Error System.NullReferenceException when trying to pass Value from one Form to another Winforms C #

I was able to resolve it as follows:

Form form1 = Application.OpenForms["FormCadastro"]; 
TabControl tabControl1 = (TabControl)form1.Controls["tbControl1"]; 
TabPage  tabPage1 = ( TabPage )tabControl1 .Controls["tabPage1 "]; 
Label LbTeste = (Label)tabPage1 .Controls["LbTeste"]; 
LbTeste.Text = "SIM";

You must inform the TabControl and TabPage that the values will be passed.

Forms are classes like any other, just create properties or make public controls to access.

The method you used works because all controls are stored in the Controls collection of the Form and are accessible in the same way as an array, informing the key (in this case the name) of the control. But in addition to getting messy code, try changing the name of one of the controls, not even the refactoring of the IDE will update the changes and will bring a terrible problem.

There are other ways to do what you need, as I said above, you can create public properties and access from another object. Example:

No Form1:

public partial class Form1 : Form
{

    public string PropriedadeForm1 { get; set; }

    public string LabelForm1 { get { return labelForm1.Text; } set { labelForm1.Text = value; } }

    public Form1()
    {
        InitializeComponent();
    }

    private void buttonAbrirForm2_Click(object sender, EventArgs e)
    {
        Form2 form2 = new Form2();
        form2.Show(this); //Passa a instância do Form1 (this) como Owner da instância do Form2
    }

    private void buttonSetPropriedadeForm2_Click(object sender, EventArgs e)
    {
        Form2 form2 = this.OwnedForms.OfType<Form2>().FirstOrDefault(); //Seleciona a instância do Form2 que é 'Owned' por esta instância do Form1

        if (form2 != null) //Se há um Form2
        {
            form2.PropriedadeForm2 = textBoxForm1.Text; //Define a propriedade
        }
    }
}

Notice public properties PropriedadeForm1 and LabelForm1 , they will be accessible from outside the Form.

Note also that when instantiating Form2, this was passed as a parameter of the Show method. This causes the instance of Form2 to be owned by this instance of Form1. Later, we can access them by the OwnedForms properties of Form1, and Owner of Form2.

Now an example of Form2:

public partial class Form2 : Form
{
    public string PropriedadeForm2 { get; set; }


    public Form2()
    {
        InitializeComponent();
    }

    private void buttonMostraLabelForm1_Click(object sender, EventArgs e)
    {
        Form1 form = this.Owner as Form1;
        if (form != null)
            MessageBox.Show(form.LabelForm1);
    }

    private void buttonSetLabelForm1_Click(object sender, EventArgs e)
    {
        Form1 form = this.Owner as Form1; //Pega a instância do Form1 que é dona desta instância do Form2

        if (form != null)
            form.LabelForm1 = textBoxForm2.Text;
    }

    private void buttonPropriedadeForm2_Click(object sender, EventArgs e)
    {
        MessageBox.Show(this.PropriedadeForm2);
    }
}

  

Result:

Thereisalsothepossibilityofmakingthefieldspublic,andaccessingthemfromoutsidetheForm:

Inthisway,toaccesslabelForm1itsuffices:

privatevoidbuttonMostraLabelForm1_Click(objectsender,EventArgse){Form1form=this.OwnerasForm1;if(form!=null)MessageBox.Show(form.labelForm1.Text);}

Ifyouusethisforminyourcode,insteadof:

TabControltabControl1=(TabControl)form1.Controls["tbControl1"]; 

would be:

form1.tbControl1;

or

form1.LbTeste.Text = "Teste Label";