Multiple values for a property on resource in Azure ARM Template - json

Question

According to https://learn.microsoft.com/en-us/azure/azure-resource-manager/resource-group-create-multiple#property-iteration it should be possible to create multiple values on a resource using copy but I can't make it work. Here is my code...
{
"$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.0",
"parameters": {
"appServiceName": {
"type": "string",
"metadata": {
"description": "Name of app service to apply SSL to."
}
},
"certificateName": {
"type": "string",
"metadata": {
"description": "User friendly certificate resource name"
}
},
"appServicePlan": {
"type": "string",
"metadata": {
"description": "App Service Plan Name"
}
},
"keyVaultId": {
"type": "string",
"metadata": {
"description": "Existing Key Vault resource Id with an access policy to allow Microsoft.Web RP to read Key Vault secrets (Checkout README.md for more information)"
}
},
"hostname": {
"type": "array",
"metadata": {
"description": "Custom hostname for creating SSL binding. This hostname should already be assigned to the Web App"
}
}
},
"resources": [
{
"apiVersion": "2017-03-30",
"type": "Microsoft.Web/sites",
"name": "[parameters('appServiceName')]",
"location": "[resourceGroup().location]",
"dependsOn": [
"[resourceId('Microsoft.Web/certificates', parameters('certificateName'))]"
],
"properties": {
"copy": [
{
"name": "hostnames",
"count": "[length(parameters('hostname'))]",
"input": {
"name": "[copyIndex('hostnames')]",
"properties": {
"hostNameSslStates": [
{
"name": "[[copyIndex(hostname)]]",
"sslState": "SniEnabled",
"thumbprint": "[reference(resourceId('Microsoft.Web/certificates', parameters('certificateName'))).Thumbprint]",
"toUpdate": true
}
]
}
}
}
]
}
},
{
"type": "Microsoft.Web/certificates",
"name": "[parameters('certificateName')]",
"apiVersion": "2016-03-01",
"location": "[resourceGroup().location]",
"properties": {
"keyVaultId": "[parameters('keyVaultId')]",
"keyVaultSecretName": "[parameters('certificateName')]",
"serverFarmId": "[resourceId('Microsoft.Web/serverfarms',parameters('appServicePlan'))]"
}
}
]
}
And it returns Error: Code=InvalidTemplate; Message=Deployment template l
anguage expression evaluation failed: 'Unable to parse language expression 'copyIndex(hostname)]': expecte
d token 'LeftParenthesis' and actual 'RightParenthesis'.'. Please see https://aka.ms/arm-template-expressi
ons for usage details.
Any ideas what am I doing wrong?
Thanks in advance!

Answer 1

the error gives is away:
change "[[copyIndex(hostname)]]", to "[copyIndex('hostname')]"
you even have it right in other places, why not here?
and you probably want to do this:
"[parameters('hostname')[copyIndex('hostname')]]"