+1: nice post @bricelam!

@anpete, after reading @pmiddleton's comment:

Currently if you provide a working implementation it will be used as a fallback when the method call can't be translated and EF tries to execute it client side.

I am wondering what this issues is exactly about.

Perhaps the point is that the function might always need to be computed in the database, so in order to use it in memory (in a query or even outside of a query) you need to "bootstrap" it with the DbContext?

Going back to @bricelam's example, we could make both of these things work (I know it seems a bit contrived):

// call outside of query
foreach(var p in posts)
{
  if (db.PostReadCount(p.Id) > 5)
  {
    // do something
  }
}

// call in query
foreach(var p in posts.Where(p => db.PostReadCount(p.Id) > 5))
{
  // do something
}

Since in the body of the method we have access to "this" DbContext, we can use it to execute a query in which the method is invoked:

public int PostReadCount(int postId)
{
  return Posts.Where(p => p.Id == postId).Select(p => db.PostReadCount(p.Id));  
}

Or perhaps something like this some day:

public int PostReadCount(int postId)
{
  return Execute(db => db.PostReadCount(postId));  
}

We actually had this kind of capability in EF6, although it wasn't at all easy to setup, and it was useful for mapping regular functions as well as TVFs.

Note that it was also safer in EF6 to invoke the method through the LINQ provider as part of the implementation of the method, because there was no automatic client evaluation. In EF Core we should probably have a way to protect against re-entrant calls.

If this is the reason it is more compelling to enable instance methods, I think we need to reflect it better in the description of the issue. If it isn't then I would like to understand better.

Otherwise, when I hear enabling mapping instance methods to functions, I think that what I would really want to have is support for this:

db.Posts.Where(p => p.ReadCount() > 5);

I tried to describe this in #9393.

DbFunctions: Consider allowing instance methods #9213

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