Converting to Strong Types

Replaces loose object and array parameter types with strong alternatives for compile-time validation and autocompletion.

Conversion Workflow

1. Identify loose types

   • Search for param <name> object and param <name> array declarations.

2. Analyze parameter usage

   • Examine how the parameter is used in the template to determine expected structure.

3. Choose conversion strategy

4. Define types and update parameters

   • Place type definitions above parameters.

5. Validate

   • Run command bicep build <.bicep file> --stdout --no-restore to verify syntax and type correctness.

Quick Reference

Simple Arrays

// Before
param addresses array

// After
param addresses string[]

Union Types (Constrained Values)

param environments ('dev' | 'staging' | 'prod')[]
param skuName 'Standard_LRS' | 'Premium_LRS'

User-Defined Types

type subnetType = {
  name: string
  addressPrefix: string
  nsgId: string?  // Optional property
}

param subnets subnetType[]

Type Best Practices

• Use ? for optional properties: description: string?
• Use @sealed() to prevent extra properties at deployment
• Use @description() on types and properties
• Use constraints: @minLength(), @maxLength(), @minValue(), @maxValue()
• Compose complex types from simpler types

MCP Tools

Edge Cases

Dynamic objects (unknown keys): Use wildcard { *: string }

Mixed-type arrays: Use union (string | int | bool)[]

Optionality and backwards compatibility: Use optional types ? for new properties

Resource-Derived Types

Use resourceInput<> and resourceOutput<> to derive types directly from Azure resource schemas. Prioritise this approach over custom user-defined types.

Syntax

resourceInput<'<resourceType>@<apiVersion>'>   // Writable properties
resourceOutput<'<resourceType>@<apiVersion>'>  // Readable properties (includes computed)

When to Use

• Parameter must match resource property structure exactly
• Want to avoid maintaining custom types that mirror resource properties

Examples

Full Properties Block

param storageAccountProps resourceInput<'Microsoft.Storage/storageAccounts@2023-01-01'>.properties = {
  accessTier: 'Hot'
  minimumTlsVersion: 'TLS1_2'
  allowBlobPublicAccess: false
  supportsHttpsTrafficOnly: true
}

resource storageAccount 'Microsoft.Storage/storageAccounts@2023-01-01' = {
  name: 'mystorageacct'
  location: resourceGroup().location
  sku: { name: 'Standard_LRS' }
  kind: 'StorageV2'
  properties: storageAccountProps
}

Output Types

output endpoints resourceOutput<'Microsoft.Storage/storageAccounts@2024-01-01'>.properties.primaryEndpoints

Do not do this

Do not use a type definition for resource-derived types but define them directly in parameters or outputs.

type managedRulesDefinitionType = resourceInput<'Microsoft.Network/ApplicationGatewayWebApplicationFirewallPolicies@2024-07-01'>.properties.managedRules
type customRulesType = resourceInput<'Microsoft.Network/ApplicationGatewayWebApplicationFirewallPolicies@2024-07-01'>.properties.customRules
type policySettingsType = resourceInput<'Microsoft.Network/ApplicationGatewayWebApplicationFirewallPolicies@2024-07-01'>.properties.policySettings
type wafPolicyTagsType = resourceInput<'Microsoft.Network/ApplicationGatewayWebApplicationFirewallPolicies@2024-07-01'>.tags