To set Option Infer in a file, type Option Infer On or Option Infer Off at the top of the file, before any other source code. If the value set for Option Infer in a file conflicts with the value set in the IDE or on the command line, the value in the file has precedence.

When you set Option Infer to On, you can declare local variables without explicitly stating a data type. The compiler infers the data type of a variable from the type of its initialization expression.

In the following illustration, Option Infer is turned on. The variable in the declaration Dim someVar = 2 is declared as an integer by type inference.

In the following illustration, Option Infer is turned off. The variable in the declaration Dim someVar = 2 is declared as an Object by type inference. In this example, the Option Strict setting is set to Off on the Compile Page, Project Designer (Visual Basic).

When a variable is declared as an Object, the run-time type can change while the program is running. Visual Basic performs operations called boxing and unboxing to convert between an Object and a value type. These operations make execution slower. For information about boxing and unboxing, see the Visual Basic Language Specification.

Type inference applies at the procedure level, and does not apply outside a procedure in a class, structure, module, or interface.

For additional information, see Local Type Inference (Visual Basic).

When an Option Infer Statement Is Not Present

Default Data Types and Values

The following examples demonstrate how the Option Infer statement enables local type inference.

' Enable Option Infer before trying these examples.

' Variable num is an Integer.
Dim num = 5

' Variable dbl is a Double.
Dim dbl = 4.113

' Variable str is a String.
Dim str = "abc"

' Variable pList is an array of Process objects.
Dim pList = Process.GetProcesses()

' Variable i is an Integer.
For i = 1 To 10
    Console.WriteLine(i)
Next

' Variable item is a string.
Dim lst As New List(Of String) From {"abc", "def", "ghi"}

For Each item In lst
    Console.WriteLine(item)
Next

' Variable namedCust is an instance of the Customer class.
Dim namedCust = New Customer With {.Name = "Blue Yonder Airlines",
                                   .City = "Snoqualmie"}

' Variable product is an instance of an anonymous type.
Dim product = New With {Key .Name = "paperclips", .Price = 1.29}

' If customers is a collection of Customer objects in the following 
' query, the inferred type of cust is Customer, and the inferred type
' of custs is IEnumerable(Of Customer).
Dim custs = From cust In customers
            Where cust.City = "Seattle"
            Select cust.Name, cust.ID

The following example demonstrates that the run-time type can differ when a variable is identified as an Object.

' Disable Option Infer when trying this example.

Dim someVar = 5
Console.WriteLine(someVar.GetType.ToString)

' If Option Infer is instead enabled, the following
' statement causes a run-time error. This is because
' someVar was implicitly defined as an integer.
someVar = "abc"
Console.WriteLine(someVar.GetType.ToString)

' Output:
'  System.Int32
'  System.String

Reference

Concepts

Para establecer Option Infer en un archivo, escriba Option Infer On o Option Infer Off en la parte superior del archivo, antes de cualquier otro código fuente. Si el valor que se establece para Option Infer en un archivo está en conflicto con el valor establecido en el IDE o la línea de comandos, tiene prioridad el valor del archivo.

Al establecer Option Infer en On, puede declarar las variables locales sin especificar explícitamente un tipo de datos. El compilador deduce el tipo de datos de una variable a partir del tipo de su expresión de inicialización.

En la siguiente ilustración, Option Infer está activada. La variable de la declaración Dim someVar = 2 se declara como un entero por inferencia de tipos.

En la siguiente ilustración, Option Infer está desactivada. La variable de la declaración Dim someVar = 2 se declara como un Object por inferencia de tipos. En este ejemplo, el valor de Option Strict se establece en Off en Página Compilación, Diseñador de proyectos (Visual Basic).

Cuando se declara una variable como un Object, puede cambiar el tipo en tiempo de ejecución mientras se ejecuta el programa. Visual Basic realiza operaciones llamadas conversión boxing y conversión unboxing para convertir entre un Object y un tipo de valor. Estas operaciones ralentizan la ejecución. Para obtener información acerca de las conversiones boxing y unboxing, vea Especificación del lenguaje de Visual Basic.

La inferencia de tipos se aplica en el nivel de procedimiento, y no se aplica fuera de un procedimiento en una clase, estructura, módulo o interfaz.

Para obtener información adicional, vea Inferencia de tipo de variable local (Visual Basic).

Cuando no existe una instrucción Option Infer

Valores y tipos de datos predeterminados

En los ejemplos siguientes se muestra cómo la instrucción Option Infer habilita la inferencia de tipo de variable local.

' Enable Option Infer before trying these examples.

' Variable num is an Integer.
Dim num = 5

' Variable dbl is a Double.
Dim dbl = 4.113

' Variable str is a String.
Dim str = "abc"

' Variable pList is an array of Process objects.
Dim pList = Process.GetProcesses()

' Variable i is an Integer.
For i = 1 To 10
    Console.WriteLine(i)
Next

' Variable item is a string.
Dim lst As New List(Of String) From {"abc", "def", "ghi"}

For Each item In lst
    Console.WriteLine(item)
Next

' Variable namedCust is an instance of the Customer class.
Dim namedCust = New Customer With {.Name = "Blue Yonder Airlines",
                                   .City = "Snoqualmie"}

' Variable product is an instance of an anonymous type.
Dim product = New With {Key .Name = "paperclips", .Price = 1.29}

' If customers is a collection of Customer objects in the following 
' query, the inferred type of cust is Customer, and the inferred type
' of custs is IEnumerable(Of Customer).
Dim custs = From cust In customers
            Where cust.City = "Seattle"
            Select cust.Name, cust.ID

El ejemplo siguiente demuestra que el tipo en tiempo de ejecución puede variar cuando se identifica una variable como un Object.

' Disable Option Infer when trying this example.

Dim someVar = 5
Console.WriteLine(someVar.GetType.ToString)

' If Option Infer is instead enabled, the following
' statement causes a run-time error. This is because
' someVar was implicitly defined as an integer.
someVar = "abc"
Console.WriteLine(someVar.GetType.ToString)

' Output:
'  System.Int32
'  System.String

Referencia

Conceptos