function Return LastOrDefault()-CodePudding

I have ExpesnseIDVM ViewModel that only contains 1 variable ExpenseID to hold the last value from the database

public IEnumerable<ExpesnseIDVM> Profile(LoginVM loginVM)
{
    var data = (from a in context.Employees
                where a.Email == loginVM.Email
                join b in context.Expenses on a.EmployeeId equals b.EmployeeId
                select new ExpesnseIDVM()
                { ExpenseID = b.ExpenseId }).ToList().LastOrDefault();

    return data;
}

I have a problem with the return type, what type of return type should I use to get the value

CodePudding user response：

You have IQueryable?<T> as return value of the linq query, then you have .ToList() That makes EF to calculate the query, getting List<T> and finally .LastOrDefault() that returns single object of T.

CodePudding user response：

Four problems here:

SQL Server do not guarantee order of the items if you do not specify OrderBy
SQL do not have LastOrDefault direct translation. EF may try to reverse defined OrderBy and call FirstOrDefault - again OrderBy required
LastOrDefault returns one instance, not enumerable
ToList() loads whole table into the memory, but you need just one record, so do not use it.

Consider to rewrite your query in the following way:

public ExpesnseIDVM Profile(LoginVM loginVM)
{
    var data = (from a in context.Employees
                where a.Email == loginVM.Email
                join b in context.Expenses on a.EmployeeId equals b.EmployeeId
                orderby b.ExpenseId descending
                select new ExpesnseIDVM()
                { ExpenseID = b.ExpenseId }).FirstOrDefault();

    return data;
}

CodePudding user response：

LastOrDefault() will return one ExpesnseIDVM element or null if none is found yet you are returning a collection of elements, you should return one element only:

 public ExpesnseIDVM Profile(LoginVM loginVM) { ... }

I believe you may have a typo in ExpesnseIDVM, shouldn't it be ExpenseIDVM?

In Visual Studio, a quick trick to find the return type of an expression is to hover your mouse cursor over the name of the assigned variable, e.g.: