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

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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)


Solution

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.

Source: StackOverflow.com

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