[Solved]-Comparing record types with LINQ in C#-LINQ,C#

In your line of code:

return _alarms.FirstOrDefault(a => a.InternalAlarm == alarm);

the a.InternalAlarm == alarm is calling operator== rather than object.Equals() to do the comparison.

You can fix this by changing the code to:

return _alarms.FirstOrDefault(a => a.InternalAlarm.Equals(alarm));

The following program demonstrates the difference:

using System;

static class Program
{
    public static void Main()
    {
        MyAlarm ma1 = new MyAlarm { Prop1 = 1, Prop2 = 2 };
        MyAlarm ma2 = new MyAlarm { Prop1 = 1, Prop2 = 2 };
        
        VisibleAlarm va1 = new VisibleAlarm { InternalAlarm = ma1, Message = "message" };
        VisibleAlarm va2 = new VisibleAlarm { InternalAlarm = ma2, Message = "message" };

        Console.WriteLine(ma1 == ma2); // True
        Console.WriteLine(va1 == va2); // True
        Console.WriteLine(va1.InternalAlarm == va2.InternalAlarm); // False
        Console.WriteLine(va1.InternalAlarm.Equals(va2.InternalAlarm)); // True
    }

    public interface IAlarm
    {
        int Prop1 { get; }
        int Prop2 { get; }
    }

    public record MyAlarm : IAlarm
    {
        public int Prop1 { get; init; }
        public int Prop2 { get; init; }
    }

    public record VisibleAlarm
    {
        public IAlarm InternalAlarm { get; init; }
        public string Message       { get; set; }
    }
}

Output:

True
True
False
True

The reason this isn't working for you is because InternalAlarm is of type IAlarm and not MyAlarm. If you change the type to MyAlarm, you'll get the comparison as true rather than false.

This is because IAlarm does not define an operator== (note: such an operator would be static).