Operation from datatable inserted values in only one textbox

[jig. Bire]

Hi , i'm new to c# programming my objective is to type an operation (operands and operators) in a single `expressionTextBox` and if I click on a `resultButton` the result will be displayed.

For example, i have 3 variables of type `float` named `var1`, `var2` and var3 (in a table of data with their content values ) I want to write in the `expressionTextBox ``var2 + var3=` (or `*`or `-`or `sin`or `cos`...) and the result of `var1` will be displayed in `ResultTextbox`.

The textbox has to be able to understand more than one variable from datatable and has to understand operands and functions for the operation. Itried with a code, can someone help me to do that my problem is in the first textbox (`expressionTextbox`) because i know how to update the result in `var1` , are there other methods using `listBox`, for example?

This is a first step below. Am I on the right path?please help !

private void ResultButton_Click(object sender, EventArgs e)
    {
        con.Open();
        string[] parts = expressionTextbox.Text.Split('+');
        double intSum = 0;

        SqlCommand cmd = con.CreateCommand();
        cmd.CommandType = CommandType.Text;
        cmd.CommandType = System.Data.CommandType.Text;
        cmd.CommandText = "select * from Table_com where Variable=@variable";

        cmd.Parameters.Add("@variable", SqlDbType.VarChar).Value = expressionTextbox.Text;
        cmd.ExecuteNonQuery();
        DataTable dt = new DataTable();
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        da.Fill(dt);

        foreach (DataRow dr in dt.Rows)
        {
            intSum = intSum + Convert.ToDouble(dr["Contents"]);
        }
        ResultTextbox.Text = intSum.ToString();
        con.Close();
    }