C# Online Compiler | .NET Fiddle

using System;
using System.Linq;
using System.Collections.Generic;
					
public class Program
{
	public static void Main()
	{
		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 = 5, StudentName = "Ron" , Age = 19 } 
		};
	
		IList<Student> studentList2 = new List<Student>() { 
				new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
				new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
			};
		
		var result = studentList1.Union(studentList2,new StudentComparer()); 
		
		foreach(var std in result)
			Console.WriteLine(std.StudentName);
			
	}
}

public class Student 
{
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public int Age { get; set; }
}

public 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.StudentID.GetHashCode();
    }
}

 
using System;
using System.Linq;
using System.Collections.Generic;
                    
public class Program
{
    public static void Main()
    {
        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 = 5, StudentName = "Ron" , Age = 19 } 
        };
    
        IList<Student> studentList2 = new List<Student>() { 
                new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
                new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
            };
        
        var result = studentList1.Union(studentList2,new StudentComparer()); 
        
        foreach(var std in result)
            Console.WriteLine(std.StudentName);

Cached Result

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