Sequence contains no matching element

1. Error in LINQ query single method

var c = DbCache.UserRoles.Single (ur = ur). Roleid = = Roleid);

2, using the method System.Linq.Enumerable the single method under the namespace to query the data, the data source count=0, or the return count of 0 will be an error.

public static TSource single<tsource> (this ienumerable<tsource> source, Func<tsource, bool> predicate);

You can use Singleordefault instead of single

var c = DbCache.UserRoles.SingleOrDefault (ur = ur). Roleid = = Roleid);

But DbCache.UserRoles.SingleOrDefault (ur = ur). Roleid = = Roleid) may be null, so it's best to write

var r = DbCache.UserRoles.SingleOrDefault (ur = ur). Roleid = = Roleid)! = null?
DbCache.UserRoles.SingleOrDefault (ur = ur). Roleid = = Roleid). UserId: "";

。

Sequence contains no matching element