Making Arrays Nullable in Azure Bicep

Comprehensive Guide to Handling Nullable Arrays in Bicep Deployments

Key Takeaways

• Use Union Types: Define array parameters with a union type that includes `null` to allow nullability.
• Handle Null Values Gracefully: Utilize Bicep functions like `coalesce` and `empty` to manage potential nulls effectively.
• Implement Safe Dereferencing: Apply the safe-dereference operator `?.` to prevent runtime errors when accessing array properties.

---

Introduction to Nullable Arrays in Azure Bicep

Azure Bicep is a domain-specific language (DSL) for deploying Azure resources declaratively. One common requirement in infrastructure-as-code is the ability to handle optional or nullable parameters, especially arrays. Making an array nullable allows templates to be more flexible and reusable, accommodating scenarios where certain configurations may or may not be provided.

Defining Nullable Arrays

Using Union Types

The most effective way to make an array nullable in Bicep is by defining the parameter with a union type that includes `null`. This approach explicitly states that the parameter can either be an array of a specific type or `null`. Here's how you can achieve this:

Example: Defining a Nullable String Array

param myNullableArray (string[] | null) = null

In this declaration:

• myNullableArray is the parameter name.
• string[] | null specifies that the parameter can be either an array of strings or null.
• = null sets the default value to null, making the parameter optional.

Using Optional Parameters

Alternatively, you can declare the parameter as optional by using the `?` suffix. This implicitly allows the parameter to be null if not provided.

Example: Optional Array Parameter

param myOptionalArray array? = null

Here, the `?` indicates that `myOptionalArray` is optional and can be omitted, defaulting to null.

Handling Nullable Arrays in Bicep

Using Conditional Logic

Once you've declared a parameter as nullable, it's essential to handle its null state within your Bicep code to avoid runtime errors. Conditional logic allows you to differentiate between a provided array and a null value.

Ternary Operator Example

var finalArray = myNullableArray != null ? myNullableArray : ['defaultValue']

This statement assigns `finalArray` to `myNullableArray` if it's not null; otherwise, it assigns a default array.

Using Bicep Functions

Bicep provides several functions to manage nullable arrays effectively:

Coalesce Function

The `coalesce` function returns the first non-null value from its arguments.

var processedArray = coalesce(myNullableArray, [])

This assigns an empty array to `processedArray` if `myNullableArray` is null.

Empty Function

The `empty` function checks whether an array is empty or null.

if (!empty(myNullableArray)) {
  // Use myNullableArray
}

This condition checks if `myNullableArray` is neither null nor empty before proceeding.

Safe Dereferencing Operator

The safe-dereference operator `?.` allows you to access properties of a potentially null object without causing runtime errors.

Example: Accessing Array Length Safely

var arrayLength = myNullableArray?.length ?? 0

This retrieves the length of `myNullableArray` if it's not null; otherwise, it defaults to `0`.

Best Practices for Nullable Arrays

Default to Empty Arrays

While allowing null values provides flexibility, defaulting to empty arrays (`[]`) is often more reliable. This approach minimizes the risk of unexpected null values causing deployment failures.

Example: Default Empty Array

param myArray array = []

Setting the default to an empty array ensures that the parameter always contains an array, even if no values are provided.

Implement Fallback Logic

Incorporate fallback logic to handle scenarios where the array might be null or empty. This ensures your templates remain robust and handle different deployment scenarios gracefully.

Example: Fallback with Coalesce

var finalArray = coalesce(myNullableArray, ['fallbackValue'])

This ensures that `finalArray` always has a value, either from `myNullableArray` or a predefined fallback.

Advanced Handling Techniques

Union Types with Specific Element Types

To enhance type safety, define union types that specify the array's element types alongside `null`. This ensures that when an array is provided, it adheres to the expected structure.

Example: Nullable Array of Objects

param myObjectArray (object[] | null) = null

This declaration allows `myObjectArray` to be either an array of objects or null, enforcing structure when values are provided.

