CODEWITHSUNDEEP
vb.netwarnings
Nick
Nick

Reputation: 5965

No warning in VB.NET when function has no return

Some comments on Stack Overflow question Why doesn't the C# compiler stop properties from referring to themselves? regarding warnings got me thinking about old issues that always goofed me up when I was writing more VB.NET code.

One of them was the fact that the compiler didn't warn if you declared a Function but never did an explicit Return statement or assign to the Function name. Running Visual Studio 2008, I just made a small experimental project, and it seems as though the behavior has never been fixed. I created a Visual Basic Console application, with the following code:

Module MainModule

    Sub Main()

        Dim test As Boolean = TestWarning()

    End Sub

    Function TestWarning() As Boolean

        Console.WriteLine("There is no Return Statement")

    End Function

End Module

I also went into the Project Settings and turned On Option Strict and Option Explicit. I also set the Warning Configurations so that "Function/Operator with no return value" was set to Error.

I compiled the project and got no warning, and no error on the TestWarning() Function. This seems like a great place to put a warning, because it will default to False, and you may have simply forgotten to do a return. C# will error without a return statement. I thought that VB.NET did the same thing with the "Function/Operator with no return value" configuration. Is this a bug, or is there something I'm missing?

Further Experimentation

Function TestWarning() As Boolean

    If DateTime.Now.DayOfWeek = DayOfWeek.Monday Then
        Return False
    Else
        Console.WriteLine("There is no Return Statement")
    End If

End Function

If I have an explicit Return in an If, and nothing in the Else, there is also no warning or error. It will simply take the default, even though you likely intended (via programming style) to have an explicit return. In this case, I explicitly returned False (which is the default for Boolean), so it's likely a hidden bug that I should have returned True in the Else.

Upvotes: 6

Views: 9352

Answers (5)

Sascha
Sascha

Reputation: 1218

I just wasted seven hours about the same problem. In my case (using Visual Studio 2012) the compilation ended with a compilation error:

Error MSB3030: File "obj\x86 Debug\<Name of the build target>.exe" could not be copied, because the file was not found. (0, 0)

(The text was translated from the German error message, and I hope it's translated properly.)

Since that message didn't help, I reverted my project back to state where it compiled and finally tracked it down to something similar to the following:

Function SomeFunction() as Boolean
  If <somecondition> then 
    Exit Function
  end if
end function

If I replace the exit function with return false or if I insert a SomeFunction = false then the compiler error won't show up.

So yes, as stated before, Visual Basic should default the return value to the default value of the return type, but it somehow crashed the compilation.

I suspect this behaviour is hard to reproduce. My bottom line is: Set return values explicitly. It's clearer to the code and might save you seven hours of debugging.

Upvotes: 0

John K
John K

Reputation: 28917

You aren't necessarily asking for a workaround, but I'm just thinking out loud: You could make the return type of the function Nullable.

At runtime, if the function reflects Nothing, you know the programmer didn't explicitly assign a return value. Of course this only works for functions that don't naturally return Nothing. And it's inefficient in many ways.

Upvotes: 0

John Knoeller
John Knoeller

Reputation: 34218

It's part of the BASIC language. All basic functions have a return type. If you do not specify a return type, then the type is assumed to be object.

So the compiler can't warn you about a missing return, because it doesn't know if you skipped the return intentionally or because you were taking advantage of the default return value feature of the language.

From this http://msdn.microsoft.com/en-us/library/sect4ck6(VS.80).aspx

if you use Exit Function without assigning a value to name, the procedure returns the default value for the data type specified in returntype. If returntype is not specified, the procedure returns Nothing, the default value for Object.

Upvotes: 6

GSerg
GSerg

Reputation: 78210

The warning will only let you know when a function is going to return Nothing by default.

You would get a warning if return value was of a reference type.

But your function has a return value of a value type, and those cannot be Nothing. Therefore, no warning.

This is because function name inside this very function acts as a result variable. You can return a value by assigning it to the function name instead of using Return. And all variables are initialized with default values, including the function-name variable. This is not the case in C, hence the different meaning of the warning.

Compare this to using variables before initializing them:

Dim x As Integer
CallFunction(x)  'No warning, x is implicitly and properly initialized to 0.

Dim y as Object
CallFunction(y)  'A warning: variable used before a value is assigned to it

Upvotes: 8

rh.
rh.

Reputation: 1721

That looks like a bug to me. I am able to reproduce it on my end in VS2008 SP1.

What is interesting is that it DOES work if it is a reference type, but does not if it is a value type.

Upvotes: 0

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