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I have a StackReference:

ref, err := pulumi.NewStackReference(ctx, "user/project/stack", nil)
		if err != nil {
			return err
		}

Then I have this code to pull out the first subnet from an array of subnets created by the first stack:

privSubnets := ref.GetOutput(pulumi.String("ref")).ApplyStringArray(func(out interface{}) []string {
			var res []string
			if out != nil {
				for _, v := range out.([]interface{}) {
					res = append(res, v.(string))
				}
			}
			return res
		})

		// Get the ID of the first public subnet
		privSubnet0 := privSubnets.ApplyString(func(a []string) string {
			var res string
			if len(a) > 0 {
				res = a[0]
			}
			return res
		})

When I try to use

privSubnet0

to create an instance, it must have a nil value because Pulumi tries to create the instance in a subnet in the default VPC (which is completely wrong). So what am I missing here?

what's the output of

pulumi stack output

for the original stack (the one used to construct the stackref "user/proj/stack")?

Current stack outputs (16):
    OUTPUT                  VALUE
    EIP                     
    amiId                   ami-0288fa05d2ca8b5c7
    bastionInstanceId       i-0b93a76f7d1db768f
    bastionSecGrpId         sg-0045840efd05c2db3
    defaultRoute            rtb-0a55c701f0d9bccb4
    gatewayId               igw-045fa2082d3efcd1e
    inetRoute               r-rtb-0a55c701f0d9bccb41080289494
    k8sSecGrpId             sg-0a4687591102705da
    natGateway              nat-027db1764b3ad56a1
    nodeSecGrpId            sg-06648ddf8796bf63c
    numOfAZs                3
    privRouteTableAssocIds  ["rtbassoc-0db084e8424e969c2","rtbassoc-068e695773ce5bc7a","rtbassoc-03da8e12e6db0ef47"]
    privRouteTableId        rtb-090cc2bd626d9f5e9
    privSubnetIds           ["subnet-0484737a31b183d10","subnet-0db65b831d7dc5603","subnet-0a7e23caa1a74d92e"]
    pubSubnetIds            ["subnet-0a1b1395860d29849","subnet-00865b0cc48b28a77","subnet-04e8dfbb0e7eba4cf"]
    vpcId                   vpc-0e9bf44c8efd1133b

All the outputs seem valid.

Can you share the error output you're seeing? and the code that creates the uses

privSubnet0

?

Yep! Here's the error output:

error: Error launching source instance: InvalidParameter: Security group sg-06648ddf8796bf63c and subnet subnet-13f5c17b belong to different networks.
    	status code: 400, request id: dbdce577-3acc-4134-821d-b1d17eb7a84d

The referenced subnet ID is in the default VPC, not the VPC created by the referenced stack. Here's the code that references `privSubnet0`:

controlPlane, err := ec2.NewInstance(ctx, "controlplane", &ec2.InstanceArgs{
			Ami:                      pulumi.String(amiID.Id),
			InstanceType:             pulumi.String("m5a.large"),
			AssociatePublicIpAddress: pulumi.Bool(false),
			KeyName:                  pulumi.String(keyPair),
			SubnetId:                 privSubnet0,
			VpcSecurityGroupIds:      pulumi.StringArray{nodeSecGrp},
			Tags: pulumi.StringMap{
				"Name": pulumi.String("controlplane"),
			},
		})

Note that

nodeSecGrp

, used in the code above, is the result of a stack reference and is working fine.

@salmon-account-74572 is this the exact code you used?

ref.GetOutput(pulumi.String("ref"))

Should this use the name of your output instead?

ref.GetOutput(pulumi.String("privSubnetIds")

Doh! You're right...I was referencing an non-existent output. Can't believe I missed that! hangs his head in shame

Awesome, glad to hear it's working. Copy paste bites us all! Eventually it would be nice to have some strongly typed way to do stack outputs and references.