10-значный калькулятор проблемы в C# ASP.NET

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Меня просят нажать кнопку "0", и она остается 0 вместо того, чтобы продолжать увеличивать "000000". Что я должен добавить в этот код?

<pre>#region EQUAL CALCULATION
        protected void Equal_Click(object sender, EventArgs e)
        {
            decimal output;
           
            //Check whether it has data or not
            try
            {
                Expression a = new Expression(lblExpression.Text);
                if (decimal.TryParse(a.Evaluate().ToString(), out output))
                {
                    lblResult.Text = output.ToString();
                    lblExpression.Text = "";
                }
            }
            catch (Exception)
            {
                lblExpression.Text =  lblExpression.Text;
                lblResult.Text = "Invalid Calculation";

            }
            
        }
        #endregion

        #region BUTTON FUNCTION
        protected void Btn1_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button1.Text;
        }

        protected void Btn2_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button2.Text;
        }

        protected void Btn3_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button3.Text;
        }

        protected void Btn4_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button4.Text;
        }

        protected void Btn5_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button5.Text;
        }

        protected void Btn6_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button6.Text;
        }

        protected void Btn7_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button7.Text;
        }

        protected void Btn8_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button8.Text;
        }

        protected void Btn9_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button9.Text;
        }

        protected void Btn0_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + button0.Text;
        }

        #endregion

        #region OPERATOR 
        protected void Decimal_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + Decimal.Text;
        }

        protected void Plus_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + Plus.Text;
        }

        protected void Minus_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + Minus.Text;
        }

        protected void Divide_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + Divide.Text;
        }

        protected void Multiply_Click(object sender, EventArgs e)
        {
            lblExpression.Text = lblExpression.Text + Multiply.Text;
        }
        #endregion

        protected void Reset_Click(object sender, EventArgs e)
        {
            lblExpression.Text = "";
            lblResult.Text = "0";

        }

        protected void Back_Click(object sender, EventArgs e)
        {
            //remove by 1 integer
            string s = "";
            if (lblExpression.Text.Length > 1)
            {
                s = lblExpression.Text;
                s = s.Substring(0, s.Length- 1);
            }
            lblExpression.Text = s;
        }

Что я уже пробовал:

Я не знаю, куда мне добавить

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protected void Button_Click(object sender, EventArgs e)
{
    var button = (Button)sender;
    lblExpression.Text = lblExpression.Text + button.Text;
}