Unable to use deploy-time parameters for privateSubnetIds for Vpc.fromVpcAttributes

Unable to pass CommaDelimitedList CfnParameter to VPC.fromVpcAttributes privateSubnetIds

Reproduction Steps

When trying to use existing VPC for a lambda function

const vpcSubnetId = new CfnParameter(this, 'VPCSubnetId', {
            description: 'VPC Subnet Id',
            type: 'CommaDelimitedList',
});
const vpc = Vpc.fromVpcAttributes(this, 'GeoVPC', {
       vpcId: vpcId.valueAsString,
       availabilityZones: ['eu-west-2a', 'eu-west-2b', 'eu-west-2c'],
       privateSubnetIds: vpcSubnetId.valueAsList,
});

When runing npn run build npx synth for a stack with the above in it it will declare the following error:

Number of privateSubnetIds (1) must be a multiple of availability zones (3).

Environment

• **CLI Version 😗*1.8.0
• **Framework Version:**1.8.0
• **OS 😗*MacOs 10.14.6
• **Language 😗*typescript

[wchaws]

Here is another nice workaround

const vpcIdParam = new CfnParameter(this, 'YourVpcId', { type: 'AWS::EC2::VPC::Id' });
const pubSubnetsParam = new CfnParameter(this, 'PubSubnets', { type: 'List<AWS::EC2::Subnet::Id>' });
const privSubnetsParam = new CfnParameter(this, 'PrivSubnets', { type: 'List<AWS::EC2::Subnet::Id>' });

const azs = ['eu-west-2a', 'eu-west-2b', 'eu-west-2c'];

const vpc = ec2.Vpc.fromVpcAttributes(this, 'VpcAttr', {
  vpcId: vpcIdParam.valueAsString,
  vpcCidrBlock: Aws.NO_VALUE,
  availabilityZones: azs,
  publicSubnetIds: azs.map((_, index) => Fn.select(index, pubSubnetsParam.valueAsList)),
  privateSubnetIds: azs.map((_, index) => Fn.select(index, privSubnetsParam.valueAsList)),
});

[eruvanos]

Hi,

we faced this issue multiple times, as a workaround we wrap the subnetIds with select statements:

const vpcSubnetIdsRef = Token.asList(Fn.Split(",", Fn.importValue('VPCSubnetId')))
// or
// const vpcSubnetIdsRef = new CfnParameter(this, 'VPCSubnetId', {
//            description: 'VPC Subnet Id',
//            type: 'CommaDelimitedList',
// });

const azs = ['eu-west-2a', 'eu-west-2b', 'eu-west-2c']

const subnetIds = [];
azs.forEach((_, index) => {
        subnetIds.push(Fn.select(index, vpcSubnetIdsRef))
});

const vpc = Vpc.fromVpcAttributes(this, 'GeoVPC', {
       vpcId: vpcId.valueAsString,
       availabilityZones: azs,
       privateSubnetIds: subnetIds,
});

This will create a list with three tokens, which will be resolved in cfn later.