How to Declare an ID Value Object in C#

cdomain-driven-design

For some more type safety I wanted to introduce a PageId type in my application. However, I am not totally sure what the best option to do so is.

I want the IDs to be GUIDs, so I thought I just create a PageId class which inherits from the Guid class. Unfortunately the Guid is a struct, which does not support inheritance. I think that would have been the nicest solution, because it would be a value type and easy to map using ORMs, but it seems that this solution is of the table.

Now my question is how should it look instead? I can think of a few options:

Build a PageId struct that contains a Guid property (I guess that can be hard to use in code at times)
The same as 1., but using a class instead of a struct
Using the new record type

Or is there another obvious option I am overlook?

Best Answer

As you already mentioned for newer C# version you can wrap Guid value with the record type. You will get required equality functionality for "free".

public record PageId(Guid Value);

For older C# versions you can wrap Guid with the dedicated struct type, but then you need to implement equality by yourself.
For multiple types you can accept possible "duplication", it would be "safe duplication", because implementation will not change.

public readonly struct PageId
{
    public static PageId New() => new PageId(Guid.NewGuid());

    public Guid Value { get; }

    public PageId(Guid value)
    {
        Value = value;
    }

    public override bool Equals(object obj)
    {
        return obj is PageId && Equals(identifier);
    }

    public override int GetHashCode()
    {
        return Value.GetHashCode();
    }

    public bool Equals(PageId identifier)
    {
        return Value == identifier.Value;
    }

    public static bool operator ==(PageId first, PageId second)
    {
        return first.Equals(second);
    }

    public static bool operator !=(PageId first, PageId second)
    {
        return first.Equals(second) == false;
    }
}

You can introduce some helper extension methods

public static class PageExtensions
{
    public static PageId AsPageId(this Guid id) => new PageId(id);

    public static PageId? AsPageId(this Guid? id) => id?.AsPageId();
}

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C# Web Development – Sharing Methods and Properties Between Custom Web Controls

As I can't use true multiple inheritance in C#, how would I design this to be able to share the methods and properties?

This is the right way to share the methods and properties. But:

The trouble now is that in my extendedTextBox class I can't cast my extendedTextBox as a 'BaseExtendedControl' to use the extension methods, as the extendedTextBox inherits from the standard TextBox class like this:
public class ExtendedTextBox : TextBox, IExtendedControl
so there's no common base to cast ExtendedTextBox as 'BaseExtendedControl' as it doesn't inherit from it.

Yes, you're right there is no common class, but you can solve the problem by casting to the IExtentedControl interface which is common to your classes and contains the methods that you need. So what about:

public static class ExtendedControlHelper
{
    public static void AddRequiredFieldValidator(this IExtendedControl control)
    {
        control.rfv = new RequiredFieldValidator();
        control.rfv.ErrorMessage = control.RequiredMessage;
        ConfigureAndAddValidator(control, control.rfv);
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    public static void ConfigureAndAddValidator(this IExtendedControl control, BaseValidator validator)
    {
        validator.ControlToValidate = control.ID;
        validator.Display = ValidatorDisplay.Dynamic;
        validator.CssClass = "validationMessage ";
        validator.CssClass += control.ValidatorCssClass;
        validator.EnableClientScript = control.ClientScript;
        control.Controls.Add(validator);
    }
}

I don't think you need to implement the IExtendedControl interface in the ExtendedControlHelper class. Moreover I corrected the grammar of the interface name from IExtendendControl to IExtendedControl.

EDIT :

Another solution could be to use the dynamic keyword. The dynamic keyword is like object but the type is checked at run time instead of compile time:

public static class ExtendedControlHelper
{
    public static void AddRequiredFieldValidator(this dynamic control)
    {
        //...
    }

    public static void ConfigureAndAddValidator(this dynamic control, BaseValidator validator)
    {
         //...
    }
}

The dynamic type doesn't exist at run time because it's solved by the compiler, so if you write this: myExtendedTextBox.AddRequiredFieldValidator(); the method is solved as

//...
public static void AddRequiredFieldValidator(this ExtendedTextBox control)
{
    //...
}

and so on. So the type can change continuously at run time.

For more informations give a look here.

C# Algorithms – Find Index of Item Closest to Value but Less Than Value

This calls for a straightforward binary search - only instead of returning failure when the bounds converge without a hit, you return the next lowest index. (Unless the total number of values is so small that it easily fits into memory - then a complete cache is the way to go.)

Best Answer

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