使用LINQ仅从表中获取所需的列以修改和保存记录会产生典型错误

Question

So what I was trying was to fetch only those columns from table which has to be updated and I tried it as below:

var user = (from u in context.tbl_user where u.e_Id == model.Id select 
           new { u.first_name, u.last_name, u.role, u.email, u.account_locked })
           .FirstOrDefault();

But when I tried to assign new value to the fetched data as below

user.first_name = model.FirstName;

I saw below error getting displayed

Property or indexer 'anonymous type: string first_name, string last_name, string role, string email, bool account_locked.first_name' cannot be assigned to -- it is read only

But when I retrieved all the values from table without filtering as below it worked fine.

var user = (from u in context.tbl_user where u.e_Id == model.Id select u).FirstOrDefault();

Why it doesn't work for first query. I've read in many sites that it is good to retrieve only required properties from database in terms of performance and security. But I am really not able to understand what's wrong with the first approach I opted. Any explanations are much appreciated.

---

Update

Are there any other ways to fetch only required column and update them and store them back?

Answer 1

Anonymous Types properties are read-only so you can not change them.

Stop doing micro-optimizing or premature-optimization on your code. Try to write code that performs correctly, then if you face a performance problem later then profile your application and see where is the problem. If you have a piece of code which have performance problem due to finding the shortest and longest string then start to optimize this part.

We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we should not pass up our opportunities in that critical 3% - Donald Knuth

If you want to just fetch specific columns you can create a custom class and fill the properties in your query like others have mentioned.

Answer 2

As others said anonymous type is read only, for achieving what you want, you will have to create a type for it with properties that are required:

public class User
   {
       public string FirstName {get;set;}
       public string LastName {get;set;}
       .....................
       .....................
   }

and then, you have to use it in your linq query:

var user = (from u in context.tbl_user 
            where u.e_Id == model.Id
            select new User 
                  { 
                     FirstName = u.first_name,
                     LastName =  u.last_name,
                     ......................,
                     ..................... 
                   }).FirstOrDefault(); 

Answer 3

Anonymous types are read-only by design.

In order to retrieve something that you can edit, you have to create a class and select your entities into that class, as done here: How to use LINQ to select into an object?

Or here:

var user = (from u in context.tbl_user where u.e_Id == model.Id select 
           new User_Mini { u.first_name, u.last_name, u.role, u.email, u.account_locked })
           .FirstOrDefault();

Note: you won't be able to call context.SubmitChnages() when editing this new object. You could do something like this though: LINQ to SQL: how to update the only field without retrieving whole entity

This will allow you to update only certain parts of the object.