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Constant expression is required

A Const statement doesn't properly initialize a constant, an array declaration uses a variable to specify the number of elements, or you're trying to initialize an array as the default value for an optional parameter.

Error ID: BC30059

To correct this error

  • If the declaration is a Const statement, check to make sure the constant is initialized with a literal, a previously declared constant, an enumeration member, or a combination of literals, constants, and enumeration members combined with operators.

  • If the declaration specifies an array, check to see if a variable is being used to specify the number of elements. If so, replace the variable with a constant expression.

  • If you're trying to initialize an array as the default value for an optional parameter, use one of the alternative approaches described in the Array initialization in optional parameters section.

  • If the preceding checks don't address the issue, try setting the Const to a different temporary value, running the program, and then resetting the Const to the desired value.

Array initialization in optional parameters

You cannot initialize an array as the default value for an optional parameter because array initialization is not a constant expression. The following code generates BC30059:

' This causes BC30059
Public Function MyFun(Optional filters() As (String, String) = New (String, String)() {}) As Boolean
    ' Function body
End Function

Solution 1: Use ParamArray instead of Optional

If you need to accept a variable number of arguments, consider using ParamArray instead of an optional parameter:

Public Function MyFun(ParamArray filters() As (String, String)) As Boolean
    ' The ParamArray automatically provides an empty array if no arguments are passed
    For Each filter In filters
        ' Process each filter
    Next
    Return True
End Function

' Can be called with any number of arguments:
MyFun() ' Empty array
MyFun(("name", "value"))
MyFun(("name1", "value1"), ("name2", "value2"))

Solution 2: Use Nothing as default and initialize in the method body

Set the default value to Nothing and check for it in your method:

Public Function MyFun(Optional filters() As (String, String) = Nothing) As Boolean
    If filters Is Nothing Then
        filters = New (String, String)() {}
    End If
    
    ' Process the filters array
    For Each filter In filters
        ' Process each filter
    Next
    Return True
End Function

' Can be called without arguments:
MyFun() ' Uses empty array
' Or with an array:
MyFun({("name", "value")})

Solution 3: Provide an overload without the parameter

Create an overloaded version of the method that doesn't require the parameter:

' Overload without the parameter
Public Function MyFun() As Boolean
    Return MyFun(New (String, String)() {})
End Function

' Main method with required parameter
Public Function MyFun(filters() As (String, String)) As Boolean
    ' Process the filters array
    For Each filter In filters
        ' Process each filter
    Next
    Return True
End Function

See also

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