Foreach of C# vs Foreach() of EF6

Question

Follows code:

ForEach() of EF6:

var result = ctx.Table.Where(x => x.User == "João").ToList();
result.ForEach(x => x.Read = true);
ctx.SaveChanges();

foreach of the C#:

var result = ctx.Table.Where(x => x.User == "João").ToList();
foreach (var item in result)
{
    item.Read = true;
}
ctx.SaveChanges();

I believe the codes above are the same functionality, what is the difference between them?

Answer 1

The method ForEach() has no relation to the Entity Framework.

He’s a class method List, see documentation. Note that he has no relation to LINQ, he is a native of List.

There is no important difference between the two. They are two different ways of doing the same thing. Basically the method uses the for to go through all the elements.

Here is the method code:

public void ForEach(Action<T> action) {
    if( action == null) {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
    }
    Contract.EndContractBlock();

    int version = _version;

    for(int i = 0 ; i < _size; i++) {
        if (version != _version && BinaryCompatibility.TargetsAtLeast_Desktop_V4_5) {
            break;
        }
        action(_items[i]);
    }

    if (version != _version && BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)
        ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);
}

^{See more in referencesource.microsfot.com}

Obviously it changes the way the code is written/read a lot, which is better or more readable is a matter of taste.

I find it interesting to read this article by Eric Lippert that deals with the subject (not directly from your question, but treats).

Answer 2

Contrary to what it may seem, the Entity Framework LINQ codes will attempt to generate an SQL expression or something equivalent that processes the database data. It will not execute exactly the code that is written there. This is called DSL (Domain Specific Language). It must occur with the Where() used earlier to acquire what is in the database.

But it should not occur with the ForEach() that does not exist in the EF LINQ provider. So the difference between them is almost syntactic.

To know which takes longer would have to do tests. The problem is that both are abstractions that have cost. The foreach need to take an iterator that is an external object to the collection to control the progress of each item. This is not free. Already the ForEach() of List is a for simple without iterator, which is faster. On the other hand it needs to call a delegate in each iteration, which is not for free.

In general he is used for processing parallelization not possible with the command foreach, It is a little more expensive, but if there are gains (there are not always) may end before running in parallel. In this case it is only a processing in the application upon the result brought by the database.

There are people who like him in any situation by getting shorter and eventually more semantic about what he wants to do. But performance is not good if it doesn’t parallelize well. And parallelization isn’t even available in every implementation of ForEach(). Contrary to what people imagine it is not a unique method, it can have several flavors.

See more:

• What is the difference between Ienumerable, Iqueryable and List?
• A LINQ query returns what type of data?
• What is LINQ technology?
• How the logic of Where no Entity Framework works?
• Source of ForEach() of List (.NET Core)
• There are other (.NET Core)