AspNetCore Options pattern with constructor dependency - asp.net-core

Question

I have an option class which like the following
public class EmailOptions
{
public EmailOptions(IEmailConfiguration account) {
this.Configuration = account;
}
public string DefaultFromAddress { get; set; }
public string DefaultFromDisplayName { get; set; }
public IEmailConfiguration Configuration { get; }
}
The IEmailConfiguration interface is there because in some cases I can have an Smtp library and so I need an Smtp based configuration while in some other cases I can use other services which needs a different configuration. Example:
public class ApiKeyConfiguration : IEmailConfiguration
{
public ApiKeyConfiguration() {
}
public string AccountName { get; set; }
public string AccountKey { get; set; }
}
or
public class SmtpConfiguration : IEmailConfiguration
{
public SmtpConfiguration() {
}
public string Host { get; set; }
public string Port { get; set; }
public string Username { get; set; }
public string Password { get; set; }
public string Domain { get; set; }
public bool EnableSsl { get; set; }
public bool UseDefaultCredentials { get; set; }
}
I am sure I am registering the correct implementation with
services.AddTransient<IEmailConfiguration, ApiKeyConfiguration>();
However when I try to inject an IOption<> into a controller I am getting the following error:
[13:56:26 ERR] An unhandled exception has occurred: Cannot create instance of type 'EmailOptions' because it is missing a public parameterless constructor.
System.InvalidOperationException: Cannot create instance of type 'EmailOptions' because it is missing a public parameterless constructor.
at Microsoft.Extensions.Configuration.ConfigurationBinder.CreateInstance(Type type)
Of course I can add a parameterless constructor to the class but how do I ensure that the DI container will create an instance of my class by using the constructor with parameter dependency?

Answer 1

Technically, you may try to create own implementation of IConfigureOptions.
Represents something that configures the TOptions type. Note: These are run before all IPostConfigureOptions.
So do something like this:
public class ConfigureEmailOptions : IConfigureOptions<EmailOptions>
{
private readonly IEmailConfiguration _account;
public ConfigureMyOptions(IEmailConfiguration account)
{
_account = account;
}
public void Configure(EmailOptions options)
{
options.Configuration = _account;
...
}
}
and register it as
services.AddTransient<IConfigureOptions<EmailOptions>, ConfigureEmailOptions>();
and your option class should be just
public class EmailOptions
{
public string DefaultFromAddress { get; set; }
public string DefaultFromDisplayName { get; set; }
public IEmailConfiguration Configuration { get; set; }
}

Answer 2

It's saying you need a public parameterless constructor.
Add
public EmailOptions() {
}

Answer 3

Based on the way configuration works in ASP.NET Core, it will always use a parameterless constructor. Although it's not the only reason why, one particularly good reason is that the configuration setup happens before any DI services are registered, meaning it couldn't inject anything if it wanted to.
Long and short, there's no way to satisfy something like IEmailConfiguration in a strongly-typed configuration class. It's frankly a bad idea anyways. Just let your configuration class be a simple entity and inject that into a service or something that handles your email stuff, instead of the other way around.