Web Forms :: Check From DB And Generate New Number Using Linq

public string ACtivationKey()
{
string allowedChars = "";
allowedChars = "a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,";
allowedChars += "A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,";
allowedChars += "1,2,3,4,5,6,7,8,9,0";

[Code] ....

I am generating a new order reference id in a class Called GeneratePdf.cs

I use it as  

GeneratePdf gp = new GeneratePdf();
string orderunique = "ORD-" + gp.ACtivationKey();
var uniqness = db.OrdersMasters.Where(x => x.OrderRefNumber == orderunique).Select(x => x.OrderRefNumber).Count();

I want to run a check if the reference number is not present in database. if its present generate new ...