named constructor idiom in c#

pwd

is there an equivalent for this in c#?

ie: a way of differentiating between different constructors for the same class with the same parameter sets

I'm using passing the parameters in different orders / passing dummy parameters for the moment until I find a neater way of doing it.

Ginger

Overload the constructor as required.. or have you the same type of paramaters for each constructor you need?

pwd

Yeah exactly that - the same same type of parameters

eg:

public constructor(int a, string b){...}
public constructor(int c, string d){...}

Ginger

Function signature will be the same so unless you use a common constructor with enums or a different name its not possible

pwd

wonder why they didn't make this possible in c#.

You can do it in c++ as described here:
http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.8

Ginger

Ahh ok

try

public functionname:this

Its the this bit that allows you to do it

pwd

I don't follow what you are saying
I'm not trying to reuse code between the two constructors.

Malice

Could you re-arrange your code so that you only need to use one constructor?

Something like:

[PHP]
public constructor(int a, string b)
{
...
if (b == "Something")
{
// Do stuff
}
else if (b == "Something Else")
{
// Do other stuff
}
...
}
[/PHP]

pwd

Not in this case.
The context is this:
I have a class with three constructors:

myclass() //used for a new instance not loaded from db
myclass(Guid tbl1_id) //used when passed Guid for a db table (loads data from db)
myclass(Guid tbl2_id) //used when passed Guid from different db table (loads data from db)

At the moment I work around it by using a dummy parameter to differentiate the latter two classes, but I thought there should be a more elegant method.

Eoin

Can you not just pass in an extra parameter that identifies the table you are loading the data from?

Malice

pwd wrote: »

Not in this case.
The context is this:
I have a class with three constructors:

myclass() //used for a new instance not loaded from db
myclass(Guid tbl1_id) //used when passed Guid for a db table (loads data from db)
myclass(Guid tbl2_id) //used when passed Guid from different db table (loads data from db)

At the moment I work around it by using a dummy parameter to differentiate the latter two classes, but I thought there should be a more elegant method.

Okay, how about defining the constructor so that it always expects a GUID. Something like this:

[PHP]
public constructor(int a, Guid b)
{
...
if (b == null)
{
// Do stuff
}
else if (b == "Table_1_GUID")
{
// Do Table 1 stuff
}
else if (b == "Table_2_GUID")
{
// Do Table 2 stuff
}
...
}
[/PHP]

stevenmu

Do you really need to do that load in the constructor, couldn't you just have one constructor and then LoadFromTableA and LoadFromTableB methods?

pwd

the constructors are very different
the guids are the primary keys of records in db tables so the if statement outlined above wouldn't work.
the different methods and a parameter indicating which one would work.

Malice

pwd wrote: »

the constructors are very different
the guids are the primary keys of records in db tables so the if statement outlined above wouldn't work.

I don't see how the if statements wouldn't work, especially if you used the LoadFromTableA(), LoadFromTableB() methods as Stevenmu suggested. Of course, I'm not looking at your code so I could be mis-understanding the scenario .

pwd wrote:

the different methods and a parameter indicating which one would work.

I don't understand what you're trying to say here.

Eoin

malice_ wrote: »

I don't understand what you're trying to say here.

I think he means what I suggested:

myclass(tablename, GUID)

Evil Phil

I would say just pass in the tablename or an enum specifying the table.

bonkey

Define classes which extend the original class, and override the constructor?

so as well as a MyClass, you'd have a Guid1MyClass and a Guid2MyClass. Assign them to variables of type MyClass, and Robert is the brother of your father.

No?

ETA: Not coming from a C++ background, I find the idea of named constructors inelegant. Subclassing seems (to me) to be a more OO way of dealing with the problem

Ginger

Sorry forgot about this.. subclassing is the best option in C# for this