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DbContext -> DbSet -> Where clause is missing (Entity Framework 6)

I've read some tutorials with entity framework 6...

The basics are easy.

using (var context = new MyContext())
{
    User u = context.Users.Find(1);
}

But how to use "Where" or something else on the "DbSet" with the users?

public class MyContext : DbContext
{
    public MyContext()
        : base("name=MyContext")
    {
        //this.Database.Log = Console.Write;
    }

    public virtual DbSet<User> Users { get; set; }
}

Users

[Table("User")]
public class User : Base
{
    public Guid Id { get; set; }

    [StringLength(100)]
    public string Username { get; set; }
}

And thats the problem which doesnt work.

string username = "Test";
using (var context = new MyContext())
{
    User u = from user in context.Users where user.Username == username select user;
}

Error: There was no implementation of the query pattern for source type 'DbSet'. 'Where' is not found. Maybe a reference or a using directive for 'System.Link' is missing.

If i try to autocomplete the methods there are none.

VS2013

Why it doesnt works? :(

// Edit: Adding System.Linq to the top of the file changes the functions of the problem above so that i havent a problem anymore.

But why the where is wrong now?

The type "System.Linq.IQueryable<User>" cant converted into "User" explicit. There already exists an explicit conversion. (Possibly a cast is missing)

above doesnt work, bottom works

like image 801
PatrickB Avatar asked Oct 24 '14 12:10

PatrickB


People also ask

What is DbContext and DbSet in Entity Framework?

DbContext generally represents a database connection and a set of tables. DbSet is used to represent a table. Your code sample doesn't fit the expected pattern.

What is DbSet in Entity Framework C#?

A DbSet represents the collection of all entities in the context, or that can be queried from the database, of a given type. DbSet objects are created from a DbContext using the DbContext. Set method.

Which of the following query syntax can be use to query in EF 6?

You can use the LINQ method syntax or query syntax when querying with EDM.


2 Answers

Thanks to @Grant Winney and @Joe.

Adding using System.Linq; to the namespace/top of the document where i'm tring the code above fixed the problem.

And using the line above it works for the first item of the list.

User user = (select user from context.Users where user.Username == username select user).First();
like image 71
PatrickB Avatar answered Oct 05 '22 21:10

PatrickB


The (second) problem is in what you expect:

User u = from user in context.Users where user.Username == username select user;

You're expecting a single element. But a Where clause returns a list (IEnumerable) of items. Even if there's only one entity that fits the where clause, it will still return a list (with a single item in it).

If you want a single item, you need to either take the .First() or the .Single() item of that list.

Some considerations:

  • Either method I just mentioned can take a clause similar to how the where clause works. This means you can skip the Where and put the clause straight in this method.
  • Single only works if only one element exists (fitting the clause). If two elements occur, an exception will be thrown.
  • First works similar to Single, but it will not throw an exception if multiple items exist (fitting the clause). It will simply return the first element it finds (fitting the clause).
like image 26
Flater Avatar answered Oct 05 '22 23:10

Flater