How can I change a column name from a SQL table on a running asp.net website?

Question

I have created a website with ASP.NET which is connected to a SQL database.

The website shows a table of this with the help of a grid view.

My target is that the user can add to this table a Column and give this a name. That with the help of a TextBox and a button.

I am so far that I can add the table a column with a button click but I don't know how I can give the column a name with the TextBox

private void disp_data()
{
    SqlCommand cmd = con.CreateCommand();
    cmd.CommandType = CommandType.Text;
    cmd.CommandText = "select * from table1";
    cmd.ExecuteNonQuery();
    DataTable dt = new DataTable();
    SqlDataAdapter da = new SqlDataAdapter(cmd);                                                
    da.Fill(dt);
    GridView3.DataSource = dt;
    GridView3.DataBind();
}

---Try1

protected void AddRow_Click(object sender, EventArgs e)
{

    SqlCommand cmd = con.CreateCommand();
    cmd.CommandType = CommandType.Text;         
    cmd.CommandText = "ALTER TABLE table1 ADD '"+TextBox3.Text+"' VARCHAR(50) NULL;";            
    cmd.ExecuteNonQuery();

    disp_data();
}

---Try2

protected void AddRow_Click(object sender, EventArgs e)
{

    SqlCommand cmd = con.CreateCommand();
    cmd.CommandType = CommandType.Text;
    string Columnname = Convert.ToString(TextBox3.Text);
    cmd.CommandText = "ALTER TABLE table1 ADD @CName VARCHAR(50)   NULL;";
    cmd.Parameters.AddWithValue(@"CName", Columnname);
    cmd.ExecuteNonQuery();

    disp_data();
}

protected void AddRow_Click(object sender, EventArgs e)
{
    SqlCommand cmd = con.CreateCommand();
    cmd.CommandType = CommandType.Text;         
    cmd.CommandText = "ALTER TABLE table1 ADD NewColumn VARCHAR(50)        NULL;";            
    cmd.ExecuteNonQuery();

    disp_data();
}                           // This works

---Try1

System.Data.SqlClient.SqlException: "Incorrect syntax near 'Textboxcontent'."

---Try2

System.Data.SqlClient.SqlException: "Incorrect syntax near '@CName'."

Answer 1

I wouldn't suggest Try 1 since is is vulnerable to an SQL Injection. Try 2 didn't work because you use a verbatim string instead of using @CName as string.

protected void AddRow_Click(object sender, EventArgs e) {

  SqlCommand cmd = con.CreateCommand();
  cmd.CommandType = CommandType.Text;
  cmd.CommandText = "ALTER TABLE table1 ADD @CName VARCHAR(50)   NULL;";
  cmd.Parameters.AddWithValue("@CName", TextBox3.Text);
  cmd.ExecuteNonQuery();
}

Answer 2

I had to add "Replace" to make @CNAME useful, now it works

protected void AddRow_Click(object sender, EventArgs e) {

                SqlCommand cmd = con.CreateCommand();
        cmd.CommandType = CommandType.Text;         
        cmd.CommandText = "ALTER TABLE table1 ADD @CNAME VARCHAR(50) NULL;";
        cmd.CommandText = cmd.CommandText.Replace("@CNAME", TextBox3.Text);
       // cmd.Parameters.AddWithValue("@CName", TextBox3.Text);
        cmd.ExecuteNonQuery();

        disp_data();
    }