DeepL .NET Library

The DeepL API is a language translation API that allows other computer programs to send texts and documents to DeepL's servers and receive high-quality translations. This opens a whole universe of opportunities for developers: any translation product you can imagine can now be built on top of DeepL's best-in-class translation technology.

The DeepL .NET library offers a convenient way for applications written in .NET to interact with the DeepL API. We intend to support all API functions with the library, though support for new features may be added to the library after they’re added to the API.

Getting an authentication key

To use the DeepL .NET Library, you'll need an API authentication key. To get a key, please create an account here. You can translate up to 500,000 characters/month for free.

Installation

Using the .NET Core command-line interface (CLI) tools:

dotnet add package DeepL.net

Using the NuGet Command Line Interface (CLI):

nuget install DeepL.net

Usage

All entities in the DeepL .NET library are in the DeepL namespace:

using DeepL;

Create a Translator object providing your DeepL API authentication key.

To avoid writing your key in source code, you can set it in an environment variable DEEPL_AUTH_KEY, then read the variable in your C# code:

var authKey = Environment.GetEnvironmentVariable("DEEPL_AUTH_KEY");
var translator = new Translator(authKey);

Translating text

To translate text, call TranslateTextAsync() with the text and the source and target language codes.

The source and target language arguments accept strings containing the language codes, for example "DE", "FR". The LanguageCode static class defines constants for the currently supported languages, for example LanguageCode.German, LanguageCode.French. To auto-detect the input text language, specify null as the source language.

The returned TextResult contains the translated text and detected source language code. Additional TextTranslateOptions can also be provided.

// Translate text into a target language, in this case, French:
var translatedText = await translator.TranslateTextAsync(
      "Hello, world!",
      LanguageCode.English,
      LanguageCode.French);
Console.WriteLine(translatedText); // "Bonjour, le monde !"
// Note: printing or converting the result to a string uses the output text.

// Translate multiple texts into British English:
var translations = await translator.TranslateTextAsync(
      new[] { "お元気ですか？", "¿Cómo estás?" }, null, "EN-GB");
Console.WriteLine(translations[0].Text); // "How are you?"
Console.WriteLine(translations[0].DetectedSourceLanguageCode); // "JA"
Console.WriteLine(translations[1].Text); // "How are you?"
Console.WriteLine(translations[1].DetectedSourceLanguageCode); // "ES"

// Translate into German with less and more Formality:
foreach (var formality in new[] { Formality.Less, Formality.More }) {
  Console.WriteLine(
        await translator.TranslateTextAsync(
              "How are you?",
              null,
              LanguageCode.German,
              new TextTranslateOptions { Formality = formality }));
}
// Will print: "Wie geht es dir?" "Wie geht es Ihnen?"

Translating documents

To translate documents, call TranslateDocumentAsync() with the input and output files as either FileInfo or Stream objects, and provide the source and target language as above. Additional DocumentTranslateOptions are also available. Note that file paths are not accepted as strings, to avoid mixing up the file and language arguments.

// Translate a formal document from English to German
try {
  await translator.TranslateDocumentAsync(
        new FileInfo("Instruction Manual.docx"),
        new FileInfo("Bedienungsanleitung.docx"),
        "EN",
        "DE",
        new DocumentTranslateOptions { Formality = Formality.More });
} catch (DocumentTranslationException exception) {
  // If the error occurs *after* upload, the DocumentHandle will contain the document ID and key
  if (exception.DocumentHandle != null) {
    var handle = exception.DocumentHandle.Value;
    Console.WriteLine($"Document ID: {handle.DocumentId}, Document key: {handle.DocumentKey}");
  } else {
    Console.WriteLine($"Error occurred during document upload: {exception.Message}");
  }
}

TranslateDocumentAsync() manages the upload, wait until translation is complete, and download steps. If your application needs to execute these steps individually, you can instead use the following functions directly:

• TranslateDocumentUploadAsync(),
• TranslateDocumentStatusAsync() (or TranslateDocumentWaitUntilDoneAsync()), and
• TranslateDocumentDownloadAsync()

Glossaries

Glossaries allow you to customize your translations using defined terms. Create a glossary containing the desired terms and then use it in translations. Multiple glossaries can be stored with your account.

// Create an English to German glossary with two terms:
var entriesDictionary = new Dictionary<string, string>{{"artist", "Maler"}, {"prize", "Gewinn"}};
var glossaryEnToDe = await translator.CreateGlossaryAsync(
    "My glossary", "EN", "DE",
    new GlossaryEntries(entriesDictionary));

// Functions to get, list, and delete glossaries from DeepL servers are also provided
var glossaries = await translator.ListGlossariesAsync();
Console.WriteLine($"{glossaries.Length} glossaries found.");

var resultWithGlossary = await translator.TranslateTextAsync(
    "The artist was awarded a prize.",
    "EN",
    "DE",
    new TextTranslateOptions { GlossaryId = glossaryEnToDe.GlossaryId });
// resultWithGlossary.Text == "Der Maler wurde mit einem Gewinn ausgezeichnet."
// Without using a glossary: "Der Künstler wurde mit einem Preis ausgezeichnet."

Check account usage

var usage = await translator.GetUsageAsync();
if (usage.AnyLimitExceeded) {
  Console.WriteLine("Translation limit exceeded.");
} else if (usage.Character != null) {
  Console.WriteLine($"Character usage: {usage.Character}");
} else {
  Console.WriteLine($"{usage}");
}

Listing available languages

// Source and target languages
var sourceLanguages = await translator.GetSourceLanguagesAsync();
foreach (var lang in sourceLanguages) {
  Console.WriteLine($"{lang.Name} ({lang.Code})"); // Example: "English (EN)"
}
var targetLanguages = await translator.GetTargetLanguagesAsync();
foreach (var lang in targetLanguages) {
  if (lang.SupportsFormality ?? false) {
    Console.WriteLine($"{lang.Name} ({lang.Code}) supports formality");
     // Example: "German (DE) supports formality"
  }
}

// Glossary languages
var glossaryLanguages = await translator.GetGlossaryLanguagesAsync();
foreach (var languagePair in glossaryLanguages) {
  Console.WriteLine($"{languagePair.SourceLanguage} to {languagePair.TargetLanguage}");
  // Example: "EN to DE", "DE to EN"
}

Development

The test suite depends on deepl-mock. Run it in another terminal while executing the tests, using port 3000. Set the mock-server listening port using the environment variable DEEPL_MOCK_SERVER_PORT.

Execute the tests using dotnet test.

Issues

If you experience problems using the library, or would like to request a new feature, please create an issue.