CS0120: An object reference is required for the nonstatic field, method, or property ‘foo’

Consider:

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            //int[] val = { 0, 0};
            int val;
            if (textBox1.Text == "")
            {
                MessageBox.Show("Input any no");
            }
            else
            {
                val = Convert.ToInt32(textBox1.Text);
                Thread ot1 = new Thread(new ParameterizedThreadStart(SumData));
                ot1.Start(val);
            }
        }

        private static void ReadData(object state)
        {
            System.Windows.Forms.Application.Run();
        }

        void setTextboxText(int result)
        {
            if (this.InvokeRequired)
            {
                this.Invoke(new IntDelegate(SetTextboxTextSafe), new object[] { result });
            }
            else
            {
                SetTextboxTextSafe(result);
            }
        }

        void SetTextboxTextSafe(int result)
        {
            label1.Text = result.ToString();
        }

        private static void SumData(object state)
        {
            int result;
            //int[] icount = (int[])state;
            int icount = (int)state;

            for (int i = icount; i > 0; i--)
            {
                result += i;
                System.Threading.Thread.Sleep(1000);
            }
            setTextboxText(result);
        }

        delegate void IntDelegate(int result);

        private void button2_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }
    }
}

Why is this error occurring?

An object reference is required for the nonstatic field, method, or property ‘WindowsApplication1.Form1.setTextboxText(int)

7 Answers

It looks like you are calling a non static member (a property or method, specifically setTextboxText) from a static method (specifically SumData). You will need to either:

  1. Make the called member static also:

    static void setTextboxText(int result)
    {
        // Write static logic for setTextboxText.  
        // This may require a static singleton instance of Form1.
    }
    
  2. Create an instance of Form1 within the calling method:

    private static void SumData(object state)
    {
        int result = 0;
        //int[] icount = (int[])state;
        int icount = (int)state;
    
        for (int i = icount; i > 0; i--)
        {
            result += i;
            System.Threading.Thread.Sleep(1000);
        }
        Form1 frm1 = new Form1();
        frm1.setTextboxText(result);
    }
    

    Passing in an instance of Form1 would be an option also.

  3. Make the calling method a non-static instance method (of Form1):

    private void SumData(object state)
    {
        int result = 0;
        //int[] icount = (int[])state;
        int icount = (int)state;
    
        for (int i = icount; i > 0; i--)
        {
            result += i;
            System.Threading.Thread.Sleep(1000);
        }
        setTextboxText(result);
    }
    

More info about this error can be found on MSDN.

You start a thread which runs the static method SumData. However, SumData calls SetTextboxText which isn’t static. Thus you need an instance of your form to call SetTextboxText.

For this case, where you want to get a Control of a Form and are receiving this error, then I have a little bypass for you.

Go to your Program.cs and change

Application.Run(new Form1());

to

public static Form1 form1 = new Form1(); // Place this var out of the constructor
Application.Run(form1);

Now you can access a control with

Program.form1.<Your control>

Also: Don’t forget to set your Control-Access-Level to Public.

And yes I know, this answer does not fit to the question caller, but it fits to googlers who have this specific issue with controls.

Your method must be static

static void setTextboxText(int result)
{
    if (this.InvokeRequired)
    {
        this.Invoke(new IntDelegate(SetTextboxTextSafe), new object[] { result }); 
    }
    else
    {
        SetTextboxTextSafe(result);
    }
}

Credit to @COOLGAMETUBE for tipping me off to what ended up working for me. His idea was good but I had a problem when Application.SetCompatibleTextRenderingDefault was called after the form was already created. So with a little change, this is working for me:


static class Program
{
    public static Form1 form1; // = new Form1(); // Place this var out of the constructor

    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(form1 = new Form1());
    }
}

From my looking you give a null value to a textbox and return in a ToString() as it is a static method. You can replace it with Convert.ToString() that can enable null value.

I actually got this error because I was checking InnerHtml for some content that was generated dynamically – i.e. a control that is runat=server.

To solve this I had to remove the “static” keyword on my method, and it ran fine.

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