C#

    public class Car
        {
        public string Registration { get; set; }
        public Color Color  { get; set; }
        }
    public class ParkingSpace
        {
        public Car Car { get; set; }
        public int Level { get; set; }
        public int Row { get; set; }
        public int Column { get; set; }
        }
...
            List<ParkingSpace> carPark = new List<ParkingSpace>();
            ...
            foreach (ParkingSpace ps in carPark)
                {
                Car c = ps.Car;
                Console.WriteLine("{0} is a {1} car, on floor {2}, Row {3}, Bay {4}.", 
                                  c.Registration, 
                                  c.Color, 
                                  ps.Level, 
                                  ps.Row, 
                                  ps.Column);
                }

List<Beton> allInstances = Beton.All;

OriginalGriff wrote:

Don't make the "raw" collection public - that lets the outside world remove instances which is a bad idea!

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]
"If you just follow the bacon Eddy, wherever it leads you, then you won't have to think about politics." -- Some Bell.

namespace WpfApp5
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            string a = "aaaaaa";
            string b = "bbbbbb";

            List<string> ab = new List<string>
            {
                a,
                b
            };

               public List<string> Ab
             { get { return new List<string>(ab); } }



        }



    }
}

  public List<string> Ab
{ get { return ab; } }

return new List<string>(ab);

 List<string> ab = new List<string>
{
    a,
    b
};

public void MyMethod(string a}
   {
   Console.WriteLine(a); // a is valid here
       {
       string b = "hello";
       Console.WriteLine(a); // a is valid here
       Console.WriteLine(b); // b is valid here
       if (a == "World")
           {
           string c = "!";
           Console.WriteLine(a); // a is valid here
           Console.WriteLine(b); // b is valid here
           Console.WriteLine(c); // c is valid here
           }
       Console.WriteLine(a); // a is valid here
       Console.WriteLine(b); // b is valid here
       Console.WriteLine(c); // c is not valid here as it is out of scope
       }
   Console.WriteLine(a); // a is valid here
   Console.WriteLine(b); // b is not valid here as it is out of scope
   Console.WriteLine(c); // c is not valid here as it is out of scope
   }
Console.WriteLine(a); // a is not valid here as it is out of scope
Console.WriteLine(b); // b is not valid here as it is out of scope
Console.WriteLine(c); // c is not valid here as it is out of scope
public void MyOtherMethod()
   {
    Console.WriteLine(a); // a is not valid here as it is out of scope
    Console.WriteLine(b); // b is not valid here as it is out of scope
    Console.WriteLine(c); // c is not valid here as it is out of scope
   }

public partial class MainWindow : Window
{
    // A field to store the value.
    // The field is available in every method and property of this class.
    private List<string> ab;
    
    public MainWindow()
    {
        InitializeComponent();

        string a = "aaaaaa";
        string b = "bbbbbb";

        // NB: Storing a value in the field, not a local variable:
        ab = new List<string>
        {
            a,
            b
        };
    }
    
    public List<string> Ab
    { 
        get { return new List<string>(ab); } 
    }
}

OriginalGriff wrote:

public, private, protected, internal, or protected internal

double number =  (double) 6 / (double) 10;
box_d.Text = number.ToString("F3") + " cm"; //returns 0.600 (correct) 
//the funny thing is, the program shows a suggestion that (double) here is an unnecessary cast........

double number =  6 / 10;
box_d.Text = number.ToString("F3") + " cm"; //returns 0.000 (WRONG AS F.)

double keyword (C# Reference) | Microsoft Docs[^]:

However, if you want an integer number to be treated as double, use the suffix d or D, for example:

double x = 3D;

double number = 6D / 10D;

public class CustomModelValidatorProvider : DataAnnotationsModelValidatorProvider
{
    protected override IEnumerable<ModelValidator> GetValidators(ModelMetadata metadata, ControllerContext context, IEnumerable<Attribute> attributes)
    {
        if (metadata.Model is ServiceFunctionCategoryLKP model && metadata.PropertyName == nameof(model.Code) && model.CodeLength >= 0)
        {
            var newAttributes = new List<Attribute>(attributes);
            var stringLength = newAttributes.OfType<StringLengthAttribute>().FirstOrDefault();
            if (stringLength != null)
            {
                newAttributes.Remove(stringLength);
                
