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.NET SDK reference (server-side)

Read time: 2 minutes
Last edited: Aug 29, 2022


This topic documents how to get started with the server-side .NET SDK, and links to reference information on all of the supported features.

SDK quick links

LaunchDarkly's SDKs are open source. In addition to this reference guide, we provide source, API reference documentation, and a sample application:

SDK API documentationSDK API docs
GitHub repositorydotnet-server-sdk
Sample applicationhello-dotnet-server
Published moduleNuGet
For use in server-side applications only

This SDK is intended for use in multi-user .NET server applications. If you have a .NET application and want to set up LaunchDarkly in a mobile, desktop, or embedded application, read the client-side .NET SDK reference.

To learn more about LaunchDarkly's different SDK types, read Client-side and server-side SDKs.

Getting started

SDK version compatibility

The LaunchDarkly .NET SDK, version 6.0 and higher, is compatible with .NET 5.0+, .NET Framework 4.5.2+, and .NET Core 2.1+.

Prior to version 6.0, the LaunchDarkly .NET SDK also supported .NET Framework 4.5.0 and .NET Core 1.0.

After you complete the Getting Started process, follow these instructions to start using the LaunchDarkly SDK in your .NET application.

First, install the LaunchDarkly SDK as a dependency in your application using your application's dependency manager.

Here's how:

Install-Package LaunchDarkly.ServerSdk

Next, import the LaunchDarkly SDK's namespaces in your application code. The namespace is not the same as the package name:

using LaunchDarkly.Sdk;
using LaunchDarkly.Sdk.Server;
// LaunchDarkly.Sdk defines general types like User, which are also used in the client-side .NET SDK.
// LaunchDarkly.Sdk.Server defines the LdClient and Configuration types for the server-side .NET SDK.

After you install and import the SDK, create a single, shared instance of LdClient. Specify your SDK key here to authorize your application to connect to a particular environment within LaunchDarkly.

To create a single instance:

LdClient client = new LdClient("YOUR_SDK_KEY");
LdClient should be a singleton

In the standard use case where there is only one SDK key, it's important to make the LdClient a singleton. The client instance maintains internal state that allows LaunchDarkly to serve feature flags without making any remote requests. Do not instantiate a new client with every request.

If you need to use more than one SDK key, the SDK allows you to create more than one LdClient instance. This is an uncommon requirement, but one the SDK supports.

You can use client to check which variation a particular user will receive for a given feature flag.

Here's how:

var user = User.Builder("example-user-key")
var flagValue = client.BoolVariation(FeatureFlagKey, user, false);
if (flagValue) {
// application code to show the feature
else {
// the code to run if the feature is off

Transport Layer Security (TLS) and other networking issues

LaunchDarkly is deprecating support for TLS versions 1.0 and 1.1. .NET applications running on older operating systems are prone to use these older, less secure versions of TLS. It is possible to increase your application's security using AppContext switches.

To learn more, read Microsoft's documentation.

If you cannot update your application platform's configurations to support TLS version 1.2, you can update your application's SDK configuration to use new LaunchDarkly endpoints that support TLS versions 1.0 and later. Here is an example:

// Use `stream-tls10.launchdarkly.com` and `events-tls10.launchdarkly.com` TLSv1 endpoints
var config = Configuration.Builder("my-sdk-key")
Potential network connectivity issues caused by DNS caching

LaunchDarkly servers operate in a load-balancing framework which may cause their IP addresses to change. This could result in the SDK failing to connect to LaunchDarkly if an old IP address is still in your system's DNS cache. In .NET, the DNS cache retains IP addresses for two minutes by default. If you notice intermittent connection failures that always resolve in two minutes, you may want to change this setting to a lower value as described in Microsoft's documentation.

Shutting down

Lastly, shut down the client when your application terminates. To learn more, read Shutting down.

Supported features

This SDK supports the following features: