04. The "accumulate" and "Calculate" functions are implemented. The textbox does not allow the input of any character other than a number, but does not include the backspace key. The first value is not allowed to be 0, and the textbox does not.

04. The "accumulate" and "Calculate" functions are implemented. The textbox does not allow the input of any character other than a number, but does not include the backspace key. The first value is not allowed to be 0, and the textbox does not.

Private void button#click (object sender, EventArgs e) {double number1, number2; if (double. tryParse (txtNumber1.Text, out number1) = false) {MessageBox. show ("Incorrect start value"); txtNumber1.Focus (); txtNumber1.SelectAll (); return;} if (double. tryParse (txtNumber2.Text, out number2) = false) {MessageBox. show ("incorrect end value"); txtNumber2.Focus (); txtNumber2.SelectAll (); return;} if (number2 <number1) {MessageBox. show ("first number The value cannot be greater than the second value. Enter "); txtNumber1.Focus (); txtNumber1.SelectAll (); return;} double sum = 0; for (double I = number1; I <= number2; I ++) {sum = sum + I; I ++;} lbResult. text = sum. toString ();} private void txtNumber1_TextChanged (object sender, EventArgs e) {lbResult. text = "";} private void btnExit_Click (object sender, EventArgs e) {this. close () ;}// the content stored in sender is the corresponding control object of the event triggered by the corresponding control. An object is a parent class that can be accessed by all subclasses. Each control corresponds to a class private void txtNumber1_KeyPress (object sender, KeyPressEventArgs e) // keypress event {TextBox txtNumber1 = sender as TextBox; // if (e. keyChar <'0' | e. keyChar> '9') // The condition is used to prevent the user from entering the corresponding code in the ascii table. You can only enter numbers {e. handled = true;} // selectionstart In the textbox attribute indicates the starting position if a piece of text is selected. If not selected, it indicates the cursor position. // Note! The first position is 0if (txtNumber1.SelectionStart = 0 & e. keyChar = '0') // if the user cursor is at the beginning of the first position and enters 0, the user cannot input {e. handled = true;} if (e. keyChar = 8) // determines whether the user inputs the backspace key {e. handled = false; // cancel e. handled function to block user input }}