option-infer-statement.md

title	Option Infer Statement
ms.date	07/20/2015
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helpviewer_keywords	variables [Visual Basic], declaring Option Infer statement [Visual Basic] Infer keyword [Visual Basic] declaring variables [Visual Basic], inferred inferred variable declaration
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Option Infer Statement

Enables the use of local type inference in declaring variables.

Syntax

Option Infer { On | Off }  

Parts

Term	Definition
On	Optional. Enables local type inference.
Off	Optional. Disables local type inference.

Remarks

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.

IntelliSense when Option Infer is on

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).

IntelliSense when Option Infer is off

Note

When a variable is declared as an Object, the run-time type can change while the program is running. [!INCLUDEvbprvb] performs operations called boxing and unboxing to convert between an Object and a value type, which makes 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.

When an Option Infer Statement Is Not Present

If the source code does not contain an Option Infer statement, the Option Infer setting on the Compile Page, Project Designer (Visual Basic) is used. If the command-line compiler is used, the /optioninfer compiler option is used.

To set Option Infer in the IDE

1. In Solution Explorer, select a project. On the Project menu, click Properties.

2. Click the Compile tab.

3. Set the value in the Option infer box.

When you create a new project, the Option Infer setting on the Compile tab is set to the Option Infer setting in the VB Defaults dialog box. To access the VB Defaults dialog box, on the Tools menu, click Options. In the Options dialog box, expand Projects and Solutions, and then click VB Defaults. The initial default setting in VB Defaults is On.

To set Option Infer on the command line

• Include the /optioninfer compiler option in the vbc command.

Default Data Types and Values

The following table describes the results of various combinations of specifying the data type and initializer in a Dim statement.

Data type specified?	Initializer specified?	Example	Result
No	No	Dim qty	If Option Strict is off (the default), the variable is set to Nothing. If Option Strict is on, a compile-time error occurs.
No	Yes	Dim qty = 5	If Option Infer is on (the default), the variable takes the data type of the initializer. See Local Type Inference. If Option Infer is off and Option Strict is off, the variable takes the data type of Object. If Option Infer is off and Option Strict is on, a compile-time error occurs.
Yes	No	Dim qty As Integer	The variable is initialized to the default value for the data type. For more information, see Dim Statement.
Yes	Yes	Dim qty As Integer = 5	If the data type of the initializer is not convertible to the specified data type, a compile-time error occurs.

Example

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

[!code-vbVbVbalrTypeInference#6]

Example

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

[!code-vbVbVbalrTypeInference#11]

See Also

Dim Statement
Local Type Inference
Option Compare Statement
Option Explicit Statement
Option Strict Statement
Visual Basic Defaults, Projects, Options Dialog Box
/optioninfer
Boxing and Unboxing