Calculator code in C# sharp

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 WindowsFormsApplication1
{
public partial class Form1 : Form
{
bool plus = false;
bool minus = false;
bool multiply = false;
bool devide = false;
bool equal = false;
public Form1()
{
InitializeComponent();
}

private void button5_Click(object sender, EventArgs e)
{
checkifequal();
textBox1.Text = textBox1.Text + 4;
}

private void button1_Click(object sender, EventArgs e)
{
checkifequal();
textBox1.Text = textBox1.Text + 7;
}

private void button3_Click(object sender, EventArgs e)
{
checkifequal();
textBox1.Text = textBox1.Text + 9;
}

private void button4_Click(object sender, EventArgs e)
{
if (textBox1.Text == "")
{
return;
}
else
{
devide = true;
textBox1.Tag = textBox1.Text;
textBox1.Text = "";
}
}



private void button6_Click(object sender, EventArgs e)
{
checkifequal();
textBox1.Text = textBox1.Text + 5;
}

private void button9_Click(object sender, EventArgs e)
{
checkifequal();


textBox1.Text = textBox1.Text + 1;

}

private void checkifequal()
{
if (equal)
{
textBox1.Text = "";
equal = false;
}
}

private void button10_Click(object sender, EventArgs e)
{
checkifequal();
textBox1.Text = textBox1.Text + 2;
}

private void button11_Click(object sender, EventArgs e)
{
checkifequal();
textBox1.Text = textBox1.Text + 3;
}

private void button7_Click(object sender, EventArgs e)
{
checkifequal();
textBox1.Text = textBox1.Text + 6;
}

private void button2_Click(object sender, EventArgs e)
{
checkifequal();
textBox1.Text = textBox1.Text + 8;
}

private void button15_Click(object sender, EventArgs e)
{
if (textBox1.Text.Contains("."))
{
return;
}
else
textBox1.Text = textBox1.Text + ".";
}

private void button16_Click(object sender, EventArgs e)
{
if (textBox1.Text == "")
{
return;
}
else
{
plus = true;
textBox1.Tag = textBox1.Text;
textBox1.Text = "";
}
}

private void button14_Click(object sender, EventArgs e)
{
equal = true;
if (plus)
{
decimal dec = Convert.ToDecimal(textBox1.Tag) + Convert.ToDecimal(textBox1.Text);
textBox1.Text = dec.ToString();
}
if (minus)
{
decimal dec = Convert.ToDecimal(textBox1.Tag) - Convert.ToDecimal(textBox1.Text);
textBox1.Text = dec.ToString();
}
if (multiply)
{
decimal dec = Convert.ToDecimal(textBox1.Tag) * Convert.ToDecimal(textBox1.Text);
textBox1.Text = dec.ToString();
}
if (devide)
{
decimal dec = Convert.ToDecimal(textBox1.Tag) / Convert.ToDecimal(textBox1.Text);
textBox1.Text = dec.ToString();
}
}

private void button12_Click(object sender, EventArgs e)
{
if (textBox1.Text == "")
{
return;
}
else
{
minus = true;
textBox1.Tag = textBox1.Text;
textBox1.Text = "";
}
}

private void button8_Click(object sender, EventArgs e)
{
if (textBox1.Text == "")
{
return;
}
else
{
multiply = true;
textBox1.Tag = textBox1.Text;
textBox1.Text = "";
}
}

private void button18_Click(object sender, EventArgs e)
{
plus = minus = multiply = devide = false;
textBox1.Text = "";
textBox1.Tag = "";
}


}

}