Is functions
isxxxx()
functions in the standard libraries of the C and C++ programming languages, see ctype.h.Function name | VB 6[1][2] | VBA[3] | VBScript[4] | VB .NET | T-SQL |
---|---|---|---|---|---|
IsArray | Yes | Yes | Yes | Yes | No |
IsDate | Yes | Yes | Yes | Yes | Yes |
IsDBNull | No | No | No | Yes | No |
IsEmpty | Yes | Yes | Yes | No | No |
IsError | Yes | Yes | No | Yes | No |
IsMissing | Yes | Yes | No | No | No |
IsNothing | No | No | No | Yes | No |
IsNull | Yes | Yes | Yes | No | Yes |
IsNumeric | Yes | Yes | Yes | Yes | Yes |
IsObject | Yes | Yes | Yes | No | No |
IsReference | No | No | No | Yes | No |
The Is
functions (also known as data information functions,[5] data inspection functions,[6] or data-testing functions[6]) are a set of functions in Microsoft's Visual Basic 6, Visual Basic for Applications, VBScript, and Visual Basic .NET. Several of them are also provided in Transact-SQL by the .NET Framework Data Provider for Microsoft SQL Server.
What the functions do
The functions are simple data validation and data type checking functions. The data validation functions determine whether it is possible to convert or coerce the data value given as an argument to the function to the type implied by the function name, and return a Boolean
value recording whether it was possible or not. (Note that the actual data conversion functions, such as Oct()
throw exceptions if conversion is not possible. The validation functions allow one to test whether a conversion would fail, and change the program's flow of control in an if statement.) True
indicates that conversion would be possible, False
indicates that it would not be. Similarly the type checking functions return a Boolean
recording whether the argument expression is of a particular type.
In Transact-SQL, the functions return zero or one rather than Boolean
values True
and False
.
IsArray(name)
- This function determines whether the variable name passed as its argument is an array. Uninitialized arrays will, note, return
False
from this function in Visual Basic .NET.[7] In Visual Basic 6, arrays are not reference types, and an uninitialized array will returnTrue
from this function just like an initialized array.[8] IsDate(expression)
- This function determines whether the expression passed as its argument can be converted to a variable of type
Date
, or is already of typeDate
. Uninitialized variables that are of typeDate
can of course be converted, despite being uninitialized, so this will always returnTrue
for such variables.[9] Note that strings that contain a day of the week in addition to a date (e.g."Sat, October 12, 2010"
) will return a failure result.[10] In VBScript and Visual Basic .NET, the conversion process employs the locale settings of Microsoft Windows, meaning that what may parse as a date on one system, configured to use one locale, may fail to parse as a date on another system, configured to use a different locale.[11][12] IsDBNull(expression)
- This function determines whether the expression passed as its argument evaluates to
System.DBNull.Value
.[9] This is equivalent to Visual Basic 6'sIsNull()
function.[10] Note that it is not possible to directly compare an expression for equality toSystem.DBNull
, because any expression of the formx = DbNull
will evaluate toDbNull
simply because it contains a null.IsDBNull()
is the only way to test for equality toSystem.DBNull
.[12] IsEmpty(expression)
- This function determines whether the expression passed as its argument is an uninitialized variant. Note that an uninitialized variant is distinct from a variant that has been initialized to hold
Null
.[13] Although the function takes an expression, rather than simply a variable name, any expression that isn't simply a variable name is considered not to be an uninitialized variant.[11] This function was available in Visual Basic 6, but has been superseded in Visual Basic .NET by theIsNothing()
function.[14] In VBScript, if a variant is assignedNothing
, this function still returnsFalse
.[15] IsError(expression)
- This function, in Visual Basic .NET, determines whether the expression passed as its argument is an exception object, i.e. an object of the
System.Exception
