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User-defined functions in ARM template

User-defined functions help simplify and reuse complex expressions in your ARM templates. By encapsulating logic in custom functions, you can improve template readability, maintainability, and scalability-especially when the same logic appears in multiple places. These functions are distinct from the built-in template functions that are always available. To define user-defined functions in Bicep, see User-defined functions in Bicep.

Define the function

Your functions require a namespace value to avoid naming conflicts with template functions. The following example shows a function that returns a unique name:

"functions": [
  {
    "namespace": "contoso",
    "members": {
      "uniqueName": {
        "parameters": [
          {
            "name": "namePrefix",
            "type": "string"
          }
        ],
        "output": {
          "type": "string",
          "value": "[concat(toLower(parameters('namePrefix')), uniqueString(resourceGroup().id))]"
        }
      }
    }
  }
],

Use the function

The following example shows a template that includes a user-defined function to get a unique name for a storage account. The template has a parameter named storageNamePrefix that is passed as a parameter to the function.

{
 "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
 "contentVersion": "1.0.0.0",
 "parameters": {
   "storageNamePrefix": {
     "type": "string",
     "maxLength": 11
   }
 },
 "functions": [
  {
    "namespace": "contoso",
    "members": {
      "uniqueName": {
        "parameters": [
          {
            "name": "namePrefix",
            "type": "string"
          }
        ],
        "output": {
          "type": "string",
          "value": "[format('{0}{1}', toLower(parameters('namePrefix')), uniqueString(resourceGroup().id))]"
        }
      }
    }
  }
],
 "resources": [
   {
     "type": "Microsoft.Storage/storageAccounts",
     "apiVersion": "2025-06-01",
     "name": "[contoso.uniqueName(parameters('storageNamePrefix'))]",
     "location": "South Central US",
     "sku": {
       "name": "Standard_LRS"
     },
     "kind": "StorageV2",
     "properties": {
       "supportsHttpsTrafficOnly": true
     }
   }
 ]
}

During deployment, the storageNamePrefix parameter is passed to the function:

  • The template defines a parameter named storageNamePrefix.
  • The function uses namePrefix because you can only use parameters defined in the function. For more information, see Limitations.
  • In the template's resources section, the name element uses the function and passes the storageNamePrefix value to the function's namePrefix.

Limitations

When defining a user function, there are some restrictions:

  • The function can't access variables.
  • The function can only use parameters that are defined in the function. When you use the parameters function within a user-defined function, you're restricted to the parameters for that function.
  • The function can't call other user-defined functions.
  • The function can't use the reference function or any of the list functions.
  • Parameters for the function can't have default values.

Next steps

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