Khi nhập 1+1 thì hiển thị đúng 1+1 ra màn label!nhưng khi nhấn button = thì programe báo lỗi:
An unhandled exception of type 'System.FormatException' occurred in mscorlib.dll

Additional information: Input string was not in a correct format.
Đây là code!Anh em coi giúp dùm mình sữa lại:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace lab3_part3
{
public partial class Form1 : Form
{
bool plus = false;

public Form1()
{
InitializeComponent();
}

private void Form1_Load(object sender, EventArgs e)
{

}

private void label_click(object sender, EventArgs e)
{

}

private void btn1_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "1";

}

private void btn2_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "2";

}

private void btn3_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "3";

}

private void btn4_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "4";

}

private void button5_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "5";

}

private void btn6_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "6";

}

private void btn7_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "7";

}


private void button8_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "8";

}

private void btn9_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "9";

}

private void btn0_Click(object sender, EventArgs e)
{
lblKetqua.Text = lblKetqua.Text + "0";

}

private void plus_Click(object sender, EventArgs e)
{
if (lblKetqua.Text=="")
{
return;
}
else
{
plus = true;
lblKetqua.Tag = lblKetqua.Text;
lblKetqua.Text +="+";
}
}

private void btnEqual_Click(object sender, EventArgs e)
{

if (plus)
{
decimal dec = Convert.ToDecimal(lblKetqua.Tag) + Convert.ToDecimal(lblKetqua.Text);
lblKetqua.Text =lblKetqua.Tag + lblKetqua.Text + "=" + dec.ToString();
}
}



}
}