returns average value of all the integers in the collection.

Example: Average Method C#

IList<int> intList = new List<int>>() { 10, 20, 30 };

var avg = intList.Average();

Console.WriteLine("Average: {0}", avg);

You can specify an int, decimal, double or float property of a class as a lambda expression of which you want to get an average value. The following example demonstrates Average method on the complex type.

Example: Average in Method Syntax C#

IList<Student> studentList = new List<Student>>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };

var avgAge = studentList.Average(s =&gt; s.Age);

Console.WriteLine("Average Age of Student: {0}", avgAge);

Output:

Average Age of Student: 17.4

The Average operator in query syntax is Not Supported in C#. However, it is supported in VB.Net

Output:

Average Age of Student: 17.4

Learn about another aggregate operator - Count.

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Aggregation Operator: Count

The Count operator returns the number of elements in the collection or number of elements that have satisfied the given condition.

The Count() extension method has the following two overloads:

Count() Overloads:

int Count<TSource>();int Count<TSource>(Func<TSource, bool> predicate);

The first overload method of Count returns the number of elements in the specified collection, whereas the second overload method returns the number of elements which have satisfied the specified condition given as lambda expression/predicate function.

The following example demonstrates Count() on primitive collection.

Example: Count() - C#

IList<int> intList = new List<int>() { 10, 21, 30, 45, 50 };

var totalElements = intList.Count();

Console.WriteLine("Total Elements: {0}", totalElements);

var evenElements = intList.Count(i => i%2 == 0);

Console.WriteLine("Even Elements: {0}", evenElements);

Output:

Total Elements: 5
Even Elements: 3

The following example demonstrates Count() method on the complex type collection.

Example: Count() in C#

IList<Student> studentList = new List<Student>>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Mathew" , Age = 15 } 
    };

var totalStudents = studentList.Count();

Console.WriteLine("Total Students: {0}", totalStudents);

var adultStudents = studentList.Count(s => s.Age >= 18);

Console.WriteLine("Number of Adult Students: {0}", adultStudents );

Output:

Total Students: 5
Number of Adult Students: 3

Example: Count() in VB.Net

Dim studentList = New List(Of Student) From {
        New Student() With {.StudentID = 1, .StudentName = "John", .Age = 13},
        New Student() With {.StudentID = 2, .StudentName = "Moin", .Age = 21},
        New Student() With {.StudentID = 3, .StudentName = "Bill", .Age = 18},
        New Student() With {.StudentID = 4, .StudentName = "Ram", .Age = 20},
        New Student() With {.StudentID = 5, .StudentName = "Ron", .Age = 15}
}

Dim totalStudents = studentList.Count()

Console.WriteLine("Total Students: {0}", totalStudents)

Output:

Total Students: 5

Count(predicate) extension method with predicate parameter is Not Supported in VB.Net.

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Count Operator in Query Syntax

Example: Count operator in query syntax - VB.Net

// Student collection
Dim studentList = New List(Of Student) From {
        New Student() With {.StudentID = 1, .StudentName = "John", .Age = 13},
        New Student() With {.StudentID = 2, .StudentName = "Moin", .Age = 21},
        New Student() With {.StudentID = 3, .StudentName = "Bill", .Age = 18},
        New Student() With {.StudentID = 4, .StudentName = "Ram", .Age = 20},
        New Student() With {.StudentID = 5, .StudentName = "Ron", .Age = 15}
}

Dim totalStudents = Aggregate st In studentList 
                    Into Count(st.Age >= 18)             

Console.WriteLine("Total Students: {0}", totalStudents)

Output:

Total Students: 3

C# Query Syntax doesn't support aggregation operators. However, you can wrap the query into brackets and use an aggregation functions as shown below.

Example: Count operator in query syntax C#

var totalAge = (from s in studentList
                select s.age).Count();

Learn about another aggregate operator - Max .

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Aggregation Operator: Max

The Max() method returns the largest numeric element from a collection.

The following example demonstrates Max() on primitive collection.

Example: Max method C#

IList<int> intList = new List<int>() { 10, 21, 30, 45, 50, 87 };

var largest = intList.Max();

Console.WriteLine("Largest Element: {0}", largest);

var largestEvenElements = intList.Max(i => {
			                        if(i%2 == 0)
				                        return i;
			
			                        return 0;
		                        });

Console.WriteLine("Largest Even Element: {0}", largestEvenElements );

Output:

Largest Element: 87
Largest Even Element: 50

The following example demonstrates Max() method on the complex type collection.

