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public static Form1 frmMain;

frmMain = new Form1();
Application.Run(frmMain);
或者
Application.Run(frmMain = new Form1());

Program.frmMain.txtBox1.Text…..

    private void button1_Click(object sender, EventArgs e)
            {
                //加法运算
                Double x, y, result;
                x = Convert.ToDouble(txtUsr.Text);
                y = Convert.ToDouble(txtPwd.Text);
                result = x + y;
                MessageBox.Show(result.ToString());
            }</textarea>

            private void button2_Click(object sender, EventArgs e)
            {
               //点击reset复位所有文本框内容
                txtUsr.Text = “”;
                txtPwd.Text = “”;
            }

            private void button6_Click(object sender, EventArgs e)
            {
                //点击QUIT退出程序
                Application.Exit();
            }

            private void button3_Click(object sender, EventArgs e)
            {
                //减法运算
                Double x, y, result;
                x = Convert.ToDouble(txtUsr.Text);
                y = Convert.ToDouble(txtPwd.Text);
                result = x – y;
                MessageBox.Show(result.ToString());
            }

            private void button4_Click(object sender, EventArgs e)
            {
                Double x, y, result;
                x = Convert.ToDouble(txtUsr.Text);
                y = Convert.ToDouble(txtPwd.Text);
                result = x * y;
                MessageBox.Show(result.ToString());
            }

            private void button5_Click(object sender, EventArgs e)
            {
                //除法，因为除数为0没有意义，所以用了条件语句
                Double x, y, result;
                x = Convert.ToDouble(txtUsr.Text);
                y = Convert.ToDouble(txtPwd.Text);
                if (y == 0)
                {
                    MessageBox.Show(“Sorry,there is an error!”);
                }
                else
                {
                    result = x / y;
                    MessageBox.Show(result.ToString());
                }
            }

Parser Error

Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately. Parser Error Message: Could not load type System.Web.UI.WebControls.RequireFieldValidator from assembly System.Web, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a.