C# Online Compiler | .NET Fiddle

using System;
using System.Linq;
using System.Collections.Generic;
					
public class Program
{
	public static void Main()
	{
		var l = new List<Range> (){
			new Range(){
				Id = 1,
				Start = DateTime.Now.AddHours(1),
				End = DateTime.Now.AddHours(3)
			},
			new Range(){
				Id = 2,
				Start = DateTime.Now.AddHours(2),
				End = DateTime.Now.AddHours(6)
			},

			new Range(){
				Id = 3,
				Start = DateTime.Now.AddHours(6),
				End = DateTime.Now.AddHours(8)
			},

		};
		DetectOverlap(l);
		//Console.WriteLine("Hello World");
	}
	static void DetectOverlap(List<Range> l)
	{
		foreach(var r in l)
		{
			var overlap = l.Any(x => x.Id != r.Id 
				  && ((r.Start == x.Start && r.End == x.End) 
					  || (r.Start >= x.Start && r.Start < x.End)
					  || (r.End > x.Start && r.End <= x.End)));
			if(overlap)
			{
				Console.WriteLine("Overlap detected");
				throw new Exception("Overlapping range detected");
			}
		}
		Console.WriteLine("Clean ranges");
	}
	
}

public class Range {
	public int Id {get; set;}
	public DateTime Start {get; set;}
	public DateTime End {get; set;}
}

 
using System;
using System.Linq;
using System.Collections.Generic;
                    
public class Program
{
    public static void Main()
    {
        var l = new List<Range> (){
            new Range(){
                Id = 1,
                Start = DateTime.Now.AddHours(1),
                End = DateTime.Now.AddHours(3)
            },
            new Range(){
                Id = 2,
                Start = DateTime.Now.AddHours(2),
                End = DateTime.Now.AddHours(6)
            },
​
            new Range(){
                Id = 3,
                Start = DateTime.Now.AddHours(6),
                End = DateTime.Now.AddHours(8)

name:Item3,Item1,Item2
letter:A,A,B
number:1,3,2