title: CS0120: An object reference is required for the nonstatic field, method, or property 'foo'
body: <p>Consider:</p>
  
  <pre><code>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 &gt; 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();
          }
      }
  }
  </code></pre>
  
  <p>Why is this error occurring?</p>
  
  <blockquote>
    <p>An object reference is required for the nonstatic field, method, or property 'WindowsApplication1.Form1.setTextboxText(int)</p>
  </blockquote>