Merge pull request #310278 from mumian/1008-type-expression

ARM template expression limitation

Author: prmerger-automator[bot]

articles/azure-resource-manager/templates/template-expressions.md

@@ -3,7 +3,7 @@ title: Template syntax and expressions
 description: Describes the declarative JSON syntax for Azure Resource Manager templates (ARM templates).
 ms.topic: article
 ms.custom: devx-track-arm-template
-ms.date: 04/28/2025
+ms.date: 01/09/2026
 ---
 
 # Syntax and expressions in ARM templates
@@ -12,6 +12,55 @@ The basic syntax of the Azure Resource Manager template (ARM template) is JavaSc
 
 A template expression can't exceed 24,576 characters.
 
+## Compile-time constrains
+
+Template expressions are evaluated at runtime, but certain properties in ARM templates must be known at compile time and therefore cannot use expressions. These properties must be specified as literal strings so the deployment engine and compiler can validate resource schemas, dependencies, and supported properties before any expressions are evaluated.
+
+Properties that must be compile-time literals include:
+
+- The `type` and `apiVersion` of a resource.
+- The `name` of variables, parameters, and outputs.
+- Extension references.
+
+The following ARM template failed because the `type` property of the resource uses an expression:
+
+```json
+{
+  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
+  "contentVersion": "1.0.0.0",
+  "parameters": {
+    "storageAccountName": {
+      "type": "string",
+      "defaultValue": "[format('storage{0}', uniqueString(resourceGroup().id))]"
+    },
+    "location": {
+      "type": "string",
+      "defaultValue": "[resourceGroup().location]"
+    },
+    "resourceProvider": {
+      "type": "string",
+      "defaultValue": "Microsoft.Storage"
+    },
+    "resourceType": {
+      "type": "string",
+      "defaultValue": "storageAccounts"
+    }
+  },
+  "resources": [
+    {
+      "type": "[format('{0}/{1}', parameters('resourceProvider'), parameters('resourceType'))]",
+      "apiVersion": "2025-06-01",
+      "name": "[parameters('storageAccountName')]",
+      "location": "[parameters('location')]",
+      "sku": {
+        "name": "Standard_LRS"
+      },
+      "kind": "StorageV2"
+    }
+  ]
+}
+```
+
 ## Use functions
 
 Azure Resource Manager provides [functions](template-functions.md) that you can use in a template. The following example shows an expression that uses a function in the default value of a parameter: