Refer this code:
HTML:
<form id="form1" runat="server">
<div>
<table border="0" cellpadding="0" cellspacing="0">
<tr>
<td>
Column Namme
</td>
<td>
<asp:TextBox ID="txtColumnName" runat="server" />
</td>
</tr>
<tr>
<td>
Column Namme
</td>
<td>
<asp:DropDownList ID="ddlDataTypes" runat="server">
<asp:ListItem Text="int" Value="int" />
<asp:ListItem Text="varchar(50)" Value="varchar(50)" />
<asp:ListItem Text="decimal(18, 0)" Value="decimal(18, 0)" />
<asp:ListItem Text="float" Value="float" />
</asp:DropDownList>
</td>
</tr>
</table>
<asp:Button runat="server" Text="Alter Table" OnClick="AlterTable" />
</div>
</form>
C#:
protected void AlterTable(object sender, EventArgs e)
{
string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
using (SqlConnection con = new SqlConnection(constr))
{
using (SqlCommand cmd = new SqlCommand(string.Format("ALTER TABLE tblPerson Add {0} {1}", this.txtColumnName.Text.Trim(), this.ddlDataTypes.SelectedItem.Value), con))
{
con.Open();
cmd.ExecuteNonQuery();
con.Close();
}
}
}
Thank You.