Question

Which of the following is more readable and preferable?

This, where the default value is assigned in the declaration:

    Dim isLastWeekOfPeriod As Boolean = False
    If periodInfo.WeekCount = weekInfo.Week Then
        isLastWeekOfPeriod = True
    End If

Or this, where the value is set in the else clause:

    Dim isLastWeekOfPeriod As Boolean
    If periodInfo.WeekCount = weekInfo.Week Then
        isLastWeekOfPeriod = True
    Else
        isLastWeekOfPeriod = False
    End If

Answer 1

Neither.

Write the assignment directly in the initialisation, don’t use If here at all:

Dim isLastWeekOfPeriod As Boolean = (periodInfo.WeekCount = weekInfo.Week)

The parentheses are not necessary but make the assignment-vs.-comparison more readable. By the way, with Option Infer On you can also safely remove the type name, it’s obvious from the initialisation expression and the variable name prefix is:

Dim isLastWeekOfPeriod = (periodInfo.WeekCount = weekInfo.Week)

Remember: Short is good (as long as it doesn’t negatively affect readability).

Rationale

Your first code is acceptable but it does more or less the same as my above code and has four times more lines of code, without any benefit whatsoever.

The second code does the initialisation twice: once in the declaration, since every variable is implicitly default-initialised in VB (thus isLastWeekOfPeriod takes on the value False in the first line). And once in the conditional block after that. Oh, and finally this code is five times as long as mine.

Furthermore, writing If a Then result = True Else result = False is always doing redundant work. There is no reason not just to write result = a directly.