Configuring the runtime host

.NET Core 3 introduced a not-so-well-known configuration mechanism that still has some use: a runtime host configuration. The idea here is that you provide configuration settings, as key-value pairs, in the .csproj file. You can retrieve them programmatically from the AppContext class. Here is an example project file:

<ProjectSdk="Microsoft.NET.Sdk.Web">
    <PropertyGroup>
<TargetFramework>netcoreapp3.0</TargetFramework>
</PropertyGroup>
<ItemGroup>
<RuntimeHostConfigurationOptionInclude="Foo"Value="Bar"/>
</ItemGroup>
</Project>

The "Foo" setting is retrievable through a call to the GetData method of the AppContext class, as illustrated in the following code snippet:

var bar = AppContext.GetData("Foo");

If the named entry does not exist, GetData just returns null. Mind you, GetData is prototyped as returning an object, but in this case, it will return a string.

Normally, you wouldn't want to do that, but should you ever want to create or modify one entry of a runtime host configuration setting, you can do that through the application domain, as follows:

AppDomain.CurrentDomain.SetData("Foo", "ReBar");

Mind you, this is not a replacement for a well-structured and properly defined configuration. What .NET Core does is, at run and deployment time, it copies the contents of the RuntimeHostConfigurationOption sections (and some more) to a generated ${project}.runtimeconfig.json file that is placed together with the generated binary.

We'll now see a new feature of ASP.NET Core: feature toggles.