It is deemed best practice to always use Option Explicit in VBA as it forces the developer to declare all their variables before use. This has other benefits too, such as auto-capitalization for declared variable names and IntelliSense.

Option Explicit

Sub OptionExplicit()
    Dim a As Integer
    a = 5
    b = 10 '// Causes compile error as 'b' is not declared
End Sub

Setting Require Variable Declaration within the VBE's Tools ► Options ► Editor property page will put the Option Explicit statement at the top of each newly created code sheet.

(opens new window)

This will avoid silly coding mistakes like misspellings as well as influencing you to use the correct variable type in the variable declaration. (Some more examples are given at ALWAYS Use "Option Explicit" (opens new window).)

Option Base is used to declare the default lower bound of array elements. It is declared at module level and is valid only for the current module.

By default (and thus if no Option Base is specified), the Base is 0. Which means that the first element of any array declared in the module has an index of 0.

If Option Base 1 is specified, the first array element has the index 1

Example in Base 0 :

Option Base 0

Sub BaseZero()

    Dim myStrings As Variant
    
    ' Create an array out of the Variant, having 3 fruits elements
    myStrings = Array("Apple", "Orange", "Peach")
    
    Debug.Print LBound(myStrings) ' This Prints "0"
    Debug.Print UBound(myStrings) ' This print "2", because we have 3 elements beginning at 0 -> 0,1,2
            
    For i = 0 To UBound(myStrings)
    
        Debug.Print myStrings(i) ' This will print "Apple", then "Orange", then "Peach"
    
    Next i

End Sub

Same Example with Base 1

Option Base 1

Sub BaseOne()

    Dim myStrings As Variant
    
    ' Create an array out of the Variant, having 3 fruits elements
    myStrings = Array("Apple", "Orange", "Peach")
    
    Debug.Print LBound(myStrings) ' This Prints "1"
    Debug.Print UBound(myStrings) ' This print "3", because we have 3 elements beginning at 1 -> 1,2,3
            
    For i = 0 To UBound(myStrings)
    
        Debug.Print myStrings(i) ' This triggers an error 9 "Subscript out of range"
    
    Next i

End Sub

The second example generated a Subscript out of range (Error 9) (opens new window) at the first loop stage because an attempt to access the index 0 of the array was made, and this index doesn't exists as the module is declared with Base 1

The correct code with Base 1 is :

   For i = 1 To UBound(myStrings)
    
        Debug.Print myStrings(i) ' This will print "Apple", then "Orange", then "Peach"
    
    Next i

It should be noted that the Split function (opens new window) always creates an array with a zero-based element index regardless of any Option Base setting. Examples on how to use the Split function can be found here (opens new window)

^{Split Function}

_{Returns a zero-based, one-dimensional array containing a specified number of substrings.}

In Excel, the Range.Value and Range.Formula properties for a multi-celled range always returns a 1-based 2D Variant array.

Likewise, in ADO, the Recordset.GetRows method always returns a 1-based 2D array.

One recommended 'best practice' is to always use the LBound (opens new window) and UBound (opens new window) functions to determine the extents of an array.

'for single dimensioned array
Debug.Print LBound(arr) & ":" & UBound(arr)
Dim i As Long
For i = LBound(arr) To UBound(arr)
    Debug.Print arr(i)
Next i

'for two dimensioned array
Debug.Print LBound(arr, 1) & ":" & UBound(arr, 1)
Debug.Print LBound(arr, 2) & ":" & UBound(arr, 2)
Dim i As long, j As Long
For i = LBound(arr, 1) To UBound(arr, 1)
    For j = LBound(arr, 2) To UBound(arr, 2)
         Debug.Print arr(i, j)
    Next j
Next i

The Option Base 1 must be at the top of every code module where an array is created or re-dimensioned if arrays are to be consistently created with an lower boundary of 1.

Option Compare Binary

Binary comparison makes all checks for string equality within a module/class case sensitive. Technically, with this option, string comparisons are performed using sort order of the binary representations of each character.

A < B < E < Z < a < b < e < z

If no Option Compare is specified in a module, Binary is used by default.

Option Compare Binary

Sub CompareBinary()

    Dim foo As String
    Dim bar As String
    
    '// Case sensitive
    foo = "abc"
    bar = "ABC"
    
    Debug.Print (foo = bar) '// Prints "False"
    
    '// Still differentiates accented characters
    foo = "ábc"
    bar = "abc"
    
    Debug.Print (foo = bar) '// Prints "False"
    
    '// "b" (Chr 98) is greater than "a" (Chr 97)
    foo = "a"
    bar = "b"
    
    Debug.Print (bar > foo) '// Prints "True"
    
    '// "b" (Chr 98) is NOT greater than "á" (Chr 225)
    foo = "á"
    bar = "b"
    
    Debug.Print (bar > foo) '// Prints "False"

End Sub

Option Compare Text

Option Compare Text makes all string comparisons within a module/class use a case insensitive comparison.

(A | a) < (B | b) < (Z | z)

Option Compare Text

Sub CompareText()

    Dim foo As String
    Dim bar As String
    
    '// Case insensitivity
    foo = "abc"
    bar = "ABC"
    
    Debug.Print (foo = bar) '// Prints "True"
    
    '// Still differentiates accented characters
    foo = "ábc"
    bar = "abc"
    
    Debug.Print (foo = bar) '// Prints "False"
    
    '// "b" still comes after "a" or "á"
    foo = "á"
    bar = "b"
    
    Debug.Print (bar > foo) '// Prints "True"

End Sub

Option Compare Database

Option Compare Database is only available within MS Access. It sets the module/class to use the current database settings to determine whether to use Text or Binary mode.

Note: The use of this setting is discouraged unless the module is used for writing custom Access UDFs (User defined functions) that should treat text comparisons in the same manner as SQL queries in that database.

Syntax

• Option optionName [value]
• Option Explicit
• Option Compare {Text | Binary | Database}
• Option Private Module
• Option Base {0 | 1}

Parameters

Remarks

It is much easier to control the boundaries of arrays by declaring the boundaries explicitly rather than letting the compiler fall back on an Option Base {0|1} declaration. This can be done like so:

Dim myStringsA(0 To 5) As String '// This has 6 elements (0 - 5)
Dim myStringsB(1 To 5) As String '// This has 5 elements (1 - 5)
Dim myStringsC(6 To 9) As String '// This has 3 elements (6 - 9)