注册用户时，用户名为什么只能输入数字？

protected void btnRegister_Click(object sender, EventArgs e)
        {
            conn.Open();
            string strsql = "select count(*) from Users where UserName=" + this.TextBox1.Text.Trim();
            SqlCommand cmd = new SqlCommand(strsql, conn);
            //string s = cmd.ExecuteScalar();
            int i = Convert.ToInt32(cmd.ExecuteScalar());
            if (i == 0)
            {
                string sql = "insert into Users values('" + this.TextBox1.Text + "','" + this.TextBox2.Text + "')";
                if (this.TextBox1.Text != "" && this.TextBox2.Text != "")
                {
                    SqlCommand cmd2 = new SqlCommand(sql, conn);
                    cmd2.ExecuteNonQuery();
                   
                    Response.Redirect("login.aspx");
                }
                else
                {
                    Response.Write("<script>alert('用户名或密码不能为空！')</script>");
                }
            }
            else
            {
                Response.Write("<script>alert('姓名冲突，请重新输入！')</script>");
                this.TextBox1.Text = "";
                this.TextBox1.Focus();
            }
          
           
        }数据库中的用户表Users，字段用户名UserName是char类型，输入字符或汉字时， “int i = Convert.ToInt32(cmd.ExecuteScalar());”这行代码出错：列名 '张三' 无效。。不知道什么原因，请高手指教一下！