Linq query or Lambda expression?

问题:

I'm using Entity Framework in my ASP.NET, C#, Web Application. If I need to select a record from DataBase (in Data Access Layer), which method should I use? Linq query or a Lambda Expression?

Eg:-

//Linq        
var result = from a in db.myTable.Take(1) where a.Id == varId  select a;
return result.First();

//Lambda
return db.myTable.FirstOrDefault(a => a.Id == varId);

Is there any preferred way in this scenario or any advantage over the other?

回答1:

Query Expression compiles into Method Expression (Lambda expression), so there shouldn't be any difference, In your code though you are accessing First and FirstOrDefault which would behave differently.

See: Query Syntax and Method Syntax in LINQ (C#)

and LINQ Query Expressions (C# Programming Guide)

At compile time, query expressions are converted to Standard Query Operator method calls according to the rules set forth in the C# specification. Any query that can be expressed by using query syntax can also be expressed by using method syntax. However, in most cases query syntax is more readable and concise.

回答2:

Linq query syntax is just a syntax sugar for expression methods. Any Linq query compiled into expression methods. Btw your first query:

var query = from a in db.myTable.Take(1) 
            where a.Id == varId  
            select a;
return query.First();

Is equivalent to

return db.myTable.Take(1).Where(a => a.Id == varId).First();

回答3:

Both of your tries use Linq.

The first takes one record and checks if the id matches.

The second takes the first record where the id matches.

That's a difference.

回答4:

I guess the result is the same. Lambda is just a bit more comfortable. If you need a result of just one table, the lambda expression is very fast and readable.

回答5:

Every query expression can be expressed as C#-code using calls to query operators as extension methods. But the opposite is not true; only a small subset of the standard query operators can be used as keywords in query expressions. In other words query expressions have some limitations that the method-call mechanism does not have:

1. Some query operators have simply no C# query expression equivalent, e.g. ToArray().
2. We can't use all kinds of overloads in C#'s query expressions. E.g. there is an overload of Select() that awaits the index of the currently iterated object; you cannot call this overload within a query expression.
3. We can't use statement lambdas in query expressions. - This is the cause why object and collection initializers have been introduced into the C# language.