Update net_standard.md

Author: sj-net

docs/dotnet_framework/net_standard.md

@@ -10,10 +10,50 @@ grand_parent: My Docs
 ---
 
 ``` csharp
-using Microsoft.
-public static class HealthCheckExtensions
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+
+namespace Microsoft.Extensions.DependencyInjection
 {
-    public static HealthCheckService
+    public class SampleDatabaseHealthCheck : IHealthCheck
+    {
+        private readonly DBContext dbContext;
+    
+        public SampleDatabaseHealthCheck(DBContext dbContext)
+        {
+            this.dbContext = dbContext;
+        }
+        public async Task<HealthCheckResult> CheckHealthAsync(HealthCheckContext context, CancellationToken cancellationToken = default)
+        {
+            try
+            {
+                // select 1 is the fastest way to detect whether the DB can be connected or not.
+                var result = await dbContext.ExecuteSqlCommandAsync("Select 1", cancellationToken);
+                return HealthCheckResult.Healthy();
+            }
+            catch (System.Exception e)
+            {
+                return HealthCheckResult.Unhealthy(exception: e);
+            }
+        }
+    }
+
+    public static class HealthCheckExtensions
+    {
+        public static HealthCheckService GetHealthCheckService(DBContext dbContext)
+        {
+            var serviceCollection = new ServiceCollection();
+            serviceCollection
+                    .AddLogging() // HealthCheckService needs ILogger
+                    .AddHealthChecks()
+                    // if the ctor needs custom arguements
+                    .AddTypeActivatedCheck<SampleDatabaseHealthCheck>("database", dbContext)
+                    // if ctor argument can be resolved by this this serviceCollection
+                    .AddCheck<SampleDatabaseHealthCheck>("database");
+            // ensure the health check can be resolved with all the required dependecies else this throws DI error.
+            var serviceProvider = serviceCollection.BuildServiceProvider();
+            return serviceProvider.GetRequiredService<HealthCheckService>();
+        }
+    }
 }
 ```