class or one of its subclasses.[7] In Visual Basic 6, the function tests whether the expression is a variant with the specialvbError
subtype.[16] IsMissing(name)
- This function determines whether the variable name passed as its argument is an optional argument that was not passed to a function by its caller. It returns
True
only of the variable is a variant that has not been initialized. This function only exists in Visual Basic 6. In Visual Basic .NET, optional parameters are required to have default initializers, and the function no longer exists.[17] IsNothing(expression)
- This function determines whether the expression passed as its argument evaluates to
Nothing
.[9] It is a simple library function (comprising just 4 CIL instructions) which can itself be written in Visual Basic as:[18][19]Public Shared Function IsNothing(ByVal Expression As Object) As Boolean Return (Expression Is Nothing) End Function
False
for all value (non-reference) expressions, because they will be wrapped up, as part of the function call, into objects, which will by their very natures, not be null objects.[18][19] To avoid this behaviour, one can use theIS
operator to compare an object directly toNothing
, writingexpression
IS Nothing
rather thanIsNothing(expression)
. The compiler will raise a compile-time error if the compared expression is a value rather than a reference type, catching the type mismatch at compile time rather than simply returningFalse
at run-time.[20] Strings are reference types in Visual Basic .NET, and so capable of being null (as opposed to simply zero-length, empty, strings). For such strings, this function returnsTrue
. (For empty strings it returnsFalse
.)[21][18] IsNull(expression)
- This function determines whether the expression passed as its argument evaluates to
Null
. A null value in any sub-expression of the expression causes the entire expression to be considered null.[15] IsNull(expression1,expression2)
- This function, taking two arguments, is specific to Transact-SQL. In contrast to the Visual Basic function by this name, it does not return a
Boolean
, but instead returns the first expression if that is notNULL
, otherwise the second expression.[22] The purpose of the function is to replace anyNULL
values with another, presumably (but not required to be) non-NULL
, value.[23] It is a two-argument version ofCOALESCE()
. IsNumeric(expression)
- This function determines whether the expression passed as its argument can be converted to a number (be that a
Short
,Integer
,Long
,Single
,Double
, orDecimal
) from a character or string, or is already a number.[24][25] In Transact-SQL, strings can be converted to numbers even if they contain characters that one might not expect in numbers. This is because Transact-SQL allows conversion frommoney
andsmallmoney
types to numbers, and monetary data in string form may contain currency indicator characters such as the '£
' or '$
' symbols.[26] The same is true of VBScript, where any string that can be converted to a currency value in the current locale is considered to be numeric. VBScript does not, however, consider dates and times to be numeric.[27] IsObject(expression)
- This function determines whether the expression passed as its argument is an object rather than a value.[4] This is equivalent to Visual Basic .NET's
IsReference()
function.[10] IsReference(expression)
- This function determines whether the expression passed as its argument is a reference rather than a value.[24] This is equivalent to Visual Basic 6's
IsObject()
function.[10]
References
What supports what
- ↑ Marcus & Levy 2002, p. 750–751.
- ↑ Roman, Petrusha & Lomax 2002b, pp. 394 et seq..
- ↑ Harris 1999, p. 402–403.
- 1 2 Knittel 2002, p. 609.
- ↑ Harris 1999, p. 402.
- 1 2 Perry & Hettihewa 1998, p. 237.
- 1 2 Roman, Petrusha & Lomax 2002a, p. 69.
- ↑ Roman, Petrusha & Lomax 2002b, pp. 394–395.
- 1 2 3 Roman, Petrusha & Lomax 2002a, p. 86.
- 1 2 3 4 Petroutsos 2002, p. 115.
- 1 2 Lomax, Childs & Petrusha 2003, p. 341.
- 1 2 Roman, Petrusha & Lomax 2002b, pp. 395.
- ↑ Harris 1999, p. 416.
- ↑ Wakefield & Sonder 2001, p. 690.
- 1 2 Lomax, Childs & Petrusha 2003, p. 342.
- ↑ Roman, Petrusha & Lomax 2002b, pp. 397.
- ↑ Petrusha 2006, p. 367,528,637.
- 1 2 3 Petrusha 2006, p. 438.
- 1 2 Steele 2005a.
