LINQ：如何使用LINQ查找Array属性包含特定值的所有对象

Question

I've been searching for a while and could not find exactly what I need.

This questioning might be devoted, since, I suspect there are some alike questions here already. But I think I have a specific case.

I have an object User that contains the property as Array of permissions.

I'm looking for the correct way to grab all the users with a particular permission.

I tried the following:

List<User> users = UserRepository.GetUser().Where(u => u.Permissions[0].Equals("MyPermission")).ToList();

When doing this, I'm getting an error:

"Index was outside the bounds of the array."

What is the correct way of doing this?

Answer 1

Using .Any() is a safe way

List<User> users = UserRepository.GetUser()
    .Where(u => u.Permissions.Any(x => x.Equals("MyPermission"))
    .ToList();

Answer 2

You get an exception because you blindly index Permissions with [0] , without checking that it has any values at all.

Adding a check for length would fix this problem, but it wouldn't catch "MyPermision" outside the initial position.

You can fix this by using Contains(...) :

// Construct a MyService.UserPermissions object for searching
var targetPermission = new MyService.UserPermissions("MyPermission");
var users = UserRepository
    .GetUser()
    .Where(u => u.Permissions.Contains(targetPermission))
    .ToList();

Edit: Since it turns out that you are looking for a permission with a specific PermissionName , you cannot use Contains . You need to use Any instead:

var users = UserRepository
    .GetUser()
    .Where(u => u.Permissions.Any(p => p.PermissionName == "MyPermission"))
    .ToList();

Answer 3

利用空传播来跳过没有权限对象的用户，并检查数组长度以避免超出范围的异常：

List<User> users = UserRepository.GetUser().Where(u => u.Permissions?.Length > 0 && u.Permission[0] == "MyPermission").ToList();