Question

I am trying to port some code from Delphi to C# and I have found a construction which I cannot implement in a reasonable manner, while complying with .NET Framework design guidelines (which I address at the end of my question).

Obviously C#, Java, C++ (and many other languages) provide meanings of method/constructor overloading, but Delphi constructors can additionally have multiple names. This way it’s possible to write code which directly represents the intent:

var
  Data, ParI, ParD, Locl: TDataElement;
begin
  Data := TDataElement.Create('Element');
  ParI := TDataElement.CreateParam('IntElement', 22);
  ParD := TDataElement.CreateParam('DblElement', 3.14);
  Locl := TDataElement.CreateLocal('LocalElement');
  // ... use the above objects ...
end;

Simplified code below:

unit DataManager;

interface

TDataElement = class
  FName: string;
  FPersistent: Boolean;
public
  constructor Create(AName: string);
  constructor CreateParam(AName: string; DefaultInt: Integer); overload;
  constructor CreateParam(AName: string; DefaultDouble: Double); overload;
  constructor CreateLocal(AName: string);
  property Name: string read FName;;
  property Persistent: Boolean read FPersistent;
end;

implementation

constructor TDataElement.Create(AName: string);
begin
  FName := AName;
  FPersistent := True;
  // ... other initialization ...
end;

constructor TDataElement.CreateParam(AName: string; DefaultDouble: Double);
begin
  Create(AName); 
  // ... use DefaultInt ...
end;

constructor TDataElement.CreateParam(AName: string; DefaultInt: Integer);
begin
  Create(AName); 
  // ... use DefaultDouble...
end;

constructor TDataElement.CreateLocal(AName: string);
begin
  Create(AName); 
  FPersistent := False;
  // ... other code for local (non-persistent) elements ...
end;

Specifically in C# constructor must have the same name as the class, so first I have tried to differentiate behavior with enumeration. Alas, I’ve stumbled upon several problems:

• first parameter in each constructor is the same type (ElementKind)
• constructors are not easily recognizable like in Delphi (Create vs. CreateParam vs. CreateLocal)
• extra care is required in subclasses of DataElement
• possibility of mistakes, like specifying ElementKind.DoubleParam and passing integer value
• an extra bool parameter is required to handle local elements

First attempt below:

public enum ElementKind
{
    Regular, IntParam, DoubleParam, Local
}

public class DataElement
{
    private string FName;
    public string Name { get { return FName; } }

    private bool FPersistent;
    public bool Persistent { get { return FPersistent; } }

    public DataElement(ElementKind kind, string name)
    {
        FName = name;
        // ugly switch :-(
        switch (kind)
        {
            case ElementKind.Regular:
            case ElementKind.IntParam:
            case ElementKind.DoubleParam:
                FPersistent = true;
                break;
            case ElementKind.Local:
                FPersistent = false;
                break;
        }
        // ... other initialization ...
    }

    public DataElement(ElementKind kind, string name, int defaultInt)
        : this(kind, name)
    {
        // ... use defaultInt ...
    }

    public DataElement(ElementKind kind, string name, double defaultDouble)
        : this(kind, name)
    {
        // ... use defaultDouble ...
    }

    // Redundant "bool local" parameter :-(
    public DataElement(ElementKind kind, string name, bool local)
        : this(kind, name)
    {
        // What to do when "local" is false ???

        // ... other code for local (non-persistent) elements ...
    }
}

public class Program
{
    public void Run()
    {
        DataElement data = new DataElement(ElementKind.Regular, "Element");
        DataElement parI = new DataElement(ElementKind.IntParam, "IntElement", 22);
        DataElement parD = new DataElement(ElementKind.DoubleParam, "DblElement", 3.14);
        DataElement locl = new DataElement(ElementKind.Local, "LocalElement");
    }
}

