Dynamically creating objects using reflection - C# Discussion Boards

17-Jan-07 17:01

Hi All,

I am trying to dynamically create object based on values that being submited from a form, but keep on getting a "No parameterless constructor defined for this object." error.

<br />
Type paramType = Type.GetType("System." + Request["type"]);  //The Type of object<br />
String paramValue = Request["val"]; //the value <br />
      <br />
          if (paramType != null)<br />
          {<br />
            Activator.CreateInstance(paramType);<br />
          }<br />

Phillip M. Hoff17-Jan-07 17:15

Phillip M. Hoff

17-Jan-07 17:15

What are the types that you are trying to create, and do they all have public, parameterless constructors?

-Phil

mcd242417-Jan-07 17:22

17-Jan-07 17:22

Hi Phil,

Right now I am just trying to create strings and ints, but will have to create other types of objects. All will be public but some will be parameterless constructors some will not.

Thanks,

tom

Phillip M. Hoff17-Jan-07 17:37

Phillip M. Hoff

17-Jan-07 17:37

The System.String type does not have a parameterless constructor (they all require at least one parameter), and therefore cannot be instantiated via the Activator.CreateInstance(Type) method.

-Phil

mcd242417-Jan-07 18:07

17-Jan-07 18:07

thanks Phil,

Would you mind showing me a quick example of how to create a string via reflection. This is what I tried using before but it allways returns null.

<br />
 Type[] ArgTypes = new Type[1] {Type.GetType("System.String") };<br />
 ConstructorInfo oConstructorInfo = paramType.GetConstructor(ArgTypes );<br />

thanks

Phillip M. Hoff17-Jan-07 19:17

Phillip M. Hoff

17-Jan-07 19:17

Here's one way to create the string "a" via reflection:

<br />
Type type = Type.GetType("System.String");<br />
<br />
object obj = Activator.CreateInstance(type, 'a', 1);<br />

In this case, the .NET runtime would use the System.String.String(char, int) constructor in order to create the instance because we passed a character and an integer to Activator.CreateInstance().

In the case of the .NET native types like strings and integers, however, if you already know what the value of the object will be, using reflection is overkill. There are far easier ways to translate between one native type and another.

-Phil

get folder content form a webservice

Russell Jones17-Jan-07 23:00

Russell Jones

17-Jan-07 23:00

You could Create a class called MyString

add a parameterless constructor and an implicit cast

public static implicit operator string (MyString A)
{
return A._internalStringVariable;
}
public static implicit operator MyString (String S)
{
return new MyString(S);
}

Then you'll have an object that behaves a bit like a string but has a parameterless constructor.

HTH

Russ

C# Dial-up manager

oren201017-Jan-07 15:49

oren2010

17-Jan-07 15:49

Hi,

As part of the application I am working on, I would like to allow the application to dial up to the closest regional office. Our company has servers in every major city in the US so basically the only thing that I need to change is the phone number of the dial-up connection that the application is using.

I have managed to connect/disconnect using an unmanaged c++ code that is based on WinINet(InternetDial, dll or using RASDial. The problem with RasDial is that it's not reliable (At least from my experience...) and the problem with the c++ function that I am using is that it doesn't let me specify the phone number...

At this point, the only option that might be viable is to use RasDialDlg. Has anyone ever tried to use this function with c#? If yes, do you have any code samples or tips you can share with me?

Regards,
Joe

gcorgnet17-Jan-07 15:40

gcorgnet

17-Jan-07 15:40

Hi,

I am trying to develop a webservice which will scan a given directory on the server and expose the result. I want to call this webservice from a WinForm application to populate a treeview.

Do you have any suggestions on the way I should do that ?

