Chaining overloaded constructors

Question

I am trying to create an efficient class with minimum code-duplication.

I have this defined:

Public Class Foo
    Private _firstName as string = ""
    Private _lastName as string = ""

    Public Sub New(ByVal userGUID As Guid)
        'query DB to get firstName and lastName
        Me.New(dt.Rows(0)("FirstName").ToString(),dt.Rows(0)("LastName").ToString())
    End Sub

    Public Sub New(ByVal firstName As String, ByVal lastName As String)
        _firstName = firstName.toUpper()
        _lastName = lastName.toUpper()
        Validate()
    End Sub

    Private Sub Validate()
        ' Throw error if something is wrong
    End Sub
End Class

The Constructor with firstName and lastName parameters is the end-point constructor that does validation. A constructor with userGUID as a parameter would query DB to obtain name and call the final constructor. This way all execution should be directed towards one of the constructors that actually does all validation etc etc. The idea behind it is that if I add new contructors, I only have to pull necessary data (firstname/lastname) and call the final constructor that does validation.

However, there is a compilation error preventing me from using this system on line Me.New(dt.Rows(0)("FirstName").ToString(),dt.Rows(0)("LastName").ToString()). Apparently this line has to be the first line in the constructor. But If I have this as first line, it will break the validation process because validation will throw an error due to no firstname/lastname. I have to query the DB in order to pull that info.

I am aware that I can assign values here and call validation from this constructor too, but this will effectively isolate this constructor from the final one, thus duplicating code and adding to maintenance a bit. FYI, in the example below I only have 2 constructors, but in reality i have several more. If each will do its own assignment it just adds up to maintenance that much.

So, is there a way to achieve my task by executing some code and THEN calling an overloaded constructor?

Thank you for any insight

UPDATE 1:

Per the_lotus comment, I am including dt definition. There is a workaround for this issue. Basically I would take the validation and assignment out of the final constructor and put it into a function. All constructors would call this function, thus eliminating the need to chain constructors. It doesn't look bad, but I would like to understand why in order to chain constructors I have to put constructor calls on the first line.

Here is new code:

Public Class Foo Private _firstName As String = "" Private _lastName As String = ""

Public Sub New(ByVal userGUID As Guid)
    Dim dt As New DataTable
    ' query DB to get firstName and lastName
    ' Assume I populate dt with at least one DataRow
    AssignAndValidate(dt.Rows(0)("FirstName").ToString(), dt.Rows(0)("LastName").ToString())
    'Me.New(dt.Rows(0)("FirstName").ToString(), dt.Rows(0)("LastName").ToString())
End Sub

Public Sub New(ByVal firstName As String, ByVal lastName As String)
    AssignAndValidate(firstName, lastName)
End Sub

Private Sub Validate()
    ' Throw error if something is wrong
End Sub

Private Sub AssignAndValidate(ByVal firstName As String, ByVal lastName As String)
    _firstName = firstName.ToUpper()
    _lastName = lastName.ToUpper()
    Validate()
End Sub

End Class

One curious not to mention: online code converters (vb.net to C#) have no issues converting chained constructor calls NOT on the first line. The C# code comes back as this.#ctor(dt.Rows(0)("FirstName").ToString(), dt.Rows(0)("LastName").ToString()); However, If I try to convert back to VB.NET, it fails.

Answer 1

What you are looking is a factory method

Public Class Foo 

    Public Shared Function GetFooFromGuid(ByVal userGUID As Guid) As Foo

        ' Query db

        return New Foo(dt.Rows(0)("FirstName").ToString(), dt.Rows(0)("LastName").ToString())
    End Function

End Class

Or an initialization function

Public Class Foo 

    Public Sub New(ByVal userGUID As Guid)
        ' query DB to get firstName and lastName
        Initialize(dt.Rows(0)("FirstName").ToString(), dt.Rows(0)("LastName").ToString())
    End Sub

    Public Sub New(ByVal firstName As String, ByVal lastName As String)
        Initialize(firstName, lastName)
    End Sub

    Private Sub Initialize(ByVal firstName As String, ByVal lastName As String)
    End Sub

End Class

Personally, I wouldn't call the database inside a New.

Answer 2

What I don't like is the fact that you accessing DB on constructor, and also that you validate in constructor. I see this as design issue. Below there are 3 examples of overloaded constructors. All three work. You may need #3. Init your dt in a static (vb - shared) method. You can also substitute your fname/lname parameters with one parameter that contains both. And that will work with #3 for you

public class A
{
    public A() : this ("xxx")
    {

    }
    public A(string x)
    {

    }
}

public class A
{
    public A() 
    {

    }
    public A(string x): this ()
    {

    }
}

public class A
{
    public A() : this(GetXxx())
    {

    }
    public A(string x)
    {

    }

    private static string GetXxx()
    {
        return "xxx";
    }
}

Why constructor chaining? Because your object can have default values in many properties and you may have many constructors, each adding a property. Internally, one constructor may set 5 properties and other 4 constructors set only 1 property.

For example:

public class Door
{
    private string _material = "wood";
    private int _locks = 1;
    private int _hinges = 3;

    public Door()
    {

    }
    public Door(int locks) : this()
    {
        _locks = locks;
    }
    public Door(int locks, int hinges) : this(locks)
    {
        _hinges = hinges;
    }
}