I am using two windows form application, and I want to set 1st form label's value from 2nd form.
But when I access 1st form label in 2nd form then application show this error
Object reference not set to an instance of an object.
I am using this statement to access
login_form.ActiveForm.Controls["label_name"].Text = "Hello World";
sometime i worked fine but some time show this error
Please solve my problem. I will be very thankful to you.
You would be wise to either:
Expose a property in the first form through which you can enact a change to the label text.
Expose a method in the first form that you can call to affect a label text change.
Example #1:
public class Form1 : Form
{
public String LabelText
{
get { return label_name.Text; }
set { label_name.Text = value; }
}
}
//from Form2...
login_form.LabelText = "Hello World";
Example #2:
public class Form1 : Form
{
public void SetLabelText(String TextToSet)
{
label_name.Text = TextToSet;
}
}
//from Form2...
login_form.SetLabelText("Hello World");
I would not advise simply changing the control to be Public. Indirect access is preferable.
Related
I have a windows Forms application with one form and a few classes.
I want to get the values of some textBoxes from the Form1 instance and extract the values.
My first way of achieving this was by using Application.OpenForms[] array to get the form but I realised that using a singleton on the class Form1 would be more efficient as I can have direct access and it would be impossible to make other instances.
Here is how I have set it up:
1. Controls class to gets controls from Form1
class Controls
{
//Request Form1 instance
private static Form1 form = Form1.GetInstance();
//Sets global values for easy access with getters and null setters
//--Variable 'form' is still null hence I get the NullReferenceException
private TextBox employer = form.Controls["textBoxEmployerName"] as TextBox;
private TextBox role = form.Controls["textBoxRole"] as TextBox;
private TextBox company = form.Controls["textBoxCompanyName"] as TextBox;
private TextBox website = form.Controls["textBoxWebsite"] as TextBox;
private TextBox refNumber = form.Controls["textBoxRefNumber"] as TextBox;
private TextBox reason = form.Controls["textBoxReason"] as TextBox;
private TextBox dateListed = form.Controls["textBoxDateListed"] as TextBox;
private Label charLimit = form.Controls["labelCharsRemaining"] as Label;
public TextBox Employer { get { return employer; } }
public TextBox Role { get { return role; } }
public TextBox Company { get { return company; } }
public TextBox Website { get { return website; } }
public TextBox RefNumber { get { return refNumber; } }
public TextBox Reason { get { return reason; } }
public TextBox DateListed { get { return dateListed; } }
public Label CharLimit { get { return charLimit; } }
}
}
2. Singleton set up inside class Form1
public partial class Form1 : Form
{
private static Form1 theInstance;
public Form1()
{
InitializeComponent();
}
//Return instance of Form1
//--This is obviously returning null for some reason
public static Form1 GetInstance()
{
if (theInstance == null)
theInstance = new Form1();
return theInstance;
}
As you can probably see I am getting the "NullReferenceException" when I attempt to get the Singleton from class Form1.
The following methods I have used are as follows:
Using Windows.OpenForms["Form1"].Controls["--somecontrol--"]
Using Windows.ActiveForm
Using a Singleton Design Pattern on class Form1
All of these ways are returning null and I cant think of a reason why it is returning null.
Any help would be appreaciated.
Thankyou
I want to get the values of some textBoxes from the Form1 instance and extract the values.
This is where you need to stop and re-think your approach. Forms represent views of your data; however, your data itself needs to be in the model, a separate place independent of the views.
Text boxes need to reflect the state of some model object, such as a Person object that has string properties for employer, company, role, web site, and so on. The form would read from that object's properties, display them in a text box, and then react to text box changes, and save values back to the model Person object.
If you make Person a singleton, or provide some other universal way of accessing it, you would be able to access person's properties from all forms, without accessing the forms themselves.
I am a complete amateur at programming and am trying to learn proper code design. I`m trying to learn how to use accessors but can't seem to get it to run properly. This is what I am trying to run:
class Classy
{
public void changeLine(string change)
{
Form1 form1 = new Form1();
form1.Line = change;
}
}
public partial class Form1 : Form
{
string line = "";
public string Line
{
get
{
return line;
}
set
{
line = value;
}
}
private void button2_Click(object sender, EventArgs e)
{
line = "";
string change = "cake";
Classy classy = new Classy();
classy.changeLine(change);
MessageBox.Show(line);
}
When I click the button, it shows blank text. From what I understand these are the steps it take:
variable change with "cake" value is passed to classy's changeLine method.
changeLine method sets Form1 line to variable change with "cake" value.
MessageBox displays line variable of "cake" value.
Why does this not work?
This is happening because you are creating a new instance of Form on the changeLine method. This new instance is different from the one the event was fired, or the button was clicked.
In order to use the correct Form reference, you may pass the actual reference as an argument:
public void changeLine(Form1 form, string line) {
form.Line = line;
}
You would call this method (from the form) like this:
classy.changeLine(this, change);
this is the current Form1 instance, it is the one you want to modify. You don't want to create a new one.
