I think you may be right about this. I tried looking through the source code for Azure Management Libraries for .NET as well and couldn't find any methods or helpers that would help with creation using system assigned or user assigned identity.
Like similar functionality is supported in case of Virtual Machines. Source code
public VirtualMachineImpl WithExistingUserAssignedManagedServiceIdentity(IIdentity identity)
{
this.virtualMachineMsiHelper.WithExistingExternalManagedServiceIdentity(identity);
return this;
}
I would have expected to see similar methods in classes belonging to ContainerInstance like ContainerGroupImpl, but I don't.
Disclaimer: As you can see, I say this based on manual search and not based on official documentation so it's possible I could be missing something.
Possible Alternative
If you're interested in doing this from .NET/C# based code (since you're looking in .NET SDK), one alternative could be to use the direct REST API.
Container Groups - Create or Update
PUT https://management.azure.com/subscriptions/subid/resourceGroups/demo/providers/Microsoft.ContainerInstance/containerGroups/demo1?api-version=2018-10-01
You can specify the identity to be used in body
"identity": {
"type": "SystemAssigned, UserAssigned",
"userAssignedIdentities": {
"/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/myResourceGroup/providers/Microsoft.ManagedIdentity/userAssignedIdentities/identity-name": {}
}
Full for REST API call example here