Then I have tried more object-oriented way to differentiate constructors by types, while keeping the same initialization code in Run() method:

public class ElementKind
{
    public class RegularElement
    {
        internal RegularElement() { /* disallow direct creation */ }
    }
    public class IntParamElement
    {
        internal IntParamElement() { /* disallow direct creation */ }
    }
    public class DoubleParamElement
    {
        internal DoubleParamElement() { /* disallow direct creation */ }
    }
    public class LocalElement
    {
        internal LocalElement() { /* disallow direct creation */ }
    }

    public static readonly ElementKind.RegularElement Regular = new RegularElement();
    public static readonly ElementKind.IntParamElement IntParam = new IntParamElement();
    public static readonly ElementKind.DoubleParamElement DoubleParam = new DoubleParamElement();
    public static readonly ElementKind.LocalElement Local = new LocalElement();
}

public class DataElement
{
    private string FName;
    public string Name { get { return FName; } }

    private bool FPersistent;
    public bool Persistent { get { return FPersistent; } }

    protected DataElement(string name)
    {
        FName = name;
        // ... other initialization ...
    }

    public DataElement(ElementKind.RegularElement kind, string name)
        : this(name)
    {
        FPersistent = true;
    }

    public DataElement(ElementKind.IntParamElement kind, string name, int defaultInt)
        : this(name)
    {
        FPersistent = true;
        // ... use defaultInt ...
    }

    public DataElement(ElementKind.DoubleParamElement kind, string name, double defaultDouble)
        : this(name)
    {
        FPersistent = true;
        // ... use defaultDouble ...
    }

    public DataElement(ElementKind.LocalElement kind, string name)
        : this(name)
    {
        FPersistent = false;

        // ... other code for "local" elements ...
    }
}

public class Program
{
    public void Run()
    {
        DataElement data = new DataElement(ElementKind.Regular, "Element");
        DataElement parI = new DataElement(ElementKind.IntParam, "IntElement", 22);
        DataElement parD = new DataElement(ElementKind.DoubleParam, "DblElement", 3.14);
        DataElement locl = new DataElement(ElementKind.Local, "LocalElement");
    }
}

Everything compiles and works very well. So what is my problem here? .NET Framework design guidelines, and a tool named Microsoft FxCop. After running the last code through this tool I got multiple breaking problems (see below).

And the question is: how to design my classes to comply with .NET design guidelines and best practices?

Breaking – Certainty 90% – Nested types should not be visible – ElementKind+RegularElement
Breaking – Certainty 90% – Nested types should not be visible – ElementKind+IntParamElement
Breaking – Certainty 90% – Nested types should not be visible – ElementKind+DoubleParamElement
Breaking – Certainty 90% – Nested types should not be visible – ElementKind+LocalElement

Breaking – Certainty 90% – Static holder types should not have constructors – ElementKind

Breaking – Certainty 75% – Identifiers should not contain type names – DataElement.#.ctor(ElementKind+IntParamElement,System.String,System.Int32)

Breaking – Certainty 75% – Identifiers should not contain type names – DataElement.#.ctor(ElementKind+DoubleParamElement,System.String,System.Double)

Non Breaking – Certainty 25% – Do not declare read only mutable reference types – ElementKind.#Regular

Non Breaking – Certainty 25% – Do not declare read only mutable reference types – ElementKind.#IntParam

Non Breaking – Certainty 25% – Do not declare read only mutable reference types – ElementKind.#DoubleParam

Non Breaking – Certainty 25% – Do not declare read only mutable reference types – ElementKind.#Local

Answer 1

For starters, I would replace your nested “ElementKind” class with an enum:

public enum ElementKind
{
    RegularElement,
    IntParamElement,
    DoubleParamElement,
    LocalElement
}

In addition, I don’t think your Delphi code needs to be mapped to a constructor. You would probably be better off using static factory methods that return a DataElement. For example:

public static DataElement Create(string name)
{
    return new DataElement(ElementKind.Regular, name);
}

public static DataElement CreateParam(string name, int defaultInt);
{
    return new DataElement(ElementKind.IntParam, name);
    // ... use defaultInt ... 
}

// similar to above
public static DataElement CreateParam(string name, double defaultDouble); 
public static DataElement CreateLocal(string name);

Since you’re using the factory functions to create your DataElement objects, you should make the constructors private.

You would then update your Run() function to use these as follows:

public void Run() 
{ 
    DataElement data = DataElement.Create("Element");
    DataElement parI = DataElement.CreateParam("IntElement", 22);
    DataElement parD = DataElement.CreateParam("DblElement", 3.14); 
    DataElement locl = DataElement.CreateLocal("LocalElement"); 
} 

Update: I included the suggested changes to your Run() function and corrected the basic Create() factory method (I believe it’s supposed to return a “Regular” DataElement).