Flattening Arrow Code

I often encounter code like this:

if (rowCount > rowIdx)
{
 if (drc[rowIdx].Table.Columns.Contains("avalId")
 {
 do
 {
 if (Attributes[attrVal.AttributeClassId] == null)
 {
 // do stuff
 }
 else
 {
 if (!(Attributes[attrVal.AttributeClassId] is ArrayList))
 {
 // do stuff
 }
 else
 {
 if (!isChecking)
 {
 // do stuff
 }
 else
 {
 // do stuff
 }
 }
 }
 rowIdx++;
 }
 while (rowIdx < rowCount && GetIdAsInt32(drc[rowIdx]) == Id);
 }
 else
 rowIdx++;
}
return rowIdx;

The excessive nesting of conditional clauses pushes the code out into an arrow formation:

if
 if
 if
 if
 do something
 endif
 endif
 endif
endif

And you know you’re definitely in trouble when the code you’re reading is regularly exceeding the right margin on a typical 1280x1024 display. This is the Arrow Anti-Pattern in action.

One of my primary refactoring tasks is "flattening" arrow code like this. Those sharp, pointy barbs are dangerous! Arrow code has a high cyclomatic complexity value – a measure of how many distinct paths there are through code:

Studies show a correlation between a program’s cyclomatic complexity and its error frequency. A low cyclomatic complexity contributes to a program’s understandability and indicates it is amenable to modification at lower risk than a more complex program. A module’s cyclomatic complexity is also a strong indicator of its testability.

Where appropriate, I flatten that arrow code by doing the following:

1. Replace conditions with guard clauses. This code...

if (SomeNecessaryCondition)
{
 // function body code
}

... works better as a guard clause:

if (!SomeNecessaryCondition)
{
throw new RequiredConditionMissingException;
}
// function body code

2. Decompose conditional blocks into separate functions. In the above example, we’re in a do... while loop which could be decomposed:

do
{
 ValidateRowAttribute(drc[rowIdx]);
 rowIdx++;
}
while(rowIdx < rowCount && GetIdAsInt32(drc[rowIdx]) == Id);

3. Convert negative checks into positive checks. As a broad rule, I prefer to put the positive comparison first and let the negative comparison fall out naturally into the else clause. I think this reads a lot better and, more importantly, avoids the "I ain't not never doing that" syndrome:

if (Attributes[attrVal.AttributeClassId] is ArrayList)
{
 // do stuff
}
else
{
 // do stuff
}

4. Always opportunistically return as soon as possible from the function. Once your work is done, get the heck out of there! This isn’t always possible – you might have resources you need to clean up. But whatever you do, you have to abandon the ill-conceived idea that there should only be one exit point at the bottom of the function.

The goal is to have code that scrolls vertically a lot... but not so much horizontally.