Short Circuiting 'Or' Evaluation in Visual Basic .NET

Some years back, in the post “A Rose By Any Other Name - Short Circuiting”, we looked at how Visual Basic.NET does not do short circuit evaluation when evaluating an AND condition.

In this post, we will look at a a similar scenario, but with the OR condition.

Take this C# code, where we have two methods:

public bool FirstCheck()
{
	Console.WriteLine("This is the first check");
	return true;
}


public bool SecondCheck()
{
	Console.WriteLine("This is the second check");
	return true;
}

We then call these methods as follows:

if (FirstCheck() || SecondCheck())
{
	Console.WriteLine("Our checks ran here");
}

The following will be printed on your console:

Here we can see that the second method, SecondCheck(), never ran.

This is short circuit behaviour, meaning the runtime decides there is no point in evaluating the second condition, given the first is true.

Let us mirror this code in VB.NET.

First, our methods:

Public Function FirstCheck() As Boolean
	Console.WriteLine("This is the first check")
	Return True
End Function

Public Function SecondCheck() As Boolean
	Console.WriteLine("This is the second check")
	Return True
End Function

Next, the invocation:

If FirstCheck() Or SecondCheck() Then
	Console.WriteLine("Our checks ran here")
End If

This will print the following:

Of interest here is that both methods have ran, as VB.NET does not short circuit.

If you want short circuit behaviour, you can use the OrElse operator, like so:

If FirstCheck() OrElse SecondCheck() Then
	Console.WriteLine("Our checks ran here")
End If

This will print the expected:

TLDR

VB.NET does not short-circuit OR evaluation. To get that behaviour, use the OrElse operator.

Happy hacking!