                if (model.CodeLength != 0)
                {
                    newAttributes.Add(new StringLengthAttribute(model.CodeLength)
                    {
                        MinimumLength = model.CodeLength, 
                        ErrorMessage = $"The field {{0}} length must be at least {model.CodeLength}."
                    });
                }
                
                attributes = newAttributes;
            }
        }
        
        return base.GetValidators(metadata, context, attributes);
    }
}

ModelValidatorProviders.Providers.Clear();
ModelValidatorProviders.Providers.Add( new CustomValidatorProvider() );

    public class CustomModelValidatorProvider : System.Web.Mvc.DataAnnotationsModelValidatorProvider
    {
        protected override IEnumerable<System.Web.Mvc.ModelValidator> GetValidators(System.Web.Mvc.ModelMetadata metadata, ControllerContext context, IEnumerable<Attribute> attributes)
        { 
            if (metadata.Model is LookupTable)
            {
                LookupTable model = metadata.Model as LookupTable;

                if (metadata.PropertyName == GetPropertyName(() => model.CodeLength) && (model.CodeLength > 0))
                {
                    var newAttributes = new List<Attribute>(attributes);
                    var stringLength = newAttributes.OfType<StringLengthAttribute>().FirstOrDefault();
                    if (stringLength != null)
                    {
                        newAttributes.Remove(stringLength);

                        if (model.CodeLength != 0)
                        {
                            newAttributes.Add(new StringLengthAttribute(model.CodeLength)
                            {
                                MinimumLength = model.CodeLength,
                                ErrorMessage = @"The field {{0}} length must be at least {model.CodeLength}."
                            });
                        }

                        attributes = newAttributes;
                    }
                }
            }            

            return base.GetValidators(metadata, context, attributes);
        }

        public string GetPropertyName<T>(Expression<Func<T>> propertyLambda)
        {
            var me = propertyLambda.Body as MemberExpression;

            if (me == null)
            {
                throw new ArgumentException("You must pass a lambda of the form: '() => Class.Property' or '() => object.Property'");
            }

            return me.Member.Name;
        }
    }

ModelValidatorProviders.Providers.Clear();
ModelValidatorProviders.Providers.Add( new CustomValidatorProvider() );

if (metadata.PropertyName == GetPropertyName(() => model.Code) && ...

if (metadata.PropertyName == GetPropertyName(() => model.Code) && (model.CodeLength > 0))

if (metadata.Model is ServiceFunctionCategoryLKP model && metadata.PropertyName == nameof(model.Code) && model.CodeLength >= 0)

    public class LookupTable
    {
        public int Id { get; set; }

        [Required(ErrorMessage = "Code is required.")]<br />
        public string Code { get; set; }

        [Required(ErrorMessage = "Description is required.")]
        public string Description { get; set; }
        public int? ForeignKeyId { get; set; }
        public string ForeignKeyValue { get; set; }
        public DateTime? CreatedDate {get;set;} 
        public string CreatedBy {get;set;}       
        public int CodeLength { get; set; }
    }

        protected override IEnumerable<System.Web.Mvc.ModelValidator> GetValidators(System.Web.Mvc.ModelMetadata metadata, ControllerContext context, IEnumerable<Attribute> attributes)
        { 
            if (metadata.Model is LookupTable)
            {
                LookupTable model = metadata.Container as LookupTable;                

                if (metadata.PropertyName == GetPropertyName(() => model.Code) && (model.CodeLength > 0))
                {
                    var newAttributes = new List<Attribute>(attributes);
                    var stringLength = newAttributes.OfType<StringLengthAttribute>().FirstOrDefault();
                    if (stringLength != null)
                    {
                        newAttributes.Remove(stringLength);

                        if (model.CodeLength != 0)
                        {
                            newAttributes.Add(new StringLengthAttribute(model.CodeLength)
                            {
                                MinimumLength = model.CodeLength,
                                ErrorMessage = @"The field {{0}} length must be at least {model.CodeLength}."
                            });
                        }

                        attributes = newAttributes;
                    }
                }
            }            

            return base.GetValidators(metadata, context, attributes);
        }