Add code example for CA2012 rule (#49037)

docs/fundamentals/code-analysis/quality-rules/ca2012.md

@@ -10,6 +10,8 @@ helpviewer_keywords:
   - "CA2012"
 author: stephentoub
 ms.author: stoub
+dev_langs:
+- CSharp
 ---
 # CA2012: Use ValueTasks correctly
 
@@ -37,6 +39,10 @@ In general, ValueTasks should be directly awaited rather than discarded or store
 
 For `ValueTask` objects returned from arbitrary member calls, the caller needs to assume that the `ValueTask` must be consumed (for example, awaited) once and only once. However, if the developer also controls the member being invoked and has complete knowledge of its implementation, the developer may know it's safe to suppress the warning, for example, if the return `ValueTask` always wraps a <xref:System.Threading.Tasks.Task> object.
 
+## Example
+
+:::code language="csharp" source="snippets/csharp/all-rules/ca2012.cs" id="snippet1":::
+
 ## Suppress a warning
 
 If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

docs/fundamentals/code-analysis/quality-rules/snippets/csharp/all-rules/ca2012.cs

@@ -0,0 +1,48 @@
+using System.Threading.Tasks;
+
+namespace ca2012
+{
+    //<snippet1>
+    public class NumberValueTask
+    {
+        public async ValueTask<int> GetNumberAsync()
+        {
+            await Task.Delay(100);
+            return 123;
+        }
+
+        public async Task UseValueTaskIncorrectlyAsync()
+        {
+            // This code violates the rule,
+            // because ValueTask is awaited multiple times
+            ValueTask<int> numberValueTask = GetNumberAsync();
+
+            int first = await numberValueTask;
+            int second = await numberValueTask; // <- illegal reuse
+
+            // ...
+        }
+
+        // This code satisfies the rule.
+        public async Task UseValueTaskCorrectlyAsync()
+        {
+            int first = await GetNumberAsync();
+            int second = await GetNumberAsync();
+
+            // ..
+        }
+
+        public async Task UseValueTaskAsTaskAsync()
+        {
+            ValueTask<int> numberValueTask = GetNumberAsync();
+
+            Task<int> numberTask = numberValueTask.AsTask();
+
+            int first = await numberTask;
+            int second = await numberTask;
+
+            // ...
+        }
+    }
+    //</snippet1>
+}