Added article Generate Unit Tests with Copilot

docs/core/testing/unit-testing-with-copilot.md

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+---
+title: Generate Unit Tests with Copilot
+author: sigmade
+description: How to generate unit tests and test projects in C# using the xUnit framework with the help of Visual Studio commands and GitHub Copilot
+ms.date: 01/12/2025
+---
+# Generate Unit Tests with GitHub Copilot and Visual Studio
+
+By [Yegor Sychev](https://github.com/sigmade)
+
+This article explores how to generate unit tests and test projects in C# using the xUnit framework with the help of Visual Studio commands and GitHub Copilot.
+
+## Generating a test project and a stub Method
+
+There is a `ProductService` class with a `GetProductById` method that depends on the `IProductDataStorage` and `ICacheClient` interfaces.
+
+```csharp
+public class ProductService(
+    IProductDataStorage productDataStorage,
+    ICacheClient cacheClient)
+{
+    private readonly IProductDataStorage _productDataStorage = productDataStorage;
+    private readonly ICacheClient _cacheClient = cacheClient;
+
+    public async Task<Product?> GetProductById(int productId)
+    {
+        var product = await _cacheClient.GetProduct(productId);
+
+        if (product != null)
+        {
+            return product;
+        }
+
+        product = await _productDataStorage.GetProduct(productId);
+
+        if (product != null)
+        {
+            await _cacheClient.SetProduct(product);
+        }
+
+        return product;
+    }
+}
+```
+
+To generate a test project and a stub method, follow these steps:
+
+* Select the method.
+* Right-click and select 𝗖𝗿𝗲𝗮𝘁𝗲 𝗨𝗻𝗶𝘁 𝗧𝗲𝘀𝘁𝘀. 
+
+:::image type="content" source="media/create-unit-test.png" lightbox="media/create-unit-test.png" alt-text="Command 𝗖𝗿𝗲𝗮𝘁𝗲 𝗨𝗻𝗶𝘁 𝗧𝗲𝘀𝘁𝘀":::
+
+In the dialog that appears, select xUnit from the Test Framework dropdown menu
+
+> [!NOTE]
+> The "Create Unit Tests" command defaults to the MSTest framework. However, since this example uses xUnit, you need to install the Visual Studio extension xUnit.net.TestGenerator2022.
+
+:::image type="content" source="media/create-unit-test-window.png" lightbox="media/create-unit-test-window.png" alt-text="Create Unit Tests window":::
+
+* If you don't have a test project yet, choose "New Test Project" or select an existing one.
+* If necessary, specify a template for the namespace, class, and method name, then click OK.
+
+After a few seconds, Visual Studio will pull in the necessary packages, and we will get a generated xUnit project with the required packages, structure, a reference to the project being tested, and with the `ProductServiceTests` class and a stub method.
+
+:::image type="content" source="media/test-mehod-stub.png" lightbox="media/test-mehod-stub.png" alt-text="Generated stub method":::
+
+## Generate the tests themselves:
+
+* Select the method being tested again.
+* Right-click - Ask Copilot.
+* Enter a simple prompt, such as: <br> `generate unit tests using xunit, nsubstitute and insert the result into ProductServiceTests`</br> You need to select your test class using #. 
+
+> [!TIP]
+> For quick search, it is desirable that `ProductServiceTests` is open in a separate tab.
+
+:::image type="content" source="media/test-copilot-prompt.png" lightbox="media/test-copilot-prompt.png" alt-text="Prompt for generate tests":::
+
+Execute the prompt, click Accept, and Copilot generates the test code. After that, it remains to install the necessary packages.
+
+When the packages are installed, the tests can be run. This example worked on the first try: Copilot knows very well how to work with NSubstitute, and all dependencies were defined through interfaces. 
+
+:::image type="content" source="media/test-mehod-stub.png" lightbox="media/test-mehod-stub.png" alt-text="Generated tests":::
+
+Thus, using 𝗩𝗶𝘀𝘂𝗮𝗹 𝗦𝘁𝘂𝗱𝗶𝗼 in combination with 𝗚𝗶𝘁𝗛𝘂𝗯 𝗖𝗼𝗽𝗶𝗹𝗼𝘁 significantly simplifies the process of generating and writing unit tests.