How to Write Interfaces for Static Classes in C#

cinterfacesstatic

I'm writing a postcode validation library, so that I can call a helper method

var result = Postcode.IsValid(postcode, country)

To that end I need to have 'classes' that represent supported countries and know how to validate each. At present I have an interface thus:

public interface IPostcode {
    bool IsValid(string postcode);
}

and I have to have classes for each country, e.g.

public class USA : IPostcode {
  public bool IsValid(string postcode) {
     // ... validate here
  }

The helper method selects the relevant IPostcode based on the country code.

The issue is that it feels wrong to have to instantiate classes that have no state or properties, just methods which would be far better if they were static. But of course static classes can't have interfaces. Is there a better pattern for this?

Best Answer

I don't think there is anything wrong with having classes without state. As long as the classes have different behavior (different postcode validation logic) they are justified.

You could kind-of achieve the same thing with static methods and using a delegate signature rather than an interface (since the interface is only a single function signature anyway) but I think using classes are more natural and straightforward.

Related Solutions

C# Validation – Best Way of Validating Class Properties

If I needed to input data and capture errors, I would probably do something more like:

public class Employee
{
    public int EmpID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Salary { get; set; }
}

public class ErrorEmployee : Employee
{
    public string[] ErrorReason { get; set; }
}

Read CSV file row by row
For each element i.e EmpId, FirstName... etc do the required validations by calling individual methods having validation logic. eg: public bool ValidateIsDecimal(string value) { } public bool ValidateIsEmpIdExists(string value) { } etc
If valid, fill corresponding property of "Employees" class. Add this Class to Employees collection (eg: List)
If NOT VALID, fill "ErrorReason" property, with appropriate reason as to what caused the validation to fail.(eg: Required filed was missing or datatype is not decimal etc) Add this Class to ErrorEmployees collection (eg: List)

I wouldn't want to pollute the regular Employee (singular, use the plural for a collection of them) object with error messages. As in the note above, check out the FluentValidation library for your validation and capture a list of errors rather than just the first found.

C# – Forcing Derived Class to Implement Static Method

Such methods shouldn't be static (what if you have to deal with two databases?) and they also shouldn't be on your entity classes.

Instead, you should have a separate type, that represents the storage for this entity. That type could be generic, so that code that's not entity-specific doesn't have to be repeated.

This way, inheritance works (storage for ClassA can inherit from some base storage) and you don't have to perform the nonsensical "I have to create a person to get a list of all persons".

This pattern is for example used by Entity Framework, there the storage type is called DbSet<T>.

Best Answer

Related Solutions

C# Validation – Best Way of Validating Class Properties

C# – Forcing Derived Class to Implement Static Method

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