User-defined data types in Bicep

Learn how to use user-defined data types in Bicep.

Bicep version 0.12.1 or newer is required to use this feature.

User-defined data type syntax

You can use the type statement to define user-defined data types. In addition, you can also use type expressions in some places to define custom types.

type <user-defined-data-type-name> = <type-expression>

The valid type expressions include:

• Symbolic references are identifiers that refer to an ambient type (like string or int) or a user-defined type symbol declared in a type statement:

  // Bicep data type reference
  type myStringType = string
  
  // user-defined type reference
  type myOtherStringType = myStringType
  

• Primitive literals, including strings, integers, and booleans, are valid type expressions. For example:

  // a string type with three allowed values.
  type myStringLiteralType = 'bicep' | 'arm' | 'azure'
  
  // an integer type with one allowed value
  type myIntLiteralType = 10
  
  // an boolean type with one allowed value
  type myBoolLiteralType = true
  

• Array types can be declared by suffixing [] to any valid type expression:

  // A string type array
  type myStrStringsType1 = string[]
  // A string type array with three allowed values
  type myStrStringsType2 = ('a' | 'b' | 'c')[]
  
  type myIntArrayOfArraysType = int[][]
  
  // A mixed-type array with four allowed values
  type myMixedTypeArrayType = ('fizz' | 42 | {an: 'object'} | null)[]
  

• Object types contain zero or more properties between curly brackets:

  type storageAccountConfigType = {
    name: string
    sku: string
  }
  

  Each property in an object consists of key and value. The key and value are separated by a colon :. The key may be any string (values that would not be a valid identifier must be enclosed in quotes), and the value may be any type syntax expression.

  Properties are required unless they have an optionality marker ? after the property value. For example, the sku property in the following example is optional:

  type storageAccountConfigType = {
    name: string
    sku: string?
  }
  

  Decorators may be used on properties. * may be used to make all values require a constraint. Additional properties may still be defined when using *. This example creates an object that requires a key of type int named id, and that all other entries in the object must be a string value at least 10 characters long.

  type obj = {
    @description('The object ID')
    id: int
  
    @description('Additional properties')
    @minLength(10)
    *: string
  }
  

  Recursion

  Object types may use direct or indirect recursion so long as at least leg of the path to the recursion point is optional. For example, the myObjectType definition in the following example is valid because the directly recursive recursiveProp property is optional:

  type myObjectType = {
    stringProp: string
    recursiveProp: myObjectType?
  }
  

  But the following would not be valid because none of level1, level2, level3, level4, or level5 is optional.

  type invalidRecursiveObjectType = {
    level1: {
      level2: {
        level3: {
          level4: {
            level5: invalidRecursiveObject
          }
        }
      }
    }
  }
  

• Bicep unary operators can be used with integer and boolean literals or references to integer or boolean literal-typed symbols:

  type negativeIntLiteral = -10
  type negatedIntReference = -negativeIntLiteral
  
  type negatedBoolLiteral = !true
  type negatedBoolReference = !negatedBoolLiteral
  

• Unions may include any number of literal-typed expressions. Union types are translated into the allowed-value constraint in Bicep, so only literals are permitted as members.

  type oneOfSeveralObjects = {foo: 'bar'} | {fizz: 'buzz'} | {snap: 'crackle'}
  type mixedTypeArray = ('fizz' | 42 | {an: 'object'} | null)[]
  

In addition to be used in the type statement, type expressions can also be used in these places for creating user-defined date types:

• As the type clause of a param statement. For example:

  param storageAccountConfig {
    name: string
    sku: string
  }
  

• Following the : in an object type property. For example:

  param storageAccountConfig {
   name: string
    properties: {
      sku: string
    }
  } = {
    name: 'store$(uniqueString(resourceGroup().id)))'
    properties: {
      sku: 'Standard_LRS'
    }
  }
  

• Preceding the [] in an array type expression. For example:

  param mixedTypeArray ('fizz' | 42 | {an: 'object'} | null)[]
  

A typical Bicep file to create a storage account looks like:

param location string = resourceGroup().location
param storageAccountName string

@allowed([
  'Standard_LRS'
  'Standard_GRS'
])
param storageAccountSKU string = 'Standard_LRS'

resource storageAccount 'Microsoft.Storage/storageAccounts@2022-09-01' = {
  name: storageAccountName
  location: location
  sku: {
    name: storageAccountSKU
  }
  kind: 'StorageV2'
}

By using user-defined data types, it can look like:

param location string = resourceGroup().location

type storageAccountSkuType = 'Standard_LRS' | 'Standard_GRS'

type storageAccountConfigType = {
  name: string
  sku: storageAccountSkuType
}

param storageAccountConfig storageAccountConfigType

resource storageAccount 'Microsoft.Storage/storageAccounts@2022-09-01' = {
  name: storageAccountConfig.name
  location: location
  sku: {
    name: storageAccountConfig.sku
  }
  kind: 'StorageV2'
}

Declare tagged union type

To declare a custom tagged union data type within a Bicep file, you can place a discriminator decorator above a user-defined type declartion. Bicep version 0.21.1 or newer is required to use this decorator. The syntax is:

@discriminator('<propertyName>')

The discriminator decorator takes a single parameter, which represents a shared property name among all union members. This property name must be a required string literal on all members and is case-sensitive. The values of the discriminated property on the union members must be unique in a case-insensitive manner.

The following example shows how to declare a tagged union type:

type FooConfig = {
  type: 'foo'
  value: int
}

type BarConfig = {
  type: 'bar'
  value: bool
}

@discriminator('type')
type ServiceConfig = FooConfig | BarConfig | { type: 'baz', *: string }

param serviceConfig ServiceConfig = { type: 'bar', value: true }

output config object = serviceConfig

The parameter value is validated based on the discriminated property value. In the preceeding example, if the serviceConfig parameter value is of type foo, it undersoes validation using the FooConfigtype. Likewise, if the parameter value is of type bar, validation is performed usin the BarConfig type, and this pattern continues for other types as well.

Import types between Bicep files (Preview)

Bicep version 0.21.1 or newer is required to use this compile-time import feature. The experimental flag compileTimeImports must be enabled from the Bicep config file.

Only user-defined data types that bear the @export() decorator can be imported to other templates. Currently, this decorator can only be used on type statements.

The following example enables you to import the two user-defined data types from other templates:

@export()
type myStringType = string

@export()
type myOtherStringType = myStringType

For more information, see Import user-defined data types.

Next steps

• For a list of the Bicep data types, see Data types.