Partial Class in C# not wokring

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C#

Second file: Class1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;



public partial class TestClass
{

    public TestClass(int x, int y)
    {

        _a = x;
        _b = y;
    }


    public int A
    {


        get
        {
            return _a;
        }


        set { _b = value; }
    }

    public int B { get { return _b; } set { _b = value; } }




}

First file: Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;



partial class TestClass
{
    private int _a, _b;
    public TestClass()
    {
        Console.WriteLine("Test Class Default Constructor");

    }

    public void Add()
    {
        Console.WriteLine("Addition Method");
        Console.WriteLine(_a+_b);

    }


    public void Sub()
    {
        Console.WriteLine("Subtraction Method");
        Console.WriteLine(_a - _b);

    }

}

    class Program
    {
        static void Main(string[] args)
        {

            TestClass t = new TestClass();

            t.A=10;

            t.B = 20;

            t.Add();
            t.Sub();


        }
    }

I am trying to understand the partial class concepts i have written simple program to add and subtract it all works but i always get the output has 20 and -20 ..I don't know why do i get the same output i have tried changing the numbers still the same