Method chaining with short-circuit parameter evaluation

[Nemanja Trifunovic]

Out of curiosity, what would you do in the else section of such a call?

utf8-cpp

[AspDotNetDev]

There are a few ways you could do this, but one thing I've been meaning to write a tip/trick on (as soon as I figure out how it can be done) is the expression tree approach. You'd do this:

var result = SafeChain(A.b.c.d.e.f.g.h.i.j.k);

The parameter would be passed as an expression tree, which would then be evaluated in steps, making sure to check for nulls along the way. The first null would cause null to be returned, otherwise the result value would be returned. I imagine you could use the same approach with your method chaining / expression trees.

Thou mewling ill-breeding pignut!

[szukuro]

In C# you can implement this k<ind of chaining using extension methods and expressions trees. The extension method (somewhat combined version of the two above):

    public static U NotNull(this T myObject, Expression expression) 
        where U : class
    {
        if (myObject == null)
            return null;
        else
        {
           var func = expression.Compile();
            return func();
        }
    }

Usage (returns either null if there's a null in the chain, or the value of MySubProperty, with almost the same syntax as above):

var value = myObject.NotNull(() => myObject.MyProperty).NotNull(() => myProperty.MySubProperty);

The only downside I found that in this case you have to declare a variable of type MyProperty for use in the second lambda expression. Maybe there's a way around it, but I didn't manage to find one as of yet. That can be changed though if the input remains myObject and the expression consists of the full path i. e. () => myObject.MyProperty.MySubProperty.

[OriginalGriff]

Chris Maunder wrote:

Am I procrastinating?

Yep! But then, you probably don't want to fix RootAdmin - it's down according to http://www.downforeveryoneorjustme.com/rootadmin.com[^]

If you get an email telling you that you can catch Swine Flu from tinned pork then just delete it. It's Spam.

[szukuro]

Actually to be passed as an expresion tree you'd have to write:

var result = SafeChain(**() =>**A.b.c.d.e.f.g.h.i.j.k);

But I agree that this is the way to do it.

[Brisingr Aerowing]

public static U NotNull<T, U>(this T myObject, Expression<Func<T, U>> expression) where U : class
{

if (myObject == null)
{
    return null;
}

try
{
    var func = expression.Compile();
    return func(myObject);
}
catch (Exception)
{
    return null;
}

}

Based off of a suggestion above. I tested this, and it seems to work.

Bob Dole

The internet is a great way to get on the net.

:doh: 2.0.82.7292 SP6a

[Andrew Rissing]

Maybe this IS the fix for the site.

[Brisingr Aerowing]

I figured out something that works. See my message below. It is based off of yours, with an extra parameter for the expression.

Bob Dole

The internet is a great way to get on the net.

:doh: 2.0.82.7292 SP6a

[Richard Deeming]

If only there was some sort of site where people could write articles on exactly this issue[^]! ;P

---

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[Paul Conrad]

You could just toss a :thumbsup: in place of a 5 vote.

"I've seen more information on a frickin' sticky note!" - Dave Kreskowiak

[Septimus Hedgehog]

Have you looked at this CP article by Dimitri Nesteruk[^] It looks very useful.

"I do not have to forgive my enemies, I have had them all shot." — Ramón Maria Narváez (1800-68). "I don't need to shoot my enemies, I don't have any." - Me (2012).

[AspDotNetDev]

Yes! :thumbsup:

Thou mewling ill-breeding pignut!

[AspDotNetDev]

You were beat to the punch. :)

Thou mewling ill-breeding pignut!

[Gary Wheeler]

In C++ you could do this with an 'instance' call like this:

if (this != NULL)
{
return this->real_stuff;
}
else
{
return default_stuff;
}

As others have mentioned, in C# you can do this with extension methods:

namespace Test
{
public class Widget
{
}
public static class WidgetExtensions
{
static public void Method(this Widget _this)
{
if (_this != null)
{
Console.WriteLine("real widget");
}
else
{
Console.WriteLine("null widget");
}
}
}
class Program
{
static void Main(string[] args)
{
Widget widget = null;

        widget.Method();
        
        widget = new Widget();
        
        widget.Method();
    }
}

}

results in the output

null widget
real widget

Software Zen: delete this;

[Jorgen Andersson]

That settles it for me, I want voting back. :thumbsup:

People say nothing is impossible, but I do nothing every day.

[Jorgen Andersson]

System.DBNull is an example of the pattern. And I can't describe in words how much I dislike it.

People say nothing is impossible, but I do nothing every day.

[Septimus Hedgehog]

So I was. Great minds think alike but in my case I'll pass up on the citation. ;) Hopefully it'll convince Muppet Maunder to read it and see if helps him.

"I do not have to forgive my enemies, I have had them all shot." — Ramón Maria Narváez (1800-68). "I don't need to shoot my enemies, I don't have any." - Me (2012).

[Chris Maunder]

I am humbled and ashamed. And have just found an endless opportunity for way, way more procrastination.

cheers, Chris Maunder The Code Project | Co-founder Microsoft C++ MVP

[Chris Maunder]

I respect a man who prefers power drills and small explosives where others would wimp out and use tweezers. :thumbsup:

cheers, Chris Maunder The Code Project | Co-founder Microsoft C++ MVP

[Chris Maunder]

Else? There is no else. Just blindly plow on!

cheers, Chris Maunder The Code Project | Co-founder Microsoft C++ MVP