C# Text don't display on another form after double clicking an item in listbox

Question

I\'m doing a winform which consist a listbox (listbox10) with some items on it. When I doubleclick an item, it will show another form (Form3) which consist a textbox (textbo

Answer 1

Supposing the textbox1 is a control inside msgForm3 then the right syntax should be

msgForm3.textBox1.Text = listBox10.SelectedItem.ToString(); 

This requires changing the property Modifiers from Private to Public for the textBox1 using the Form Designer property window.
I don't recommended this change. It is better to pass the new value inside the constructor of msgForm3 like this

msgForm3 = new Form3(listBox10.SelectedItem.ToString());  
msgForm3.Show();  

and then in the constructor of msgForm3 add

public Form3(string initialValue)  
{  
    InitializeComponent();  
    this.textBox1.Text = initialValue;  
}  

or you can add a public method to your Form3 class like this

public void InitText(string initialValue) { this.textBox1.Text = initialValue;
}

and call in this way

msgForm3 = new Form3();  
msgForm3.InitText(listBox10.SelectedItem.ToString());
msgForm3.Show();  

Answer 2

The textBox1 you are accessing is not on msgForm3. to access the one on msgForm3 use (as the other answers have mentioned)

msgForm3.textBox1.Text = listBox10.SelectedItem.ToString(); 

but since all form controls are private by default you can either change its protection level inside Form3.Designer.cs to public (or internal ) :

public System.Windows.Forms.TextBox textBox1;

or add the text that should go in textbox1 as a parameter in Form3 constructor :

public Form3(string text)
{
    InitializeComponent();
    this.textBox1.Text = text;
}

and when your create an instance of Form3 use

msgForm3 = new Form3(listBox10.SelectedItem.ToString());

Answer 3

This is how you can access the member of the other form.

msgForm3 = new Form3();
msgForm3.textBox1.Text = listBox10.SelectedItem.ToString();
msgForm3.Show();

EDIT:

From the Form3.Designer.cs file just change protection level from private to public.

Replace

private System.Windows.Forms.TextBox textBox1;

To

public System.Windows.Forms.TextBox textBox1;