rspec/rules/S1944/vbnet/rule.adoc

== Why is this an issue?

A cast is an https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/data-types/implicit-and-explicit-conversions[explicit conversion], which is a way to tell the compiler the intent to convert from one type to another.

In Visual Basic, there are two explicit conversion operators:

[source,vbnet]
----
Public Sub Method(Value As Object)
    Dim i As Integer
    i = DirectCast(Value, Integer)  ' Direct casting from object holding an integer type to Integer
    i = CType(Value, Integer)       ' Conversion from the underlying type to Integer
End Sub
----

In most cases, the compiler will be able to catch invalid casts between incompatible value types or reference types.

However, the compiler will not be able to detect invalid casts to https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/interfaces/[interfaces].

=== What is the potential impact?

Invalid casts will lead to unexpected behaviors or runtime errors such as https://learn.microsoft.com/en-us/dotnet/api/system.invalidcastexception[InvalidCastException].

=== Exceptions

No issue is reported if the interface has no implementing class in the assembly.

== How to fix it

To prevent an `InvalidCastException` from raising during an explicit conversion, it is recommended to use the https://learn.microsoft.com/en-us/dotnet/visual-basic/language-reference/operators/trycast-operator[`TryCast` operator].
When the conversion is not possible, the `TryCast` operator returns `Nothing` and will never raise an exception.

=== Code examples

==== Noncompliant code example

[source,vbnet,diff-id=1,diff-type=noncompliant]
----
Public Interface IMyInterface
End Interface

Public Class Implementer
    Implements IMyInterface
End Class

Public Class AnotherClass
End Class

Module Program
    Sub Main()
        Dim Another As New AnotherClass
        Dim x As IMyInterface = DirectCast(Another, IMyInterface)   ' Noncompliant: InvalidCastException is being thrown
    End Sub
End Module
----

==== Compliant solution

[source,vbnet,diff-id=1,diff-type=compliant]
----
Public Interface IMyInterface
End Interface

Public Class Implementer
    Implements IMyInterface
End Class

Public Class AnotherClass
End Class

Module Program
    Sub Main()
        Dim Another As New AnotherClass
        Dim x = TryCast(Another, IMyInterface)                      ' Compliant: but will always be Nothing
    End Sub
End Module
----

== Resources

=== Documentation
* https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/data-types/type-conversions[Type Conversions in Visual Basic]
** https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/data-types/implicit-and-explicit-conversions[Implicit and Explicit Conversions in Visual Basic]
** https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/data-types/how-to-convert-an-object-to-another-type[How to: Convert an Object to Another Type in Visual Basic]
* https://learn.microsoft.com/en-us/dotnet/visual-basic/language-reference/operators/directcast-operator[`DirectCast` operator]
* https://learn.microsoft.com/en-us/dotnet/visual-basic/language-reference/functions/ctype-function[`CType` function]
* https://learn.microsoft.com/en-us/dotnet/visual-basic/language-reference/operators/trycast-operator[`TryCast` operator]
* CWE - https://cwe.mitre.org/data/definitions/588[CWE-588 - Attempt to Access Child of a Non-structure Pointer]
* CWE - https://cwe.mitre.org/data/definitions/704[CWE-704 - Incorrect Type Conversion or Cast]

include::../rspecator-dotnet.adoc[]