Thanks,

Guillaume

Re: get folder content form a webservice

mcd242417-Jan-07 17:10

Inconsistent Exceptions While Printing

17-Jan-07 17:10

Hi,

Not sure what part you need help with, but the easiest way to consume a webserivice is build a proxy class in vs.net. Just right click on the name of your project in the soloutions explorer and the select add webreference. Then enter the url to the WSDL of your webservice and click on the add web reference button. You can then call the methods of your webservice by doiing th following.

webreferenceNameyoucreatedabove.methodName();

Regards,

tom

mfgalizia17-Jan-07 14:29

17-Jan-07 14:29

Hello all,

Hopefully someone can help me solve this problem. I'm trying to print a PrintDocument, but I get inconsistent results. Sometimes it works, and sometimes I get "The handle is invalid" or "Incorrect parameter" exceptions. I stripped it down to the following code:

using System;
using System.Collections.Generic;
using System.Windows.Forms;
using System.Drawing;
using System.Drawing.Printing;

namespace PrintTest
{
    static class Program
    {
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            PrintDocument pd = new PrintDocument();

            PageSetupDialog psd = new PageSetupDialog();
            psd.Document = pd;

            if (psd.ShowDialog() == DialogResult.OK)
            {
                PrintPreviewDialog ppd = new PrintPreviewDialog();
                ppd.Document = pd;
                try
                {
                    ppd.ShowDialog();
                }
                catch (Exception e)
                {
                    MessageBox.Show(e.Message, "Exception");
                    ppd.Close();
                }
            }
        }
    }
}

Thanks in advance!

Luc Pattyn17-Jan-07 16:24

Luc Pattyn

17-Jan-07 16:24

I havent had any printing problems, except for the fact that some details are strange at least (where and when to set properties in order to choose color, landscape, duplex...).

But I strongly suggest you throughly process any exception; rather than looking at
exc.Message, you really should look at the multi-line exc.ToString()
it gives you, depending on the kind of exception, much more information, and also
a stack trace, so you can tell in exactly what line things fail.

Good luck !

Luc Pattyn

netJP12L18-Jan-07 4:53

netJP12L

18-Jan-07 4:53

try to comment pagesetupdialog object and just try working around with objects printpreviewdialog and printdocument

printPriewdialog ppd = new printpreviewdialog();
ppd.document=pd;
ppd.showDialog();

mfgalizia18-Jan-07 11:28

18-Jan-07 11:28

Thanks for the reply! If I don't use a PageSetupDialog the problem persists when I actually try to print. Just like before sometimes the document prints and sometimes I get exceptions.

TIA

mfgalizia21-Jan-07 12:45

21-Jan-07 12:45

I'm still getting exceptions, which actually come from the ShowDialog command. The document is rendered correctly, but if I try to print from the print preview dialog I get an exception.

Thanks in advance for any help on the subject!

mfgalizia21-Jan-07 13:23

21-Jan-07 13:23

For what it's worth, I think this is a bug somewhere in Microsoft's code. Even if I use the sample code provided on the Microsoft website (http://msdn2.microsoft.com/en-us/library/system.drawing.printing.printdocument.aspx) I still get invalid handle exceptions!

Luc Pattyn21-Jan-07 13:52

Luc Pattyn

21-Jan-07 13:52

Well, your previous post triggered my memory:

When I earlier said I did not have any printing problems, I was forgotten that I once
had major problems, if clicking the Prin button in the Preview Dialog.
This occurred some 2 years ago, when I added printing capabilities to my major application,
using either .NET 1.0 or 1.1 (without SP), I dont recall.
Since then I have been using my app and have printed and previewed hundreds of pages.

I do not recall what the exact problems were, but I concluded not to use that Print button
anymore.

I should also add I had some dificulties in understanding (or getting to work without
understanding) the printer settings regarding:
- page orientation
- color support
- duplex printing

And finally I have always been puzzled how one is supposed to do a Print Preview without
selecting a printer (with its margins, page orientation, etc). I would find it simpler
to have a print dialog with its normal choices, and both a Preview and a Print button.

Following your input I just finished a couple of experiments, and was unable to make
the Print Button on Print Preview Dialog fail; I have tried .NET 1.1 WITH SP1, as well as
NET 2.0 So maybe there has been a bug and it got fixed either in SP1 or in 2.0

What is your .NET version ? and the one your bug info refers to ?

Smile | :)

Luc Pattyn

mfgalizia21-Jan-07 14:11