[.NET Generator] Auto-generate ClientSettings class and IConfiguration-enabled constructors

[Copilot]

The .NET generator did not produce IConfiguration-based client construction support, requiring manual implementation per client. This adds automatic generation of the three artifacts needed for every root client to support standard .NET configuration patterns (appsettings.json, env vars, DI containers).

New: ClientSettingsProvider

Generates {Client}Settings : ClientSettings (System.ClientModel.Primitives) with:

• Uri? {EndpointName} and {Client}Options? Options properties (credential handled by base CredentialProvider)
• BindCore(IConfigurationSection section) override binding endpoint and options from config
• [Experimental("SCME0002")] on the class

Modified: ClientOptionsProvider

Adds an internal IConfigurationSection constructor forwarding to base(section), with an early-exit null/exists guard, binding non-version service-specific properties.

Modified: ClientProvider

Adds a public {Client}({Client}Settings settings) constructor delegating to the primary constructor — only for root clients with a configurable endpoint:

[Experimental("SCME0002")]
public SecretClient(SecretClientSettings settings)
    : this(settings?.VaultUri, settings?.CredentialProvider as TokenCredential, settings?.Options)
{ }

Modified: ScmOutputLibrary / ClientProvider

• ClientProvider exposes a ClientSettings property (ClientSettingsProvider?)
• ScmOutputLibrary.BuildClient registers the ClientSettingsProvider in the output type set alongside ClientOptions

Notes

• Sub-clients do not get settings classes (only root clients with ClientOptions != null)
• ClientSettings and IConfigurationSection are referenced by name via a new CSharpType factory for external types not yet in the pinned System.ClientModel 1.9.0 — generated code will compile once the upstream package ships these types

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Original prompt

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This section details on the original issue you should resolve

<issue_title>[.NET Generator] Auto-generate ClientSettings class and IConfiguration-enabled constructors for clients</issue_title>
<issue_description>## Overview

Parent issue: Azure/azure-sdk-for-net#55491

The .NET generator should automatically produce IConfiguration-based client construction support for every generated client. This enables developers to configure clients using standard .NET configuration patterns (appsettings.json, environment variables) and register them with dependency injection containers.

Two services have been manually implemented as reference:

• Azure.Data.AppConfiguration PR microsoft/typespec#56537
• Azure.Security.KeyVault.Secrets PR microsoft/typespec#56409

The generator should produce these automatically so every client gets configuration support without manual customization.

What the Generator Needs to Produce

Three artifacts per client:

A. {Client}Settings class

• Extends ClientSettings (from System.ClientModel.Primitives)
• Properties derived from the client's primary constructor parameters (excluding credential)
• Implements BindCore(IConfigurationSection section) override
• Marked [Experimental("SCME0002")]

B. internal {Client}Options(IConfigurationSection section) constructor

• Calls base(section) — the base options class handles common properties (see Forwarding Binding)
• Guards with if (section is null || !section.Exists()) { return; } before binding any properties
• Binds each option-specific property from the section
• Marked [Experimental("SCME0002")]

C. public {Client}({Client}Settings settings) constructor on the client

• Delegates to the client's primary constructor (the one with the body implementation) — e.g., (AuthenticationPolicy, Options) or (Uri, AuthenticationPolicy, Options) depending on the client
• Creates the authentication policy via AuthenticationPolicy.Create(settings) to pass to the primary constructor
• Extracts other properties from settings and maps them to the primary constructor parameters (e.g., settings?.Endpoint, settings?.Options)
• Not all clients have a URI parameter — the settings class properties mirror whatever the primary constructor requires
• Marked [Experimental("SCME0002")]

How to Determine Settings Properties

The generator should inspect the client's primary public constructor (the one with the body implementation) parameters:

• Endpoint/URI parameters (if present) → become Uri? properties on Settings (e.g., VaultUri, Endpoint)
• Options parameter → becomes {Client}Options? property on Settings
• Credential/AuthenticationPolicy parameter → handled by base ClientSettings.Credential / CredentialProvider, NOT duplicated
• Any other non-credential constructor parameters may become settings properties

How to Implement BindCore

`csharp
protected override void BindCore(IConfigurationSection section)
{
if (section is null || !section.Exists())
{
return;
}

// String → use 'is string' pattern match
if (section["Endpoint"] is string endpoint)
{
    Endpoint = new Uri(endpoint);
}

// Options → GetSection + Exists check + internal ctor
IConfigurationSection optionsSection = section.GetSection("Options");
if (optionsSection.Exists())
{
    Options = new {Client}Options(optionsSection);
}

}
`

Key rule: If a configuration segment doesn't exist, bail early — don't set the property. No validation in BindCore. Let the client constructor that receives the settings do all required parameter validation (e.g., Argument.AssertNotNull).

Type Binding Patterns

The generator should use these patterns to bind properties from IConfigurationSection:

Type	Pattern	Example
string	section[name] is string val	TenantId = val
bool	bool.TryParse(section[name], out bool val)	DisableChallengeResourceVerification = val
Uri	Uri.TryCreate(section[name], UriKind.Absolute, out Uri val)	RedirectUri = val
TimeSpan	TimeSpan.TryParse(section[name], out TimeSpan val)	NetworkTimeout = val
int	int.TryParse(section[name], out int val)	MaxRetries = val
string[] / List<string>	section.GetSection(name).GetChildren().Where(c => c.Value is not null).Select(c => c.Value!).ToList()	AdditionallyAllowedTenants
Complex object	section.GetSection(name) + .Exists() + new Type(section)	BrowserCustomization = new BrowserCustomizationOptions(browserSection)
Custom struct/enum (extensible)	new TypeName(section[name]) after null check	Audience = new AppConfigurationAudience(audience)

For a comprehensive example of bin...

• Fixes [.NET Generator] Auto-generate ClientSettings class and IConfiguration-enabled constructors for clients #9842

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