如何检查LINQ中的数组是否不是null？

Question

I Have this where statement in LINQ:

(users != null && users.Contains(x.Appointment.UserId))

users is an int[] . When i run this code i have an exception:

Unable to create a null constant value of type 'System.Int32[]'. Only entity types, enumeration types or primitive types are supported in this context.

What should i do?

Answer 1

I had a similar problem. Assuming you are starting with something like this:

var filtered = appointments
    .Where(x => users != null && users.Contains(x.Appointment.UserId));

As @CarstenKönig suggests, try moving the null check outside of the Where clause:

var filtered = users != null ? 
    appointments.Where(x => users.Contains(x.Appointment.UserId)) :
    appointments;

That means the null check is handled by C#, and LINQ to Entities doesn't attempt to convert it to SQL.

Answer 2

As others suggested you could move bool check outside the closure and use resulting bool in your Where statement:

var usersNotNull = users != null;
var users = users == null ? new string[0] : users;
query.Where(x => usersNotNull && users.Contains(x.Appointment.UserId));