Type Casting in Visual Basic .NET
Provided by: Molly Pell, Technical Project Manager
Visual Basic .NET provides two options for casting:
CType: Casts or converts one type into another type. If the
types do not match, coercion may be performed.
DirectCast: Casts one type to another type with better performance
than CType, but does not perform coercion if the types do not match.
Note that CType includes all of the VB conversion functions. These are CBool,
CByte, CChar, CDate, CDec, CDbl, CInt, CLng, CObj, CShort, CSng, and CStr.
The main difference between the two is that DirectCast only works if the
specified type and the run-time type of the expression are the same. This
difference only appears when converting from an object type to a value type,
or unboxing a value. For instance, the following DirectCast operation will
fail because the boxed type of the object O is not an integer:
Dim O As
Object = 1.5
Dim I As
Integer =
DirectCast(O, Integer)
' Causes a run-time error.
Dim I As
Integer = CType(O,
Integer)
' Does not cause a run-time error.
The DirectCast operation in this example, on the other hand, will succeed:
Dim O As
Object = 1
Dim I As
Integer =
DirectCast(O, Integer)
' Succeeds.
Dim I As
Integer = CType(O,
Integer)
' Succeeds, but is slower than DirectCast
When converting from an object to a value type, or unboxing, the DirectCast
operator has better performance than the CType operator. The Visual Basic
.NET compiler generates four lines of IL code for DirectCast. However, using
CType causes the Visual Basic .NET compiler to generate a call to a
conversion method that is well over one hundred lines of IL code. This
method in turn calls other methods. In performance critical code, the
difference can be substantial. If coercion is necessary, however, you must
still use the CType operation, or perform the coercion manually. For
example:
Dim O As
Object = 1.5
Dim f As
Single = DirectCast(O,
Single)
' Cast to the runtime type
Dim i As
Integer = CType(f,
Integer)
' CType is OK here, since this
' cast doesn't unbox 'f'
Conclusion:
When type casting from an object type to a value type, or unboxing, you
should first determine whether type coercion is necessary. If no coercion is
necessary (i.e. the boxed type of the value is the same as the type
specified in the cast expression), use the DirectCast expression to increase
application performance.
This tip and more detected for you with Total .NET Analyzer! See
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