Type checking in VB.Net ~ andyfrench.info

Friday, 10 September 2010

Type checking in VB.Net

OK, here’s another little reminder about VB.Net syntax.

Take the following C# fragment:

if (instance is TestDomainObject)
{
    var testInstance = instance as TestDomainObject;
    _data.Remove(testInstance.Id);
}

In VB.Net the equivalent is:

If (TypeOf instance Is TestDomainObject) Then
    Dim testInstance = TryCast(instance, TestDomainObject)
    _data.Remove(testInstance.Id)
End If

Note the use of the TypeOf keyword.

Note also that casting in VB.Net takes a bit of getting used to. There are essentially 3 ways to perform casting:

Keyword	Data types	Argument relationship	Run-time failure
CType Function	Any data types	Widening or narrowing conversion must be defined between the two data types	Throws InvalidCastException
DirectCast	Any data types	One type must inherit from or implement the other type	Throws InvalidCastException
TryCast	Reference types only	One type must inherit from or implement the other type	Returns Nothing (Visual Basic)