C# - How can I convert the string value to decimal places?

Basically, ive created a form that I can select different shapes that when a value on the track bar is selected, works out both the area and boundary length of either a circle, triangle or square.

The values are currently coming through with a lot of decimal places and I want to set up radio buttons to choose whether 2, 3 or 4 decimal places.

private void sliderBar(object sender, EventArgs e)
{

        textBox3.Text = trackBar1.Value.ToString();

        if(circleButton.Checked == true)
        {
            textBox2.Text = (circle.getArea(trackBar1.Value)).ToString();
            textBox1.Text = (circle.getBoundLength(trackBar1.Value)).ToString();
        }
        else if(squareButton.Checked == true)
        {
            textBox2.Text = (square.getArea(trackBar1.Value)).ToString();
            textBox1.Text = (square.getBoundLength(trackBar1.Value)).ToString();
        }
        else
        {
            textBox2.Text = (triangle.getArea(trackBar1.Value)).ToString();
            textBox1.Text = (triangle.getBoundLength(trackBar1.Value)).ToString();
        }

        if (decimalPlaces2Button.Checked == true)
        {
            TextBox2.Text = decimal.Round(textBox2, 2, MidpointRounding.AwayFromZero).ToDouble();

        }
}