Combining Multiple Nullable Parameters

In complex templates, you might have multiple nullable array parameters. It's crucial to handle each appropriately to maintain consistency and reliability.

Example: Multiple Nullable Arrays

param firstArray (string[] | null) = null
param secondArray (int[] | null) = null

var combinedArray = [
  ... (firstArray ?? [])
  ... (secondArray ?? [])
]

This combines `firstArray` and `secondArray`, defaulting to empty arrays if either is null.

Common Pitfalls and How to Avoid Them

Direct Use of Null Values

Assigning `null` directly to parameters without handling can lead to deployment failures. Always implement checks or defaults to mitigate this risk.

Ignoring Optional Parameters

Declaring parameters as optional but not handling their null state can cause unexpected behavior. Ensure that your template logic accounts for scenarios where these parameters are omitted.

Overlooking Type Safety

When defining union types, neglecting to specify element types can reduce type safety, making your templates more error-prone. Always define clear and specific types for union parameters.

Practical Examples

Defining a Nullable Array Parameter

Here's a practical example demonstrating how to define and handle a nullable array parameter in a Bicep template:

Example Template

param storageAccountNames (string[] | null) = null

var finalStorageAccounts = coalesce(storageAccountNames, [
  'defaultStorage1'
  'defaultStorage2'
])

resource storageAccounts 'Microsoft.Storage/storageAccounts@2021-04-01' = [for name in finalStorageAccounts: {
  name: name
  location: resourceGroup().location
  kind: 'StorageV2'
  sku: {
    name: 'Standard_LRS'
  }
}]

This template:

• Defines a nullable array parameter `storageAccountNames`.
• Uses the `coalesce` function to assign a default array if `storageAccountNames` is null.
• Creates storage account resources based on the final array.

Handling Null Arrays with Conditional Resources

Sometimes, you may want to conditionally deploy resources based on whether an array is provided. Here's how to achieve this:

Example: Conditional Deployment

param optionalArray (string[] | null) = null

resource optionalResource 'Microsoft.Example/resource@2021-01-01' = if (!empty(optionalArray)) {
  name: 'optionalResource'
  properties: {
    items: optionalArray
  }
}

This template deploys `optionalResource` only if `optionalArray` is not null or empty.

Advanced Topics

Null-Forgiving Operator

In scenarios where you're certain that a nullable array won't be null at runtime, you can use the null-forgiving operator `!` to suppress nullable warnings. However, use this with caution to avoid runtime errors.

Example: Using Null-Forgiving Operator

var arrayLength = length(myNullableArray!)

This treats `myNullableArray` as non-null, allowing you to access its `length` property without additional checks.

Leveraging Custom Functions

For complex scenarios, creating custom Bicep functions to handle nullable arrays can enhance code readability and maintainability.

Example: Custom Function for Nullable Arrays

@description('Ensures the array is not null')
function ensureArray(input: string[] | null) returns string[] {
  return coalesce(input, ['default1', 'default2'])
}

var safeArray = ensureArray(myNullableArray)

This function `ensureArray` ensures that `safeArray` always contains a valid array.

Best Practices Summary

• Prefer union types to explicitly allow `null` for array parameters.
• Always handle potential null values using Bicep functions like `coalesce` and `empty`.
• Use the safe-dereference operator to prevent runtime errors.
• Default to empty arrays where possible to enhance reliability.
• Implement fallback logic to maintain robustness in deployments.
• Leverage custom functions for complex array handling scenarios.

Conclusion

Making arrays nullable in Azure Bicep is a powerful technique that enhances the flexibility and reusability of your infrastructure-as-code templates. By defining parameters with union types, handling null values gracefully, and implementing best practices, you can create robust and scalable Bicep configurations. Whether deploying simple resources or managing complex environments, understanding how to effectively manage nullable arrays ensures your deployments are both reliable and maintainable.

References