Error accessing data on main form

I'm working on a C# program, and right now I have one Form and a couple of classes. I would like to be able to access some of the Form controls (such as a TextBox) from my class. When I try to change the text in the TextBox from my class I get the following error:

An object reference is required for the non-static field, method, or property 'Project.Form1.txtLog'

How can I access methods and controls that are in Form1.cs from one of my classes?

Replay

You are trying to access the class as opposed to the object. That statement can be confusing to beginners, but you are effectively trying to open your house door by picking up the door on your house plans.

If you actually wanted to access the form components directly from a class (which you don't) you would use the variable that instantiates your form.

Depending on which way you want to go you'd be better of either sending the text of a control or whatever to a method in your classes eg

public void DoSomethingWithText(string formText)
{
   // do something text in here
}

or exposing properties on your form class and setting the form text in there - eg

string SomeProperty
{
   get
   {
      return textBox1.Text;
   }
   set
   {
      textBox1.Text = value;
   }
}

Another solution would be to pass the textbox (or control you want to modify) into the method that will manipulate it as a parameter.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        TestClass test = new TestClass();
        test.ModifyText(textBox1);
    }
}

public class TestClass
{
    public void ModifyText(TextBox textBox)
    {
        textBox.Text = "New text";
    }
}

  1. you have to have a reference to the form object in order to access its elements
  2. the elements have to be declared public in order for another class to access them
  3. don't do this - your class has to know too much about how your form is implemented; do not expose form controls outside of the form class
  4. instead, make public properties on your form to get/set the values you are interested in
  5. post more details of what you want and why, it sounds like you may be heading off in a direction that is not consistent with good encapsulation practices

You need access to the object.... you can't simply ask the form class....

eg...

you would of done some thing like

Form1.txtLog.Text = "blah"

instead of

Form1 blah = new Form1();
blah.txtLog.Text = "hello"

If the form starts up first, in the form Load handler we can instantiate a copy of our class. We can have properties that reference whichever controls we want to reference. Pass the reference to the form 'this' to the constructor for the class.

public partial class Form1 : Form
{
    public ListView Lv
    {
        get { return lvProcesses; }
    }

    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        Utilities ut = new Utilities(this);
    }
}

In your class, the reference from the form is passed into the constructor and stored as a private member. This form reference can be used to access the form's properties.

class Utilities
{
    private Form1 _mainForm;
    public Utilities(Form1 mainForm)
    {
        _mainForm = mainForm;
        _mainForm.Lv.Items.Clear();
    }
}

You need to make the members in the for the form class either public or, if the service class is in the same assembly, internal. Windows controls' visibility can be controlled through their Modifiers properties.

Note that it's generally considered a bad practice to explicitly tie a service class to a UI class. Rather you should create good interfaces between the service class and the form class. That said, for learning or just generally messing around, the earth won't spin off its axis if you expose form members for service classes.

rp

I'm relatively new to c# and brand new to stackoverflow. Anyway, regarding the question on how to access controls on a form from a class: I just used the ControlCollection (Controls) class of the form.

        //Add a new form called frmEditData to project.
        //Draw a textbox on it named txtTest; set the text to
        //something in design as a test.
        Form frmED =  new frmEditData();
        MessageBox.Show(frmED.Controls["txtTest"].Text);

Worked for me, maybe it will be of assistance in both questions.

JUST YOU CAN SEND FORM TO CLASS LIKE THIS

Class1 excell = new Class1 (); //you must declare this in form as you want to control

excel.get_data_from_excel(this); // And create instance for class and sen this form to another class

INSIDE CLASS AS YOU CREATE CLASS1

class Class1
{
    public void get_data_from_excel (Form1 form) //you getting the form here and you can control as you want
    {
        form.ComboBox1.text = "try it"; //you can chance Form1 UI elements inside the class now
    }
}

IMPORTANT : But you must not forgat you have declare modifier form properties as PUBLIC and you can access other wise you can not see the control in form from class

Category: c# Time: 2008-10-20 Views: 1
Tags: winforms

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