entity framework - The value can not be null.

Does "actividad" have a non-null EntityKey?

which verify that?

Put a breakpoint on the line of code where it reads
      db.GetObjectByKey(actividad.EntityKey)

when the breakpoint is hit, use QuickWatch to inspect "actividad".

See if EntityKey was set.

it does not. View file
this is the code the table

CREATE TABLE [certifica].[Actividad](
      [Codigo] [int] IDENTITY(1,1) NOT NULL,
      [CodigoActividad] [bigint] NOT NULL,
      [NombreActividad] [varchar](200) NULL,
      [NombreSede] [varchar](50) NULL,
      [FechaInicio] [varchar](50) NULL,
      [FechaFin] [varchar](50) NULL,
      [DirigidoA] [varchar](200) NULL,
      [NombrePrograma] [varchar](200) NULL,
      [NumeroResolucion] [varchar](100) NULL,
      [HorasLectivas] [varchar](5) NULL,
PRIMARY KEY CLUSTERED
(
      [CodigoActividad] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
enitysetKEYname.jpg