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Option Infer Statement

2015-07-20

.net

devlang-visual-basic

article

vb.OptionInfer

vb.Infer

variables [Visual Basic], declaring

Option Infer statement

Infer keyword

declaring variables, 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

In Solution Explorer, select a project. On the Project menu, click Properties. For more information, see NIB: Managing Project Properties with the Project Designer.
Click the Compile tab.
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]

dotnet/docs

Option Infer Statement

Syntax

Parts

Remarks

When an Option Infer Statement Is Not Present

To set Option Infer in the IDE

To set Option Infer on the command line

Default Data Types and Values

Example

Example

See Also