Getting an error in conversion

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C#

i am calculating total amount of after calculating the percentage applied but getting an error of conversion as
:

Unable to cast object of type 'System.Web.UI.WebControls.TextBox' to type 'System.IConvertible'.

What I have tried:

public partial class caltxt : System.Web.UI.Page
{
   double amt1;
    double exc;
    public void Page_Load(object sender, EventArgs e)
    {
       
    }

    public void txtamt_TextChanged(object sender, EventArgs e)
    {

         amt1 = Math.Round(Convert.ToDouble (txtamt));
        
        //decimal PercentageRate = Convert.ToDecimal(this.txtamt.Text);
   
    }


    protected void txtexc_TextChanged(object sender, EventArgs e)
    {
             
        double exc = Math.Round (Convert.ToDouble(this.txtexc.Text));
       
        
        // decimal    temp = (Math.Round(Convert.ToDecimal(txtexc.Text) / Convert.ToDecimal(txtamt.Text)*100));
       // txttotalexc.Text = temp.ToString();
    }

    protected void txttotalexc_TextChanged(object sender, EventArgs e)
    {
        double totat = Math.Round (Convert.ToDouble(exc) /100)* Convert.ToDouble(amt1); //+ Convert.ToInt32(this.txtexc.Text);
        this.txttotalexc.Text = totat.ToString();
    }
}