LINQ Equality Operator: SequenceEqual

There is only one equality operator: SequenceEqual. The SequenceEqual method checks whether the number of elements, value of each element and order of elements in two collections are equal or not.

If the collection contains elements of primitive data types then it compares the values and number of elements, whereas collection with complex type elements, checks the references of the objects. So, if the objects have the same reference then they considered as equal otherwise they are considered not equal.

The following example demonstrates the SequenceEqual method with the collection of primitive data types.

Example: SequenceEqual in Method Syntax C#

IList<string> strList1 = new List<string>(){"One", "Two", "Three", "Four", "Three"};

IList<string> strList2 = new List<string>(){"One", "Two", "Three", "Four", "Three"};

bool isEqual = strList1.SequenceEqual(strList2); // returns true
Console.WriteLine(isEqual);

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Output:

true

If the order of elements are not the same then SequenceEqual method returns false.

Example: SequenceEqual in Method Syntax C#

IList<string> strList1 = new List<string>(){"One", "Two", "Three", "Four", "Three"};

IList<string> strList2 = new List<string>(){ "Two", "One", "Three", "Four", "Three"};

bool isEqual = strList1.SequenceEqual(strList2); // returns false
Console.WriteLine(isEqual);

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Output:

false

The SequenceEqual extension method checks the references of two objects to determine whether two sequences are equal or not. This may give wrong result. Consider following example:

Example: SequenceEqual in C#

Student std = new Student() { StudentID = 1, StudentName = "Bill" };

IList<Student> studentList1 = new List<Student>(){ std };

IList<Student> studentList2 = new List<Student>(){ std };
       
bool isEqual = studentList1.SequenceEqual(studentList2); // returns true

Student std1 = new Student() { StudentID = 1, StudentName = "Bill" };
Student std2 = new Student() { StudentID = 1, StudentName = "Bill" };

IList<Student> studentList3 = new List<Student>(){ std1};

IList<Student> studentList4 = new List<Student>(){ std2 };
       
isEqual = studentList3.SequenceEqual(studentList4);// returns false

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In the above example, the studentList1 and studentList2 contains the same student object, std. So studentList1.SequenceEqual(studentList2) returns true. But, stdList1 and stdList2 contains two seperate student object, std1 and std2. So now, stdList1.SequenceEqual(stdList2) will return false even if std1 and std2 contain the same value.

To compare the values of two collection of complex type (reference type or object), you need to implement IEqualityComparer<T> interface as shown below.

Example: IEqualityComparer C#:

class StudentComparer : IEqualityComparer<Student>
{
    public bool Equals(Student x, Student y)
    {
        if (x.StudentID == y.StudentID && x.StudentName.ToLower() == y.StudentName.ToLower())
            return true;

        return false;
    }

    public int GetHashCode(Student obj)
    {
        return obj.GetHashCode();
    }
}

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Now, you can use above StudentComparer class in SequenceEqual extension method as a second parameter to compare the values:

Example: Compare object type elements using SequenceEqual C#

IList<Student> studentList1 = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
        new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
    };

IList<Student> studentList2 = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
        new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
    };
// following returns true
bool isEqual = studentList1.SequenceEqual(studentList2, new StudentComparer());

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