Example: Max in Method Syntax C#

IList<Student> studentList = new List<Student>>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };

var oldest = studentList.Max(s => s.Age);

Console.WriteLine("Oldest Student Age: {0}", oldest);

Example: Max in Method Syntax VB.Net

Dim studentList = New List(Of Student) From {
        New Student() With {.StudentID = 1, .StudentName = "John", .Age = 13},
        New Student() With {.StudentID = 2, .StudentName = "Moin", .Age = 21},
        New Student() With {.StudentID = 3, .StudentName = "Bill", .Age = 18},
        New Student() With {.StudentID = 4, .StudentName = "Ram", .Age = 20},
        New Student() With {.StudentID = 5, .StudentName = "Ron", .Age = 15}
}

Dim oldest = studentList.Max(Function(s) s.Age)

Console.WriteLine("Oldest Student Age: {0}", oldest)

Output:

Oldest Student Ag: 21

Max returns a result of any data type. The following example shows how you can find a student with the longest name in the collection:

Example: Max() in C#

public class Student : IComparable<Student> 
{
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public int Age { get; set; }
    public int StandardID { get; set; }

    public int CompareTo(Student other)
    {
        if (this.StudentName.Length >= other.StudentName.Length)
            return 1;

        return 0;
    }
}

class Program
{
    static void Main(string[] args)
    {
        // Student collection
        IList<Student> studentList = new List<Student>>() { 
                new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
                new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
                new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
                new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
                new Student() { StudentID = 5, StudentName = "Steve" , Age = 15 } 
            };

        var studentWithLongName = studentList.Max();

        Console.WriteLine("Student ID: {0}, Student Name: {1}", 
                                        .StudentID, studentWithLongName.StudentName)
    }
}

Output:

Student ID: 5, Student Name: Steve

You can use Min extension method/operator the same way as Max.

As per the above example, to find the student with the longest name, you need to implement IComparable<T> interface and compare student names' length in CompareTo method. So now, you can use Max() method which will use CompareTo method in order to return appropriate result.

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Max Operator in Query Syntax

Max operator is Not Supported in C# Query syntax. However, it is supported in VB.Net query syntax as shown below.

Example: Max Operator in Query Syntax VB.Net

Dim studentList = New List(Of Student) From {
        New Student() With {.StudentID = 1, .StudentName = "John", .Age = 13},
        New Student() With {.StudentID = 2, .StudentName = "Moin", .Age = 21},
        New Student() With {.StudentID = 3, .StudentName = "Bill", .Age = 18},
        New Student() With {.StudentID = 4, .StudentName = "Ram", .Age = 20},
        New Student() With {.StudentID = 5, .StudentName = "Ron", .Age = 15}
}

Dim maxAge = Aggregate st In studentList Into Max(st.Age)

Console.WriteLine("Maximum Age of Student: {0}", maxAge);

Output:

Maximum Age of Student: 21

Learn about another aggregate operator - Sum.

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Aggregation Operator: Sum

The Sum() method calculates the sum of numeric items in the collection.

The following example demonstrates Sum() on primitive collection.

Example: LINQ Sum() C#

IList<int> intList = new List<int>() { 10, 21, 30, 45, 50, 87 };

var total = intList.Sum();

Console.WriteLine("Sum: {0}", total);

var sumOfEvenElements = intList.Sum(i => {
			                    if(i%2 == 0)
				                    return i;
			
			                    return 0;
		                        });

Console.WriteLine("Sum of Even Elements: {0}", sumOfEvenElements );

Output:

Sum: 243
Sum of Even Elements: 90

The following example calculates sum of all student's age and also number of adult students in a student collection.

Example: LINQ Sum() - C#

IList<Student> studentList = new List<Student>>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };

var sumOfAge = studentList.Sum(s => s.Age);

Console.WriteLine("Sum of all student's age: {0}", sumOfAge);
		
var numOfAdults = studentList.Sum(s => {
			
	if(s.Age >= 18)
	    return 1;
	else
	    return 0;
});
 
Console.WriteLine("Total Adult Students: {0}", numOfAdults);

Example: Sum in Method Syntax VB.NET

// Student collection
Dim studentList = New List(Of Student) From {
        New Student() With {.StudentID = 1, .StudentName = "John", .Age = 13},
        New Student() With {.StudentID = 2, .StudentName = "Moin", .Age = 21},
        New Student() With {.StudentID = 3, .StudentName = "Bill", .Age = 18},
        New Student() With {.StudentID = 4, .StudentName = "Ram", .Age = 20},
        New Student() With {.StudentID = 5, .StudentName = "Ron", .Age = 15}
}

Dim sumOfAge = studentList.Sum(Function(s) s.Age)

Console.WriteLine("Total Age of Student: {0}", sumOfAge)

Dim numOfAdults = studentList.Sum(Function(s) 
            if(s.Age >= 18)
                return 1
            else
                return 0
            end if
        end function)
            
Console.WriteLine("Total Adult Students: {0}", numOfAdults)

Output:

Total Age of Student: 87
Total Adult Students: 3

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Sum operator in query syntax

Sum operator is Not Supported in C# Query syntax.

Example: Sum Operator in Query Syntax VB.Net

// Student collection
Dim studentList = New List(Of Student) From {
        New Student() With {.StudentID = 1, .StudentName = "John", .Age = 13},
        New Student() With {.StudentID = 2, .StudentName = "Moin", .Age = 21},
        New Student() With {.StudentID = 3, .StudentName = "Bill", .Age = 18},
        New Student() With {.StudentID = 4, .StudentName = "Ram", .Age = 20},
        New Student() With {.StudentID = 5, .StudentName = "Ron", .Age = 15}
}

Dim totalAge = Aggregate st In studentList Into Sum(st.Age)

Console.WriteLine("Sum of all student's age: {0}", totalAge);

Output:

Sum of all student's age: 87