how to fetch record from dataset

you can use for loop :

for (int i=0;i<ds.tables[0].rows.count;i++)>
{
string value=ds.Tables[0].Rows[i]["Col1"].Tostring();
string vlue2=ds.Tables[0].Rows[i]["Col2"].Tostring();

}

ds.table[0].AsDataView().ToTable(false,string[] columnnames)

SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["mycon"].ConnectionString);
con.Open();
SqlDataAdapter da = new SqlDataAdapter("select column_name from table_name", con);
DataSet ds = new DataSet();
da.Fill(ds);

string userid;
for (int i=0;i<=ds.tables[0].rows.count-1;i++)
{
userid=ds.Tables[0].Rows[i]["userid"].Tostring();
}

You are fetching a single column "userid" by your adapter:

SqlDataAdapter da = new SqlDataAdapter("select userid from user_detail", con);

If you want to use it somewhere then apply a foreach loop and fetch the "userid".

foreach(DataRow row in ds.Tables[0].Rows) {
   int uid = Convert.ToInt(row["userid"]);
}

You can use a code like this:

foreach(DataRow row in ds.Tables[0].Rows) {
   string col1 = row["Col1"].ToString();
}