Enhance BC30059 documentation with array initialization guidance for optional parameters (#48258)

* Initial plan

* Enhance BC30059 documentation with array initialization guidance

Co-authored-by: BillWagner <[email protected]>

* Apply suggestions from code review

Co-authored-by: Genevieve Warren <[email protected]>

* Change numbered list to bullets in BC30059 error correction steps

Co-authored-by: BillWagner <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>
Co-authored-by: Genevieve Warren <[email protected]>

Author: 3 people

docs/visual-basic/misc/bc30059.md

@@ -11,18 +11,97 @@ ms.assetid: fdd5e7bb-6370-4a63-bbb6-23b15badb4c8
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 # Constant expression is required
 
-A `Const` statement doesn't properly initialize a constant, or an array declaration uses a variable to specify the number of elements.  
+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  
 
-1. 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 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.  
 
-2. 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 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](#array-initialization-in-optional-parameters) section.
 
-3. 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.
+- 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:
+
+```vb
+' 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:
+
+```vb
+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:
+
+```vb
+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:
+
+```vb
+' 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
 
 - [Const Statement](../language-reference/statements/const-statement.md)
+- [Optional Parameters](../programming-guide/language-features/procedures/optional-parameters.md)
+- [Parameter Arrays](../programming-guide/language-features/procedures/parameter-arrays.md)
+- [How to: Initialize an Array Variable in Visual Basic](../programming-guide/language-features/arrays/how-to-initialize-an-array-variable.md)
+- [Arrays in Visual Basic](../programming-guide/language-features/arrays/index.md)