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.

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

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

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

using DeepL;

Create a Translator object providing your DeepL API authentication key.

Be careful not to expose your key, for example when sharing source code.

var authKey = "f63c02c5-f056-..."; // Replace with your key
var translator = new Translator(authKey);

This example is for demonstration purposes only. In production code, the authentication key should not be hard-coded, but instead fetched from a configuration file or environment variable.

Translator accepts options as the second argument, see Configuration for more information.

To translate text, call TranslateTextAsync(). The first argument is a string containing the text to translate, or IEnumerable of strings to translate multiple texts.

The second and third arguments are case-insensitive language codes for the source and target language respectively, 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.

Additional TextTranslateOptions can also be provided, see Text translation options below.

TranslateTextAsync() returns a TextResult or TextResult array corresponding to the input text(s). The TextResult contains the translated text, the detected source language code, and the number of characters billed for the text.

// 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[0].BilledCharacters); // 7 - the number of characters in the source text "お元気ですか？"
Console.WriteLine(translations[1].Text); // "How are you?"
Console.WriteLine(translations[1].DetectedSourceLanguageCode); // "ES"
Console.WriteLine(translations[1].BilledCharacters); // 12 - the number of characters in the source text "¿Cómo estás?"

// 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?"

TextTranslateOptions has the following properties that impact text translation:

• SentenceSplittingMode: specifies how input text should be split into sentences, default: 'SentenceSplittingMode.All'.
  • SentenceSplittingMode.All: input text will be split into sentences using both newlines and punctuation.
  • SentenceSplittingMode.Off: input text will not be split into sentences. Use this for applications where each input text contains only one sentence.
  • SentenceSplittingMode.NoNewlines: input text will be split into sentences using punctuation but not newlines.
• PreserveFormatting: controls automatic-formatting-correction. Set to true to prevent automatic-correction of formatting, default: false.
• Formality: controls whether translations should lean toward informal or formal language. This option is only available for some target languages, see Listing available languages.
  • Formality.Less: use informal language.
  • Formality.More: use formal, more polite language.
  • Formality.Default: standard level of formality.
  • Formality.PreferLess: less formality, if available for the specified target language, otherwise default.
  • Formality.PreferMore: more formality, if available for the specified target language, otherwise default.
• GlossaryId: specifies a glossary to use with translation, as a string containing the glossary ID.
• Context: specifies additional context to influence translations, that is not translated itself. Characters in the context parameter are not counted toward billing. See the API documentation for more information and example usage.
• TagHandling: type of tags to parse before translation, options are "html" and "xml".

The following options are only used if TagHandling is set to 'xml':

• OutlineDetection: set to false to disable automatic tag detection, default is true.
• SplittingTags: List of XML tags that should be used to split text into sentences. Tags may be specified individually (['tag1', 'tag2']), or a comma-separated list of strings ('tag1,tag2'). The default is an empty list.
• NonSplittingTags: List of XML tags that should not be used to split text into sentences. Format and default are the same as for splitting tags.
• IgnoreTags: List of XML tags that containing content that should not be translated. Format and default are the same as for splitting tags.

For a detailed explanation of the XML handling options, see the API documentation.

To translate documents, call TranslateDocumentAsync() with the input and output files as FileInfo objects, and provide the source and target language as above.

Additional DocumentTranslateOptions are also available, see Document translation options below. 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}");
  }
}

Alternatively the input and output files may be provided as Stream objects; in that case the input file name (or extension) is required, so the DeepL API can determine the file type:

...
  await translator.TranslateDocumentAsync(
        new MemoryStream(buffer),
        "Input file.docx", // An extension like ".docx" is also sufficient
        File.OpenWrite(outputDocumentPath),
        "EN",
        "DE",
        new DocumentTranslateOptions { Formality = Formality.More });

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()

DocumentTranslateOptions has the following properties that impact text translation:

• Formality: same as in Text translation options.
• GlossaryId: same as in Text translation options.

Glossaries allow you to customize your translations using defined terms. Multiple glossaries can be stored with your account, each with a user-specified name and a uniquely-assigned ID.

You can create a glossary with your desired terms and name using CreateGlossaryAsync(). Each glossary applies to a single source-target language pair. Note: Glossaries are only supported for some language pairs, see Listing available glossary languages for more information. The entries should be specified as a Dictionary.

If successful, the glossary is created and stored with your DeepL account, and a GlossaryInfo object is returned including the ID, name, languages and entry count.

// 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));

Console.WriteLine($"Created {glossaryEnToDe.Name}' ({glossaryEnToDe.GlossaryId}) " +
    $"{glossaryEnToDe.SourceLanguageCode}->{glossaryEnToDe.TargetLanguageCode} " +
    $"containing {glossaryEnToDe.EntryCount} entries"
);
// Example: Created 'My glossary' (559192ed-8e23-...) en->de containing 2 entries

You can also upload a glossary downloaded from the DeepL website using CreateGlossaryFromCsvAsync(). Instead of supplying the entries as a dictionary, specify the CSV data as a Stream containing file content:

var csvStream =  File.OpenRead("myGlossary.csv");
var csvGlossary = await translator.CreateGlossaryFromCsvAsync("My CSV glossary", "EN", "DE", csvStream);

The API documentation explains the expected CSV format in detail.

