C#での電卓作成について

現在C#で電卓を作成しているのですが、Funcを使用しても上手に動きません。

やりたいこととしては、1*2*3のような、連続した計算になります。
演算子ボタンを押したとき、答えが出るようにしたいです。

初めは演算子ボタンの中にイコールの処理を入れたのですが、Funcを使えば短く簡潔に書けると言われました。SUBとADDはまだ書き換えていません。クリアボタンを押したときの処理も書き換え中です。

以下が該当のコードになります。

  namespace WindowsFormsApp1
{
    public partial class Form1 : Form
    {
        public Form1(){
            InitializeComponent();
        }

        bool isFirst = true;
        decimal num1 = 0;

        private void Number_Click(object sender, EventArgs e)
        {
            if (sign != arithmetic.undefined && isFirst) {
                num1 = Convert.ToDecimal(textDisplay.Text);
                textDisplay.Text = "";
                isFirst = false;
            }
            if (textDisplay.TextLength == 12) {
                return;
            }
            textDisplay.Text =
               textDisplay.Text + ((Button)sender).Text;
        }
        private void Dot_Click(object sender, EventArgs e) {
            if (textDisplay.Text.IndexOf(".") >= 0) {
                return;
            }
            textDisplay.Text = textDisplay.Text + ".";
        }

        enum arithmetic{
            undefined, ADD, SUB, MUL, DIV
        };
        arithmetic sign = arithmetic.undefined;

        private void ADD_Click(object sender, EventArgs e){
            if (sign != arithmetic.undefined) {
                decimal num2 = Convert.ToDecimal(textDisplay.Text);
                decimal Result = 0;
                Result = num1 + num2;
                textDisplay.Text = Result.ToString();
            }
            sign = arithmetic.ADD;
            isFirst = true;
        }
        private void SUB_Click(object sender, EventArgs e)
        {
            if (sign != arithmetic.undefined) {
                decimal num2 = Convert.ToDecimal(textDisplay.Text);
                decimal Result = 0;
                Result = num1 - num2;
                textDisplay.Text = Result.ToString();
            }
            sign = arithmetic.SUB;
            isFirst = true;
        }
        private void MUL_Click(object sender, EventArgs e){
            Func<decimal, decimal, decimal> MUL = (num1, num2) => {
                return num1 * num2;
            };
            sign = arithmetic.MUL;
            isFirst = true;          
        }  

        private void DIV_Click(object sender, EventArgs e){
            Func<decimal,decimal,decimal> DIV = (num1,num2) => {
                return num1 / num2;
            }
;            sign = arithmetic.DIV;
            isFirst = true;
        }
        private void Equal_Click(object sender, EventArgs e){
            decimal num2 = Convert.ToDecimal(textDisplay.Text);
            decimal Result = 0;
            try {
                switch (sign) {
                    case arithmetic.ADD:
                        Result = num1 + num2;
                        break;
                    case arithmetic.SUB:
                        Result = num1 - num2;
                        break;
                    case arithmetic.MUL:
                        Result = num1 * num2;
                        break;
                    case arithmetic.DIV:
                        Result = num1 / num2;
                        break;
                }
            }
            catch(DivideByZeroException) {
                textDisplay.Text = "0では割れません";{
                    return;
                }
            }

            textDisplay.Text = Result.ToString();
            isFirst = true;
　　　　　　　  if (textDisplay.TextLength >= 13) {
                textDisplay.Text = "桁数超過";  

                this.button0.Enabled = false;
                this.button1.Enabled = false;
                this.button2.Enabled = false;
                this.button3.Enabled = false;
                this.button4.Enabled = false;
                this.button5.Enabled = false;
                this.button6.Enabled = false;
                this.button7.Enabled = false;
                this.button8.Enabled = false;
                this.button9.Enabled = false;
                this.buttonEq.Enabled = false;
                this.Dot.Enabled = false;
                this.ADD.Enabled = false;
                this.SUB.Enabled = false;
                this.MUL.Enabled = false;
                this.DIV.Enabled = false;
               }
        }
        private void Clear_Click(object sender, EventArgs e){

            this.button0.Enabled = true;
            this.button1.Enabled = true;
            this.button2.Enabled = true;
            this.button3.Enabled = true;
            this.button4.Enabled = true;
            this.button5.Enabled = true;
            this.button6.Enabled = true;
            this.button7.Enabled = true;
            this.button8.Enabled = true;
            this.button9.Enabled = true;
            this.buttonEq.Enabled = true;
            this.Dot.Enabled = true;
            this.ADD.Enabled = true;
            this.SUB.Enabled = true;
            this.MUL.Enabled = true;
            this.DIV.Enabled = true;
        }
    }
}