Why is this an issue?

Short-circuit evaluation is an evaluation strategy for Boolean operators, that doesn’t evaluate the second argument of the operator if it is not needed to determine the result of the operation.

VB.NET provides logical operators that implement short-circuiting evaluations AndAlso and OrElse, as well as the non-short-circuiting versions And and Or. Unlike short-circuiting operators, the non-short-circuiting operators evaluate both operands and afterward perform the logical operation.

For example False AndAlso FunctionCall always results in False even when the FunctionCall invocation would raise an exception. In contrast, False And FunctionCall also evaluates FunctionCall, and results in an exception if FunctionCall raises an exception.

Similarly, True OrElse FunctionCall always results in True, no matter what the return value of FunctionCall would be.

The use of non-short-circuit logic in a boolean context is likely a mistake, one that could cause serious program errors as conditions are evaluated under the wrong circumstances.

How to fix it

Code examples

Noncompliant code example

If GetTrue() Or GetFalse() Then ' Noncompliant: both sides evaluated
End If

Compliant solution

If GetTrue() OrElse GetFalse() Then ' Compliant: short-circuit logic used
End If

Resources

Documentation

• Logical and Bitwise Operators in Visual Basic
• Short-circuit evaluation
• Boolean operators

Articles & blog posts

• Eric Lippert’s blog - When would you use & on a bool?