Okay. You've got a Form1 object and the window it represents and controls is displayed on screen. You click a button and it invokes a method.
line = "";
This line is a private field which happens to be the backing field to the Line property. At this point accessing either the line field or the Line property will return "".
string change = "cake";
A string local to this method.
Classy classy = new Classy();
Okay, we've a new object of type Classy called classy.
classy.changeLine(change);
Let's look at what this call does:
Form1 form1 = new Form1();
You now have a different Form1 object. If you called form1.Show() you would now have two windows on the screen`.
form1.Line = change;
Sets the Line property (and hence the line field) of this new, different Form1.
We now return back to the calling method:
MessageBox.Show(line);
Shows the value of the line field of the original Form1 object.
To demonstrate a property being set from outside the class you could do something like:
class Classy
{
public void changeLine(Form1 form1, string change)
{
form1.Line = change;
}
}
public partial class Form1 : Form
{
string line = "";
public string Line
{
get
{
return line;
}
set
{
line = value;
}
}
private void button2_Click(object sender, EventArgs e)
{
string change = "cake";
Classy classy = new Classy();
classy.changeLine(this, change);
MessageBox.Show(line);
}
}
Here because the Form1 passes itself (this refers to the object of the method running) to classy its own Line property will now be used.
Alternatively:
class LineStorer
{
private string _line;
public string Line
{
get { return _line; }
set { _line = value; }
}
}
public partial class Form1 : Form
{
private void button2_Click(object sender, EventArgs e)
{
var storer = new LineStorer();
storer.Line = "cake";
MessageBox.Show(storer.Line);
}
}
Here the form is first setting and then getting a property in another object.
Note also that objects can use their own properties instead of the backing field:
public partial class Form1 : Form
{
private string _line;
public string Line
{
get { return _line; }
set { _line = value; }
}
private void button2_Click(object sender, EventArgs e)
{
Line = "cake";
MessageBox.Show(Line);
}
}
From the inside, so to speak, there's really not much difference whether one uses a field or a property, but using properties does have an advantage in that if some day you changed from this simpler property (read and write from and to a field and do nothing else) to one that is more complicated (common cases including validation checks on all setting, but you are far from limited to that) then you'd only have to change the one place where the property is defined rather than the, perhaps very many, places where it is used.
(It may seem like using Line is more work than just using line, because it does after all call into more code, and therefore using properties rather than fields would be a slight inefficiency that would add up over time. Luckily the jitter is smart enough to "inline" simple getters and setters when the code is run, so there is in fact no cost to them).
What you do is what you get. You want to show the value of line, but you never set it to another value before showing it. Whatever you do in these three lines of code has nothing to do with changing the value of line IN THIS PARTICULAR INSTANCE OF THE CLASS FORM1.
string change = "cake";//You declare and initialize a variable
Classy classy = new Classy();//You create an instance of class Classy
classy.changeLine(change);//You invoke a method in that class that has no clue of what is going on this side
Yes, it does set the value of line to "cake", but on ANOTHER OBJECT. But you could still get the behavior you need in at least two ways:
You could have an instance variable in the Classy class of type Form1 and have a constructor that would receive a Form1 object as a parameter. Then in your changeLine method, instead of creating a brand new Form1 object you could change the line of that particular instance variable, like this:
public class Classy{
private Form1 _form1;
public Classy(Form1 form1){
_form1=form1;
}
public void changeLine(string change){
_form1.Line=change;
}
}
You could change your changeMethod to accept an additional parameter of type Form1 and then pass it this when invoking it inside Form1
public void changeLine(string change,Form1 form1){
form1.line=change;
}
And on the Form1 side:
classy.changeLine("cake",this);
IDE: Visual Studio, C# .net 4.0
I have two identical user control uc1 and uc2 and both are having a textbox called txtbox1
now see this code and textbox1 is public in designer so it is assessable in form1.cs, Form 1 is simple windows form which is having uc1 and uc2.
In form1 see this function which i am calling in onLoad_form1 method.
UserControl currentUC; //Global variable;
public void loadUC(string p1)
{
//Here I want:
if(p1 == "UC1)
{
currentUC = uc1;
}
if(p1 == "UC2)
{
currentUC = uc2;
}
}
Than another function which calls update the textbox1 based on currentUC value
//on load
currentUC.textbox1.text = "UC1 called";
//Here I am getting error "currentUc does not contains definition for textbox1"
If i do:
uc1.textbox1.text = "UC1 text";
uc2.textbox1.text = "UC1 text"; //it works, But based on p1 string variable I want to make control as uc1 or uc2 than I want to access its child control. please suggest how to perform this.
please don't tell if else blocks, because This functionality I have to use in various places.
Thanks.
#Lee Answer: - works just for textbox, but I am having two usercontrols i.e. two different usercontrols not instance of it. UserControlLeft and UserControlRight and both are having same textboxes, listboxes etc (with minor design changes), and I want to access/load this based on some string "left" and "right".
