docs(dotnet): selectors example (#6580)

docs/src/api/class-selectors.md

@@ -76,6 +76,31 @@ browser.close();
 # FIXME: add snippet
 ```
 
+```csharp
+using var playwright = await Playwright.CreateAsync();
+// Script that evaluates to a selector engine instance.
+await playwright.Selectors.RegisterAsync("tag", @"{
+// Returns the first element matching given selector in the root's subtree.
+query(root, selector) {
+    return root.querySelector(selector);
+  },
+  // Returns all elements matching given selector in the root's subtree.
+  queryAll(root, selector) {
+    return Array.from(root.querySelectorAll(selector));
+  }
+}");
+
+await using var browser = await playwright.Chromium.LaunchAsync();
+var page = await browser.NewPageAsync();
+await page.SetContentAsync("<div><button>Click me</button></div>");
+// Use the selector prefixed with its name.
+var button = await page.QuerySelectorAsync("tag=button");
+// Combine it with other selector engines.
+await page.ClickAsync("tag=div >> text=\"Click me\"");
+// Can use it in any methods supporting selectors.
+int buttonCount = await page.EvalOnSelectorAllAsync<int>("tag=button", "buttons => buttons.length");
+```
+
 ### param: Selectors.register.name
 - `name` <[string]>