Error: the entity type requires a primary key

Question

This exception message doesn't mean it requires a primary key to be defined in your database, it means it requires a primary key to be defined in your class.

Although you've attempted to do so:

private Guid _id;
[Key]
public Guid ID
{
    get { return _id; }
}

This has no effect, as Entity Framework ignores read-only properties. It has to: when it retrieves a Fruits record from the database, it constructs a Fruit object, and then calls the property setters for each mapped property. That's never going to work for read-only properties.

You need Entity Framework to be able to set the value of ID. This means the property needs to have a setter.

I came here with similar error:

System.InvalidOperationException: 'The entity type 'MyType' requires a primary key to be defined.'

After reading answer by hvd, realized I had simply forgotten to make my key property 'public'. This..

namespace MyApp.Models.Schedule
{
    public class MyType
    {
        [Key]
        int Id { get; set; }

        // ...

Should be this..

namespace MyApp.Models.Schedule
{
    public class MyType
    {
        [Key]
        public int Id { get; set; }  // must be public!

        // ...

I found a bit different cause of the error. It seems like SQLite wants to use correct primary key class property name. So...

Wrong PK name

public class Client
{
  public int SomeFieldName { get; set; }  // It is the ID
  ...
}

Correct PK name

public class Client
{
  public int Id { get; set; }  // It is the ID
  ...
}

public class Client
{
  public int ClientId { get; set; }  // It is the ID
  ...
}

It still posible to use wrong PK name but we have to use [Key] attribute like

public class Client
{
   [Key]
   public int SomeFieldName { get; set; }  // It is the ID
   ...
}

This worked for me:

using System.ComponentModel.DataAnnotations;

[Key]
public int ID { get; set; }

Make sure you have the following condition:

1. Use [key] if your primary key name is not Id or ID.
2. Use the public keyword.
3. Primary key should have getter and setter.

Example:

public class MyEntity {
   [key]
   public Guid Id {get; set;}
}

Your Id property needs to have a setter. However the setter can be private. The [Key] attribute is not necessary if the property is named "Id" as it will find it through the naming convention where it looks for a key with the name "Id".

public Guid Id { get; }              // Will not work
public Guid Id { get; set; }         // Will work
public Guid Id { get; private set; } // Will also work