- ↑ Steele 2005b.
- ↑ Roman, Petrusha & Lomax 2002b, pp. 394.
- ↑ Israel & Jones 2001, p. 476.
- ↑ Price 2003, p. 119.
- 1 2 Roman, Petrusha & Lomax 2002a, p. 87.
- ↑ Petroutsos 2002, p. 114.
- ↑ MSDN 1.
- ↑ Lomax, Childs & Petrusha 2003, p. 343.
Sources used
- Harris, Matthew (1999). Sams teach yourself Microsoft Excel 2000 programming in 21 days. Sams Teach Yourself Series. Sams. ISBN 978-0-672-31543-5.
- Israel, Marc; Jones, J. Steven (2001). MCSE: SQL Server 2000 design study guide. Certification Study Guide Series. John Wiley and Sons. ISBN 978-0-7821-2942-7.
- Knittel, Brian (2002). "VBScript Reference". Windows XP under the hood. Que Publishing. ISBN 978-0-7897-2733-6.
- Lomax, Paul; Childs, Matt; Petrusha, Ronald (2003). VBScript in a nutshell. In a Nutshell (2nd ed.). O'Reilly Media, Inc. ISBN 978-0-596-00488-0.
- Marcus, William; Levy, Alex (2002). Visual Basic 6 Programming. Tata McGraw-Hill. ISBN 978-0-07-040290-4.
- Perry, Greg M.; Hettihewa, Sanjaya (1998). Sams teach yourself Visual Basic 6 in 24 hours. Teach Yourself in 24 Hours Series (2nd ed.). Sams Publishing. ISBN 978-0-672-31533-6.
- Petroutsos, Evangelos (2002). Mastering Visual Basic.NET. Mastering Series. John Wiley and Sons. ISBN 978-0-7821-2877-2.
- Petrusha, Ronald (2006). Visual Basic 2005: the complete reference. McGraw-Hill Professional. ISBN 978-0-07-226033-5.
- Price, Jason (2003). Mastering C# database programming. John Wiley and Sons. ISBN 978-0-7821-4183-2.
- Roman, Steven; Petrusha, Ron; Lomax, Paul (2002a). VB.NET Language Pocket Reference. O'Reilly Media, Inc. ISBN 978-0-596-00428-6.
- Roman, Steven; Petrusha, Ron; Lomax, Paul (2002b). VB.NET Language in a Nutshell. In a Nutshell (2nd ed.). O'Reilly Media, Inc. ISBN 978-0-596-00308-1.
- Steele, Patrick (2005-05-31). "IsNothing? Definitely!". Patrick Steele's .NET Blog.
- Steele, Patrick (2005-06-03). "Avoiding IsNothing()". Patrick Steele's .NET Blog.
- Wakefield, Cameron; Sonder, Henk-Evert (2001). Lee, Wei Meng, ed. VB.net Web developer's guide. Syngress. ISBN 978-1-928994-48-0.
- "ISNUMERIC (Transact-SQL)". MSDN: SQL Server 2008: Transact-SQL Reference. Microsoft.
Further reading
Microsoft's on-line documentation
- "
IsArray
method". MSDN: .NET Framework Class Library:Microsoft.VisualBasic
namespace. Microsoft. - "
IsDate
method". MSDN: .NET Framework Class Library:Microsoft.VisualBasic
namespace. Microsoft. - "
IsDBNull
method". MSDN: .NET Framework Class Library:Microsoft.VisualBasic
namespace. Microsoft. - "
IsError
method". MSDN: .NET Framework Class Library:Microsoft.VisualBasic
namespace. Microsoft. - "
IsNothing
method". MSDN: .NET Framework Class Library:Microsoft.VisualBasic
namespace. Microsoft. - "
IsNumeric
method". MSDN: .NET Framework Class Library:Microsoft.VisualBasic
namespace. Microsoft. - "
IsReference
method". MSDN: .NET Framework Class Library:Microsoft.VisualBasic
namespace. Microsoft.