尝试按照ToTraceString（）的示例进行操作，但失败。 为什么？

Question

I've got to find a way to find out what the SQL SELECT that is being generated when interacting with EF, is. I found the MSDN online help describing it ObjectQuery.ToTraceString Method() and I am certain that I've followed the example exactly, but it is failing. I am getting the following error:

CS1503  Argument 2: cannot convert from 'string' to
'System.Linq.Expressions.Expression<System.Func<ResearchTableUpdates.Model.Template, bool>>'

I don't get why I'm getting this error. Here's the code that generates the error:

using (var db = new MyDbContext())
{
    int idNum = 1;
    ObjectQuery<Model.Template> oqTemplate = db.Templates.Where("it.ID = @idNum");
    oqTemplate.Parameters.Add(new ObjectParameter("idNum", idNum));
    Console.WriteLine(oqTemplate.ToTraceString());

The MyDbContext is a class I defined. Here is it's definition:

public class MyDbContext : Model.CoreFrameworkEntities
{
    public override int SaveChanges()
    {
        var modifiedEntities = ChangeTracker.Entries()
                .Where(p => p.State == EntityState.Modified)
                .Select(p => p.Entity);

        foreach (var modified in modifiedEntities)
        {
            Console.WriteLine();
            //modified.LastModifiedAt = now;
            Console.WriteLine(modified.ToString());
            Console.WriteLine();
        }
        return base.SaveChanges();
    }
}

And finally Model.CoreframeworkEntities is an EF model defined creating a .edmx file. It is one of the entities in the .edmx. (It is my understanding that any entity defined in a .edmx file is a DbContext data type. Now, if that's wrong, I'd love to know.)

I'm not sure why I'm getting the error that I am. It seems to me as though I've followed the MSDN article in code. Either the MSDN article is no longer valid or I've made a mistake, possibly in my understanding as to what the entities in a entity-relationship/.edmx file are all about.

Answer 1

That's because that article uses context inherited from ObjectContext . You use context inherited from DbContext . That old ObjectContext has ObjectSets which indeed has Where overload which accepts string and returns ObjectQuery . However DbContext (which you use) has DbSets and Where methods on them do not accept strings, but only Expressions hence your error. With DbContext you usually either use interceptors, or Database.Log property, like this:

db.Database.Log = Console.WriteLine; // or any other handler
var templates = db.Templates.Where(c => c.ID == idNum).ToArray(); // sql will be written to console.

As Gert Arnold mentions in comments, you can also call ToString() on query:

db.Templates.Where(c => c.ID == idNum).ToString()

However in some cases you cannot do that (for example you end your query with Count() , or First() and so on). But when you can do that, and you need only sql for a certain single query - of course ToString method is better.