## Why is this an issue?

A cast is an 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:

    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 interfaces.

### What is the potential impact?

Invalid casts will lead to unexpected behaviors or runtime errors such as 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 `TryCast` operator. When the conversion is not possible, the `TryCast` operator returns `Nothing` and will never raise an exception.

### Code examples

#### Noncompliant code example

    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

    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

- Type Conversions in Visual Basic
  - Implicit and Explicit Conversions in Visual Basic
  - How to: Convert an Object to Another Type in Visual Basic
- `DirectCast` operator
- `CType` function
- `TryCast` operator
- CWE - CWE-588 - Attempt to Access Child of a Non-structure Pointer
- CWE - CWE-704 - Incorrect Type Conversion or Cast