How to use Parameter in Order By clause in SQL Server and C# .Net

My query is given below:

protected void ddlProject_SelectedIndexChanged(object sender, EventArgs e)
{
   SqlConnection con = new SqlConnection(ConfigurationManager.AppSettings["SqlConn"].ToString());
   string str="select * from Project_Details where IsActive=0 and Project is not NULL order by '"+ddlProject.SelectedItem.Text+"' asc";        
   SqlDataAdapter da = new SqlDataAdapter();
   SqlCommand cmd = new SqlCommand(str, con);
   //cmd.Parameters.AddWithValue("@orderby", ddlProject.SelectedItem.Text.Trim());
   cmd.CommandType = CommandType.Text;
   da.SelectCommand = cmd;
   DataSet ds = new DataSet();
   da.Fill(ds);
   if (ds.Tables[0].Rows.Count > 0)
   {
     gvPageDetails.DataSource = ds;
     gvPageDetails.DataBind();
    }
}

Here I am showing the data order by table fields (Projectname, ClientName, ContractorName, ConsultantName, year, Area)selected in dropdownlist “ddlProject”.

How I will write the above Sql Query in C# to prevent SQL Injection Attack.