使用 Linq 和 C# 比较列表数据

Question

I have two lists of class, one called lsEmployee and another called lsAccount. The account list is just a list of account names, wheras the employee list has employee and account. I am attempting a linq query to see if the employee's account matches an account in the account list.

I am new to linq so it's likely just a syntax issue. I have attempted the following code, but I cannot seem to get this to work. Any suggestions would be appreciated.

var lsEmployeeAccount = (lsAccount.Where(a => a.Account_Name == employee.Account_Name).SingleOrDefault()).ToString();

string lsEmployeeAccountString = lsEmployeeAccount.ToString() ?? "";

if (string.IsNullOrWhiteSpace(lsEmployeeAccountString))
{
//If account is not found, do stuff here

}
else
{
//If account is found, do stuff here
}

Answer 1

Not sure if you are looping through accounts or employees first, but assuming you are going through each employee:

foreach(var employee in lsEmployee)
{
  var isExist = lsAccount.Contain(employee.Account_Name);

  if(isExist)
  {}
  else
  {}
}

Answer 2

You could do something like this:

var lsEmployeeAccount = lsAccount.SingleOrDefault(a => a.Account_Name == employee.Account_Name);
var lsEmployeeAccountString = (lsEmployeeAccount != null) ? lsEmployeeAccount.ToString() : "";

if (string.IsNullOrWhiteSpace(lsEmployeeAccountString))
{
    //If account is not found, do stuff here
}
else
{
    //If account is found, do stuff here
}

But notice that lsEmployeeAccountString might not come out as you expect.

Answer 3

Consider using the SQL style LINQ syntax when joining multiple IEnumerable s containing different kinds of data; for example:

        var validAccounts = new[] {"1234", "2468", "1357", "6789", "9753", "9630"};
        var employees = new[]
        {
            new Employee {Name = "Bob", Account = "1357"},
            new Employee {Name = "Dave", Account = "1030"},
            new Employee {Name = "Fred", Account = "6789"},
            new Employee {Name = "Emma", Account = "2469"},
        };


        var employeesWithValidAccounts =
            from emp in employees
            join validAccount in validAccounts on emp.Account equals validAccount
            select emp.Name;

        foreach (var name in employeesWithValidAccounts)
        {
            Console.WriteLine(name);
        }

Output:

Bob
Fred

Answer 4

When you have SingleOrDefault and there is no matching account then your lsEmployeeAccount is null and you will get a NullReferenceException.

Change your comparison to

var lsEmployeeAccount = lsAccount.Where(a => a.Account_Name == employee.Account_Name).SingleOrDefault()
if (lsEmployeeAccount == null)
{
    //If account is not found, do stuff here
}
else
{
    //If account is found, do stuff here
}

If you didn't need the account then another option would be to

if (!lsAccount.Any(a => a.Account_Name == employee.Account_Name))
{
    //If account is not found, do stuff here
}
else
{
    //If account is found, do stuff here
}