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To use models like GPT-3.5 or GPT-4 you would need OpenAI api key and model name. 

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using LangChain.Providers.OpenAI;
using LangChain.Providers.OpenAI.Predefined;
using OpenAI;
using static LangChain.Chains.Chain;

var model = new OpenAiLatestFastChatModel("your_openAI_key");
var chain =
    Set("Hello!", outputKey: "request")          // set context variable `request` to "Hello"
    | LLM(model, inputKey: "request", outputKey: "text"); // get text from context variable `request`, pass it to the model and put result into `text`

var result = await chain.RunAsync("text");  // execute chain and get `text` context variable
Console.WriteLine(result);
// Hello! How can I assist you today?

inputKey and outputKey here is more for understanding of what goes where. They have default values and can be omitted. Also there is classes like Gpt35TurboModel for simplicity. 

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Additional options

You can pass custom HttpClient/HttpClientHandler by using OpenAiProvider constructor overload. 

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var httpClientHandler = new HttpClientHandler();
using var httpClient = new HttpClient(httpClientHandler, disposeHandler: true);
using var api = new OpenAiApi(
    httpClient,
    baseUri: new Uri("https://api.openai.com/v1/") // default value, can be omitted
    );
var provider = new OpenAiProvider(api);
var tunedModel = new OpenAiLatestFastChatModel(provider);

Complete code

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using LangChain.Providers.OpenAI;
using LangChain.Providers.OpenAI.Predefined;
using OpenAI;
using static LangChain.Chains.Chain;

var model = new OpenAiLatestFastChatModel("your_openAI_key");
var chain =
    Set("Hello!", outputKey: "request")          // set context variable `request` to "Hello"
    | LLM(model, inputKey: "request", outputKey: "text"); // get text from context variable `request`, pass it to the model and put result into `text`

var result = await chain.RunAsync("text");  // execute chain and get `text` context variable
Console.WriteLine(result);
// Hello! How can I assist you today?


var httpClientHandler = new HttpClientHandler();
using var httpClient = new HttpClient(httpClientHandler, disposeHandler: true);
using var api = new OpenAiApi(
    httpClient,
    baseUri: new Uri("https://api.openai.com/v1/") // default value, can be omitted
    );
var provider = new OpenAiProvider(api);
var tunedModel = new OpenAiLatestFastChatModel(provider);