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When i click on a button,text will appear in textbox1 but then i want it to change focus to another textbox(textbox2) and when i click the same button again,display the same text but in textbox2.

 private void btna_Click(object sender, EventArgs e)
 {
 textBox1.Text = ("A");
 if (textBox1.Text.Length > 0)
 
 textBox2.Focus();
 

If you want to alternate between different TextBoxes in your click event to determine which one to update, you can track them in a private variable, and then just switch which one you're using based on the current value, for example:

private TextBox textBoxToUpdate = null;

private void button1_Click(object sender, EventArgs e)

 // Swap the textBoxToUpdate between textBox1 and textBox2 each time
 textBoxToUpdate = (textBoxToUpdate == textBox1) ? textBox2 : textBox1;

 textBoxToUpdate.Text = "A";

Another way to do this if you're updating multiple controls is to store them in a list and increment an integer that defines the index to the next item.

// holds the text boxes we want to rotate between
private List<TextBox> textBoxesToUpdate = new List<TextBox>();

private void Form1_Load(object sender, System.EventArgs e) 

 // add some text boxes to our list
 textBoxesToUpdate.Add(textBox1);
 textBoxesToUpdate.Add(textBox2);
 textBoxesToUpdate.Add(textBox3);
 textBoxesToUpdate.Add(textBox4);


// stores the index of the next textBox to update
private int textBoxToUpdateIndex = 0;

private void button1_Click(object sender, EventArgs e)

 textBoxesToUpdate[textBoxToUpdateIndex].Text = "A";

 // increment the index but set it back to zero if it's equal to the count
 if(++textBoxToUpdateIndex == textBoxesToUpdate.Count) textBoxToUpdateIndex = 0;