Functions to get, list, and delete stored glossaries are also provided:

• GetGlossaryAsync() takes a glossary ID and returns a GlossaryInfo object for a stored glossary, or raises an exception if no such glossary is found.
• ListGlossariesAsync() returns a List of GlossaryInfo objects corresponding to all of your stored glossaries.
• DeleteGlossaryAsync() takes a glossary ID or GlossaryInfo object and deletes the stored glossary from the server, or raises an exception if no such glossary is found.

// Retrieve a stored glossary using the ID
var myGlossary = await translator.GetGlossaryAsync("559192ed-8e23-...");

// Find and delete glossaries named 'Old glossary'
var glossaries = await translator.ListGlossariesAsync();
foreach (var glossaryInfo in glossaries) {
  if (glossaryInfo.Name == "Old glossary")
    await translator.DeleteGlossaryAsync(glossaryInfo);
}

The GlossaryInfo object does not contain the glossary entries, but instead only the number of entries in the EntryCount property.

To list the entries contained within a stored glossary, use GetGlossaryEntriesAsync() providing either the GlossaryInfo object or glossary ID:

var entries = translator.GetGlossaryEntriesAsync(myGlossary);

foreach (KeyValuePair<string, string> entry in entries.ToDictionary()) {
  Console.WriteLine($"{entry.Key}: {entry.Value}");
}
// prints:
//   artist: Maler
//   prize: Gewinn

You can use a stored glossary for text (or document) translation by setting the TextTranslationOptions (or DocumentTranslationOptions) GlossaryId property to the glossary ID. You must also specify the source_lang argument (it is required when using a glossary):

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."

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

You can request the list of languages supported by DeepL for text and documents using the GetSourceLanguagesAsync() and GetTargetLanguagesAsync() functions. They both return a list of Language objects.

The Name property gives the name of the language in English, and the Code property gives the language code. The SupportsFormality property only appears for target languages, and indicates whether the target language supports the optional Formality parameter.

// 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"
  }
}

Glossaries are supported for a subset of language pairs. To retrieve those languages use the GetGlossaryLanguagesAsync() function, which returns an array of GlossaryLanguagePair objects. Use the SourceLanguage and TargetLanguage properties to check the pair of language codes supported.

// 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", etc.
}

You can also find the list of supported glossary language pairs in the API documentation.

Note that glossaries work for all target regional-variants: a glossary for the target language English ("EN") supports translations to both American English ("EN-US") and British English ("EN-GB").

All library functions may raise DeepLException or one of its subclasses. If invalid arguments are provided, they may raise the standard exceptions ArgumentException.

If you use this library in an application, please identify the application with TranslatorOptions.appInfo, which needs the name and version of the app:

var options = new TranslatorOptions {
  appInfo =  new AppInfo { AppName = "my-dotnet-test-app", AppVersion = "1.2.3"}
};
var translator = new Translator(AuthKey, options);

This information is passed along when the library makes calls to the DeepL API. Both name and version are required. Please note that setting the User-Agent header via TranslatorOptions.Headers will override this setting, if you need to use this, please manually identify your Application in the User-Agent header.

The Translator constructor accepts TranslatorOptions as a second argument, for example:

var options = new TranslatorOptions {
      MaximumNetworkRetries = 5,
      PerRetryConnectionTimeout = TimeSpan.FromSeconds(10),
};
var translator = new Translator(authKey, options);

See the TranslatorOptions class for details about the available options.

To use the library with a proxy, override the ClientFactory with a function returning a custom HttpClient:

var proxyUrl = "http://localhost:3001";
var handler = new System.Net.Http.HttpClientHandler {
      Proxy = new System.Net.WebProxy(proxyUrl), UseProxy = true,
};
var options = new TranslatorOptions {
      ClientFactory = () => new HttpClientAndDisposeFlag {
            HttpClient = new HttpClient(handler), DisposeClient = true,
      }
};
var translator = new Translator(authKey, options);

By default, we send some basic information about the platform the client library is running on with each request, see here for an explanation. This data is completely anonymous and only used to improve our product, not track any individual users. If you do not wish to send this data, you can opt-out when creating your Translator object by setting the sendPlatformInfo flag in the TranslatorOptions to false like so:

var options = new TranslatorOptions { sendPlatformInfo = false };
var translator = new Translator(authKey, options);

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

We welcome Pull Requests, please read the contributing guidelines.

Execute the tests using dotnet test. The tests communicate with the DeepL API using the auth key defined by the DEEPL_AUTH_KEY environment variable.

Be aware that the tests make DeepL API requests that contribute toward your API usage.

The test suite may instead be configured to communicate with the mock-server provided by deepl-mock. Although most test cases work for either, some test cases work only with the DeepL API or the mock-server and will be otherwise skipped. The test cases that require the mock-server trigger server errors and test the client error-handling. To execute the tests using deepl-mock, run it in another terminal while executing the tests. Execute the tests using dotnet test with the DEEPL_MOCK_SERVER_PORT and DEEPL_SERVER_URL environment variables defined referring to the mock-server.