Question

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)

Answer 1

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. If you can, make setTextboxText static:
   static void setTextboxText(int result)

HOWEVER, in this case, setTextboxText REQUIRES access to instance variables, so cannot be static.
Instead do:

2. Call setTextboxText via a static singleton of Form1:

class Form1 {     public static Form1 It;   // Singleton.      public Form1()     {         It = this;     }      private static void SumData(object state)     {         ...         It.setTextboxText(result);     } } 

3. 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); } 

   BUT that won’t do what you want, if an instance of Form1 already exists.
   Passing in an instance of Form1 would be an option also.

4. 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.