A Rose By Any Other Name - Short Circuiting

Suppose you have the following C# program:

It is composed of a main function, and two functions that are called by the main function.

void Main()
{
    var config = new LoggerConfiguration().WriteTo.Console();
    Log.Logger = config.CreateLogger();

    if (firstChecK() && secondCheck())
    {
    	Log.Information("Do work");
    }
    else
    {
    	Log.Information("Skip work");
    }
}

bool firstChecK()
{
    Log.Information("Accessing first check");
    return false;
}

bool secondCheck()
{
    Log.Information("Accessing second check");
    return false;
}

If you run this program, given that the first function returns false, you would expect it to ultimately print ‘Skip Work’

Which it does:

[18:20:51 INF] Accessing first check
[18:20:51 INF] Skip work

Now, let us take the exact same program in VB.NET

Sub Main
    Dim config = New LoggerConfiguration().WriteTo.Console()
    Log.Logger = config.CreateLogger()
    
    If firstChecK() And secondCheck() Then
        Log.Information("Do work")
    Else
    	Log.Information("Skip work")
    End If
End Sub

Function firstChecK() As Boolean
    Log.Information("Accessing first check")
    Return False
End Function

Function secondCheck() As Boolean
    Log.Information("Accessing second check")
    Return False
End Function

If we run this program, it also prints ‘Skip work’. But if you look closely you will find a difference in the outputs; to whit:

In the C# program, that line was not printed.

What does this mean?

In C#, if the first check is false, it does not bother to evaluate the second check. This is called short circuiting.

VB.NET does not short circuit the AND operator. It will evaluate every condition in a logic statement.

Which begs the question: what if you in fact wanted to short circuit in VB.NET?

This can be achieved using the AndAlso operator.

You can rewrite the logic check as follows:

If firstChecK() AndAlso secondCheck() Then
    Log.Information("Do work")
Else
    Log.Information("Skip work")
End If

It now short circuits.

Correspondingly, if you do NOT want short-circuiting in C#, how do you achieve that?

You use the logical AND operator, &, which is different from the && operator, the conditional AND.

You can rewrite the logic check in C# as follows:

if (firstChecK() & secondCheck())
{
    Log.Information("Do work");
else
{
    Log.Information("Skip work");
}

This will now perform all the checks.

When is this useful?

A simple case in use interface development is to validate a value only if it has been provided.

For instance to check if a text is a certain length in a TextBox:

if(textEdit.HasValue && textEdit.Length > 10)
{
    Log.Warning("Your provided text is too long");
}

This works out because the length is only checked if there is in fact a provided value.

The same code in VB is as follows:

If textEdit.HasValue AndAlso textEdit.Length > 10 Then
    Log.Warning("Your provided text is too long");
End If

Happy hacking!