using System;

    public class Test

{   

    public DateTime defaultValue { get; set; } = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day, 8, 0, 0);

    public DateTime? RealValue { get; set; }

    public DateTime Default

    {

        get => RealValue ?? defaultValue;

        set {RealValue = value;}

    }

}   

​

public class Program

{

​

​

    public static void Main()

    {

        var Test = new Test();

        Test.Default = Test.Default;

        Console.WriteLine(Test.RealValue);

    }

}