新手 c#.net 用update读取编辑框内的数字在减

代码如下
protected void Button1_Click(object sender, EventArgs e)
{

string str= DropDownList1.SelectedValue;
Session["A"] = str;
TextBox1.Text = str.ToString();

SqlConnection conn = new SqlConnection();
string connectionString = "server=.;database=Userinfo;uid=zh; pwd=123456";
conn.ConnectionString = connectionString;
conn.Open();
SqlCommand com = new SqlCommand("Update [Commodity information] set Number = Number - ('" + TextBox1.Text + "') where [Commodity number]=1 ", conn);
com.ExecuteNonQuery();

conn.Close();

}
点击按钮以后就会报错内容是这个：
Invalid operator for data type. Operator equals subtract, type equals char.

数据库的Number 字段类型错了

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