test: Add write API tests

Author: JanEbbing

DeepLTests/BaseDeepLTest.cs

@@ -43,6 +43,13 @@ static BaseDeepLTest() {
       };
     }
 
+    protected static DeepLClient CreateTestClient(bool randomAuthKey = false) {
+      var authKey = randomAuthKey ? Guid.NewGuid().ToString() : AuthKey;
+      return ServerUrl == null
+            ? new DeepLClient(authKey)
+            : new DeepLClient(authKey, new DeepLClientOptions { ServerUrl = ServerUrl });
+    }
+
     protected static Translator CreateTestTranslator(bool randomAuthKey = false) {
       var authKey = randomAuthKey ? Guid.NewGuid().ToString() : AuthKey;
       return ServerUrl == null

DeepLTests/RephraseTextTest.cs

@@ -0,0 +1,54 @@
+// Copyright 2025 DeepL SE (https://www.deepl.com)
+// Use of this source code is governed by an MIT
+// license that can be found in the LICENSE file.
+
+using System.Threading.Tasks;
+using DeepL;
+using DeepL.Model;
+using Xunit;
+
+namespace DeepLTests {
+  public sealed class RephraseTextTest : BaseDeepLTest {
+    [Fact]
+    public async Task TestSingleText() {
+      var client = CreateTestClient();
+      var inputText = ExampleText("en");
+      var result = await client.RephraseTextAsync(ExampleText("en"), LanguageCode.EnglishAmerican);
+      checkSanityOfImprovements(inputText, result);
+    }
+
+    [Fact]
+    public async Task TestTextArray() {
+      var client = CreateTestClient();
+      var texts = new[] { ExampleText("en"), ExampleText("en") };
+      var inputText = ExampleText("en");
+      var results = await client.RephraseTextAsync(texts, LanguageCode.EnglishAmerican);
+      foreach (var result in results) {
+        checkSanityOfImprovements(inputText, result);
+      }
+    }
+
+    [RealServerOnlyFact]
+    public async Task TestBusinessStyle() {
+      var client = CreateTestClient();
+      var inputText = "As Gregor Samsa awoke one morning from uneasy dreams he found himself transformed in his bed into a gigantic insect.";
+      var result = await client.RephraseTextAsync(
+        inputText, LanguageCode.EnglishAmerican, new TextRephraseOptions { WritingStyle = "business" }
+      );
+      checkSanityOfImprovements(inputText, result);
+    }
+
+    private void checkSanityOfImprovements(
+        string inputText,
+        WriteResult result,
+        string expectedSourceLangUppercase="EN",
+        string expectedTargetLangUppercase="EN-US",
+        float epsilon=0.2f) {
+      Assert.Equal(expectedSourceLangUppercase, result.DetectedSourceLanguageCode.ToUpper());
+      Assert.Equal(expectedTargetLangUppercase, result.TargetLanguageCode.ToUpper());
+      var ratio = ((float) result.Text.Length) / inputText.Length;
+      Assert.True(1 / (1.0 + epsilon) <= ratio, $"Rephrased text is too short compared to input text.\n{inputText}\n{result.Text}");
+      Assert.True(ratio <= (1.0 + epsilon), $"Rephrased text is too long compared to input text.\n{inputText}\n{result.Text}");
+    }
+  }
+}