Use DTNS

create function dbo.GetPatientFirstName()
Returns Table
as
Return select firstname from tblpatientdetails

Using system.data.sqlClient;

SqlConnection sqlConn=new SqlConnection("You connection");

SqlCommand sqlCmd=new SqlCommand();

SqlDataReader dr;

sqlCmd.Connection=sqlConn;
sqlCmd.CommandText="select * from dbo.GetPatientFirstName()";
sqlConn.Open();
dr=sqlCmd.ExecuteReader();
DropDownList1.DataSource=dr;
DropDownList1.DataTextField="firstname";
DropDownList1.DataTextValue="firstname";
DropDownList1.DataBind();