Since the textboxes have the same name you can look them up in the Controls collection:
TextBox tb = (TextBox)currentUC.Controls["textbox1"];
tb.Text = "UC1 called";
a better solution would be to add a property to your user control class which sets the internal text property e.g.
public class MyUserControl : UserControl
{
public string Caption
{
get { return this.textbox1.Text; }
set { this.textbox1.Text = value; }
}
}
I think you're mixing a couple of things here.
First of all, you say that you have 2 exactly the same usercontrols, do you mean the ascx files are the same, or that you have 2 instances of the same usercontrol on the page?
Let's go with all the valid options:
1. To find a control and cast it:
Assume you have the following aspx snippet:
<div>
<uc1:MyCustomUserControl id="myControl" runat="server" />
<uc1:MyCustomUserControl id="myControl2" runat="server" />
</div>
If you now want to access the control, you should do the following:
public void Page_Load()
{
var myControl ((MyCustomUserControl)FindControl("MyControlName"));
// On 'myControl' you can now access all the public properties like your textbox.
}
In WPF you can do it like this:
//on load MAINFORM
public void SetText(string text)
{
CLASSOFYOURCONTROL ctrl = currentUC as CLASSOFYOURCONTROL ;
ctrl.SetText(text);
}
// in your control SUB
public void SetText(string text)
{
textbox1.text = "UC1 called"
}
i think this should work in winforms also. And is more clean than accessing the controls from your sub-control directly
#Lee's method is good. Another method will be to use a public property with a public setter (and textbox doesn't need to be public this way).
or an interface (this way you don't care what class you have at the given moment - and no ifs):
public interface IMyInterface
{
void SetTextBoxText(string text);
}
public partial class UC1: UserControl, IMyInterface
{
public void SetTextBoxText((string text)
{
textBox1.Text=text;
}
//...
}
public partial class UC2: UserControl, IMyInterface
{
public void SetTextBoxText((string text)
{
textBox1.Text=text;
}
//...
}
using the code:
((IMyInterface)instanceOfUC1).SetTextBoxText("My text to set");
((IMyInterface)instanceOfUC2).SetTextBoxText("My text to set");
I want to change text in textbox on parentform from childform.
I set textbox
modifiers= public
i have extra written a function in parentform
public TextBox txtbox
{
get
{
return mybox;
}
set
{
mybox= value;
}
}
in child form on writing
this.ParentForm. ( can't see mybox).
what i am missing.
regards,
Since ParentForm will return a Form and not your form, you need to cast it before you can access any of your custom properties:
((MyForm)this.ParentForm).textbox = "new text!";
Additionally, you are setting the whole control, not just the text.
Try this, to expose the text property only:
public string txtbox
{
get
{
return mybox.Text;
}
set
{
mybox.Text = value;
}
}
I think the problem is that ParentForm is of type Form which does not have a member txtbox. You need to cast ParentForm to your form (suppose it is Form1), like:
((Form1)this.ParentForm).txtbox
Random guess without seeing any actual code: mybox is likely not declared public.
Edit: Or, ah, yes, as Andrei says - you havn't cast the ParentForm to your parent form's type.
I have a method that executes inside one form, but I need to retrieve data from another form to pass into the method.
Whats the best way of doing this?
You can expose a property on one form and call it from the other. Of course you'll need some way of getting the instance of form1. You could keep it as a static property in the program class or some other parent class. Usually in this case I have a static application class that holds the instance.
public static class Application
{
public static MyForm MyFormInstance { get; set; }
}
Then when you launch the first form, set the application MyFormInstance property to the instance of the first Form.
MyForm instance = new MyForm();
Application.MyFormInstance = instance;
Add a property to the second form.
public String MyText
{ get { return textbox1.Text; }
set { textbox1.Text = value; }
}
And then you can access it from your second form with:
Application.MyFormInstance.MyText
On the form that has the textbox you need data from, expose either a Property or a Method that returns the text. IE:
internal string TextBoxTest
{
get{ return this.textBox1.Text;}
}
There is a similar post here
The videos below will clear up a lot of your concepts about passing data between 2 forms.
There are multiple ways to pass data between 2 forms check these links which has example videos to do this
FormToForm Using Properties - http://windowsclient.net/learn/video.aspx?v=108089
FormToForm Using Parameters - http://windowsclient.net/learn/video.aspx?v=105861
HTH
Assuming that formB is initialized in formA I would recommend adding a string to the constructor of formB sending the Texbox1.Text
as in
class formB: Form{
private string data;
public formB(string data)
{
InitializeComponent();
this.data = data;
}
//rest of your code for the class
}
Don't do this.
Longer version: Why is your view directly interacting with another view?
Much longer version:
Rather than making a public property that exposes the field, it would provide better encapsulation and insulation from change if the form with the field of interest interacted with some form of data object, which was then passed to the interested method.
The location of the interested method should be carefully considered - if it controls aspects of the view (WinForm, in your case), then it may be appropriately a member of that class - if not, perhaps its real home is in the data object?