BC42104: Variable ‘X’ is used before it has been assigned a value.

1 minute read

My dearest VB.NET programmers (and C# programmers up to a point), I once again have to inform you of one of my pet peeves. This is pretty much a follow-up to one of my previous posts, but more than two years after, I still work in projects from various online sources as well as professionally that are littered with the warnings about undefined variables:

warning BC42104: Variable 'X' is used before it has been assigned a value. A null reference exception could result at runtime.

At runtime, these warnings usually translate into the dreaded NullReferenceException:

NullReferenceException was unhandled, Object Reference not set to an instance of an object.

So, what causes this and how can can we fix this? Let’s take a look at a piece of code that demonstrates something incredibly common, especially in VB circles:

Dim MyString As String

If ThisAndThat Then
    MyString = "Some Value"
End If

...

SomeFunction(MyString)

If the boolean ThisAndThat evaluates to False, MyString will never be defined, and a NullReferenceException will arise when the SomeFunction function tries to do anything with MyString. Remember that The System.String class does not have a default constructor, so no constructor is implicitly called. MyString is NOT String.Empty, but NULL.

To fix this, all you need is an Else statement in your code and some default value that you’d like to assign to MyString:

Dim MyString As String

If ThisAndThat Then
    MyString = "Some Value"
Else
    MyString = String.Empty
End If

...

SomeFunction(MyString)

In fact, the best way to handle default values is to define the variable upon declaration:

Dim MyString As String = String.Empty

If ThisAndThat Then
    MyString = "Some Value"
End If

...

SomeFunction(MyString)

Or you could even eliminate the If statement entirely by using the IIf function:

Dim MyString As String = IIf(ThisAndThat, "Some Value", String.Empty).ToString()

...

SomeFunction(MyString)

And even though I’m using the class System.String as an example, this should be applicable to any variable of any data type. It’s a good practice and will eliminate many of those pesky runtime errors that can be difficult to trace.

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