Custom Extension Method to Compare List in C#

In this article, we will see how to create a custom extension method to compare 2 lists in C#.

Custom Extension Method

Create a static class ExtensionDemo. Inside that, have a static method "CompareList" of generic type and return type of bool. Make sure to have 2 List parameters and the first one should have "this" keyword. This parameter here refers to the list in which you are able to see this custom extension method.

I have used the system.reflection.propertyinfo namespace to compare each property in an object. To know more about comparing properties inside 2 objects using Properties, please check this article.

public static class ExtensionDemo

{
    public static bool CompareList<T>(this List<T> list1, List<T> list2)
    {
        //if any of the list is null, return false

        if ((list1 == null && list2 != null) || (list2 == null && list1 != null))
            return false;

        //if both lists are null, return true, since its same
        else if (list1 == null && list2 == null)
            return true;
        //if count don't match between 2 lists, then return false
        if (list1.Count != list2.Count)
            return false;
        bool IsEqual = true;
        foreach (T item in list1)
        {
            T Object1 = item;
            T Object2 = list2.ElementAt(list1.IndexOf(item));
            Type type = typeof(T);
            //if any of the object inside list is null and other list has some value for the same object  then return false
                if ((Object1 == null && Object2 != null) || (Object2 == null && Object1 != null))
{
    IsEqual = false;
    break;
}
 
foreach (System.Reflection.PropertyInfo property in type.GetProperties())
{
    if (property.Name != "ExtensionData")
    {
        string Object1Value = string.Empty;
        string Object2Value = string.Empty;
        if (type.GetProperty(property.Name).GetValue(Object1, null) != null)

                            Object1Value = type.GetProperty(property.Name).GetValue(Object1, null).ToString();
                        if (type.GetProperty(property.Name).GetValue(Object2, null) != null)
                            Object2Value = type.GetProperty(property.Name).GetValue(Object2, null).ToString();
                        //if any of the property value inside an object in the list didnt match, return false
                    if (Object1Value.Trim() != Object2Value.Trim())
                    {
                        IsEqual = false;
                        break;
                    }
                }

            }
 
        }
        //if all the properties are same then return true
        return IsEqual;
    }

}

How to use the created custom extension method.

To test this, we have to create a new Web/Window/Console project. I am using a web application. I have created a student class as shown below in the same namespace.

public class Student
{
    public int? studentId { get; set; }
    public int age { get; set; }
}

Now in page_load event, create a list of objects for students.

protected void Page_Load(object sender, EventArgs e)
{
    List<Student> s1 = new List<Student>()
    {
        new Student(){studentId=100,age=21},
        new Student(){studentId=101,age=21},
        new Student(){studentId=102,age=21}
    };
    List<Student> s2 = new List<Student>()
    {
        new Student(){studentId=100,age=21},
        new Student(){studentId=101,age=21},
        null
    };
    List<Student> s3 = new List<Student>()
    {
        new Student(){studentId=100,age=21},
        new Student(){studentId=102,age=21},
        new Student(){studentId=102,age=21}

    };
    List<Student> s4 = new List<Student>()
    {
        new Student(){studentId=100,age=21},
        new Student(){studentId=101,age=21},
        new Student(){studentId=102,age=21}
    };
    List<Student> s5 = new List<Student>()
    {
        new Student(){studentId=100,age=21},
        new Student(){studentId=101,age=21},
        new Student(){studentId=102,age=22}
    };

Now we are going to compare 2 lists using the extension method. You will be able to see the created custom extension in the list of extension methods for list s1 as below.



Now I compared a few combinations to verify that the method is working in the page_load event.

//compare 2 lists with different counts
            bool same = s1.com
            //bool same = s1.CompareList(s2);
            //compare 2 lists with same number of objects but one of the property inside an object is
different. Student id property
            bool same1 = s1.CompareList(s3);
            //compare 2 lists with same count and values
            bool same2 = s1.CompareList(s4);
            //compare 2 lists with same number of objects but one of the property inside an object is different.Age property
            bool same3 = s1.CompareList(s5);
            //compare 2 lists in which a list is null
            List<Student> s6= null;         
            bool same4 = s1.CompareList(s6);
            List<Student> s7= null;
            //compare 2 lists and both are null
            bool same5 = s6.CompareList(s7);
            s1 = new List<Student>()
            {
                new Student(){studentId=100,age=21},
                new Student(){studentId=101,age=21},
                null
            };
            //compare 2 lists in which one of the object inside a list is null
            bool same6 = s1.CompareList(s4);


While debugging, same2 and same5 will return true, all the remaining items will return false.

Please find the attached complete source code.