使用Linq，存在检查数据库中记录的最有效方法

Question

I am using Linq and to check whether or not a record exists in the database and that it is the latest version. I can do this using Linq

bool query = db.foo.Any(x => x.Id == list.Id);

And then checking to see if this produced a true or false result.

The issue I have with this approach is that if I want to check all the attributes of the table to determine if the existing data differs from the new copy then it requires me to specify a mapping for each attribute and list item.

Is there a way I can iterate through or are there other alternatives available?

x refers to the Entity framework object that is representing the DB
list is a list that is mapped 1 to 1 to the Entity Framework object.

Answer 1

foo query = db.foo.Where(x => x.Id == list.Id).FirstOrDefault();

if(query != null)
{
    properties = query.GetType().GetProperties();
    foreach(PropertyInfo prop in properties)
    {
        if (list.GetType().GetProperty(prop.Name) != null)
        {
            var val1 = list.GetType().GetProperty(prop.Name).GetValue(list);
            var val2 = prop.GetType().GetProperty(prop.Name).GetValue(prop);
            // do your checking here
        }
    }
}

Answer 2

If I understand you correctly, you want a sort of for each loop to iterate over all the columns for a row, and check that they match the corresponding data from the list - is that correct?

The short answer is: Sorry, I don't think you can do that (at least not without using some reflection tricks, which might be rather messy, and will not perform as well as you probably want).

With SQL you can write: SELECT * FROM table (...) , but there is no such option in Linq; you need to specify the names of each column explicitly.

What you could and probably should do instead, is what Jon Skeet mentioned briefly in a comment: Add a version number, or possibly a timestamp that you can check. Write your logic in such a way that you can be certain that if the version is correct/updated, then the row is valid.

You can then check the validity of a specific row by looking at it's version number. You could for instance do something like this:

// Does a row with specified ID exist and the newest version exist:
bool isRowValid = db.foo.Any(x => x.Id == list.Id && 
                                  x.VersionNr == list.newestVersionNr);

Answer 3

bool chechExists = db.foo.Exists(x => x.Id == list.Id);

这将根据需要返回布尔值