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Option Explicit and Option Strict in Visual Basic .NET and in Visual Basic

Article ID: 311329 - View products that this article applies to.

This article was previously published under Q311329

SUMMARY

This article describes two Option statements:

Option Strict
Option Explicit

Option Strict is new in Microsoft Visual Basic .NET, Microsoft Visual Basic 2005 and in later versions of Visual Basic. Option Explicit is available in earlier versions of Visual Basic.

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By default, the Visual Basic .NET or Visual Basic compiler enforces explicit variable declaration, which requires that you declare every variable before you use it. To change this default behavior, see the Change the Default Project Values section.

The Option Strict statement

By default, the Visual Basic .NET or Visual Basic compiler does not enforce strict data typing. To change this default behavior, see the Change the Default Project Values section.

Option Strict restricts implicit data type conversions to only widening conversions. Widening conversions explicitly do not permit any data type conversions in which data loss may occur and any conversion between numeric types and strings. For more information about widening conversions, see the Widening Conversions section.

When you use the Option Strict statement, the statement must appear before any other code. In Visual Basic .NET, you can typically convert any data type to any other data type implicitly. Data loss can occur when the value of one data type is converted to a data type with less precision or with a smaller capacity. However, you receive a run-time error message if data will be lost in such a conversion. Option Strict notifies you of these types of conversions at compile time so that you can avoid them.

Option Strict also generates an error message in the following scenarios:

For any undeclared variable. This is because Option Strict also implies Option Explicit.
Late binding.

Widening Conversions

The following table lists the standard widening conversions.

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Data Type	Widens to Data Types
Byte	Byte, Short, Integer, Long, Decimal, Single, Double
Short	Short, Integer, Long, Decimal, Single, Double
Integer	Integer, Long, Decimal, Single, Double
Long	Long, Decimal, Single, Double
Decimal	Decimal, Single, Double
Single	Single, Double
Double	Double
Any enumerated type	Its underlying integer type and any type to which it will widen
Char	Char, String
Any type	Object, any interface that it implements
Any derived type	Any base type from which it is derived
Nothing	Any data type or object type

The following conversions may lose precision:

Integer to Single
Long to Single or Double
Decimal to Single or Double

However, these conversions do not lose information or magnitude.

Widening conversions always succeed, and you can always perform widening conversions implicitly.

Explicit Conversion with Casting

An explicit conversion uses a type conversion keyword. Visual Basic .NET or Visual Basic 2005 provides several such keywords, which coerce an expression in parentheses to the data type that you want. These keywords behave as functions, but the compiler generates the code inline. Therefore, execution is a little faster with explicit conversion than with a function call.

The following table lists the available conversion keywords.

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Type Conversion Keyword	Converts Expression to Data Type	Permitted Data Types of Expression to Be Converted
CBool	Boolean	Any numeric type (including Byte and enumerated types), String, Object
CByte	Byte	Any numeric type, any enumerated type, Boolean, String, Object
CChar	Char	String, Object
CDate	Date	String, Object
CDbl	Double	Any numeric type (including Byte and enumerated types), Boolean, String, Object
CDec	Decimal	Any numeric type (including Byte and enumerated types), Boolean, String, Object
CInt	Integer	Any numeric type (including Byte and enumerated types), Boolean, String, Object
CLng	Long	Any numeric type (including Byte and enumerated types), Boolean, String, Object
CObj	Object	Any type
CShort	Short	Any numeric type (including Byte and enumerated types), Boolean, String, Object
CSng	Single	Any numeric type (including Byte and enumerated types), Boolean, String, Object
CStr	String	Any numeric type (including Byte), Boolean, Char, Char array, Date, Object
CType	Type specified following the comma (,)	When you convert to an elementary type (including an array of an elementary type), the same types as are permitted for the corresponding conversion keyword. When you convert to a composite type, the interfaces it implements and the classes from which it inherits.

Change the Default Project Values

You can change the default values of Option Explicit and Option Strict. For example, when you create a new application by using Visual Basic .NET, Visual Basic 2005, or Visual Basic 2008, the value for Option Explicit is set to On. You can change this default value to Off.

To change the default values of Option Explicit and Option Strict in Visual Basic .NET, click Options on the Tools menu to open the Options dialog box. Then, change the default values for Option Explicit and Option Strict in the VB Defaults section of Projects and Solutions.

To change the default values of Option Explicit and Option Strict in Visual Studio, click Options on the Tools menu to open the Options dialog box. Then, change the default values for Option Explicit and Option Strict in the VB Defaults section of Projects and Solutions.

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