Применение статического конструктора

public static class Math
{
    public const double PI = 3.14159265358979323846;
    public const double E = 2.7182818284590452354;
}

public static class Console
{
    public static int CursorLeft
    {
        [System.Security.SecuritySafeCritical]  // auto-generated
        [ResourceExposure(ResourceScope.None)]
        [ResourceConsumption(ResourceScope.Process, ResourceScope.Process)]
        get
        {
            Win32Native.CONSOLE_SCREEN_BUFFER_INFO csbi = GetBufferInfo();
            return csbi.dwCursorPosition.X;
        }
        [ResourceExposure(ResourceScope.Process)]
        [ResourceConsumption(ResourceScope.Process)]
        set
        {
            SetCursorPosition(value, CursorTop);
        }
    }
}

public sealed class Singleton
    {
    private static readonly Singleton instance = new Singleton();
 
    // Explicit static constructor to tell C# compiler
    // not to mark type as beforefieldinit
    static Singleton()
    {
    }
 
    private Singleton()
    {
    }
 
    public static Singleton Instance => instance;
    
    }

static int PI = 3.14

static class Sql
{
    // Данные для SQL
    static SqlConnection connect;
    static SqlCommand command;
    static SqlDataReader reader;
 
    static Sql()
    {
        // Подключение к SQL
        connect = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\DataBase\Database.mdf;Integrated Security=True");
        connect.Open();
 
        // Класс для запросов к SQL
        command = new SqlCommand();
        command.Connection = connect;
    }
}