Substitute parts of a typed array in ASP.NET core appsettings.json from secrets/environment variables? - asp.net-core

Question

We have an ASP.NET Core web app with this appsettings.json:
{
"Subscriptions": [
{
"Name": "Production",
"PublishSettings": "<PublishData>SECRET</PublishData>",
"Environments": [
{
"Name": "Prod",
"DeploymentServiceNames": [
"api1",
"api2",
"api3"
]
}
]
},
{
"Name": "Test",
"PublishSettings": "<PublishData>SECRET</PublishData>",
"Environments": [
{
"Name": "Test1",
"DeploymentServiceNames": [
"api1",
"api2"
]
},
{
"Name": "Test2",
"DeploymentServiceNames": [
"api1",
"api2"
]
}
]
}
]
}
The PublishSettings values are secret so I want these in my local user secrets file, and in environment variables for my deployments. But, because Subscriptions is an array I'm not sure how. I don't particularly want to swap in the entire Subscriptions section. Is there a way to swap in a single property for each item in such an array, perhaps by defining a key property on the strongly typed subscription model?

Answer 1

When you load configuration in .NET Core, under the hood it's represented as a set of key-value pairs (both key and value have string type) supplied by added configuration providers.
For example, appsettings.json will be represented by JsonConfigurationProvider as the following settings list:
{Subscriptions:0:Environments:0:DeploymentServiceNames:0, api1}
{Subscriptions:0:Environments:0:DeploymentServiceNames:1, api2}
{Subscriptions:0:Environments:0:DeploymentServiceNames:2, api3}
{Subscriptions:0:Environments:0:Name, Prod}
{Subscriptions:0:Name, Production}
{Subscriptions:0:PublishSettings, <PublishData>SECRET</PublishData>}
{Subscriptions:1:Environments:0:DeploymentServiceNames:0, api1}
{Subscriptions:1:Environments:0:DeploymentServiceNames:1, api2}
{Subscriptions:1:Environments:0:Name, Test1}
{Subscriptions:1:Environments:1:DeploymentServiceNames:0, api1}
{Subscriptions:1:Environments:1:DeploymentServiceNames:1, api2}
{Subscriptions:1:Environments:1:Name, Test2}
{Subscriptions:1:Name, Test}
{Subscriptions:1:PublishSettings, <PublishData>SECRET</PublishData>}
As you see JSON structure was flattened and keys are built by joining inner section names with a colon. Array element are added with appropriate index as a name.
If you add another configuration source, e.g. environment variables or another secrets json file, which will have settings with the same keys, it will overwrite the setting.
So if you want to add or overwrite PublishSettings, you could add either another JSON file as configuration source:
{
"Subscriptions": [
{
"PublishSettings": "<PublishData>SECRET</PublishData>"
},
{
"PublishSettings": "<PublishData>SECRET</PublishData>"
}
]
}
Or add it as environment variables with the following keys:
Subscriptions:0:PublishSettings
Subscriptions:1:PublishSettings
Such setting override (or addition) is transparent for .NET Core configuration binder. Settings POCO will contain value of PublishSettings from the last configuration source that provides such value.