This feature would solve the problem of customizing or request pipeline and help common logic to share across Azure functions. If you are a .NET developer, you would have implemented ASP.NET MVC filters in your web application. The Function Filters are way similar to ASP.NET filters.

In the following, let us see the various filter
types available in Functions and how to implement it in a sample HTTP trigger
Function.
Below are the two filters which are available for Functions;

1. FunctionInvocationFilterAttribute
2. FunctionExceptionFilterAttribute

FunctionInvocationFilterAttribute

This filter can be executed before and after reprocessing
logic in the particular function.

FunctionExceptionFilterAttribute

This Filter will be executed if there is an
exception thrown by the Azure Function.

The above
mentioned two filter can be applied globally to all functions, it can applied
either to a class level or Function level respectively.

Now, let us implement the two filters in a HTTP
trigger Function. For this purpose, create a new Function in Visual
Studio with HTTP trigger. Then, make necessary changes to the template code
as below or you may just copy paste it.

// Modified Template Code
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.Azure.WebJobs.Host;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;
using System;
using System.Diagnostics;
using System.IO;
using System.Threading;
using System.Threading.Tasks;

namespace FunctionFilters
{
    public class Function1:IFunctionExceptionFilter, IFunctionInvocationFilter
    {
        [FunctionName("Function1")]
        public  async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
            ILogger log)
        {

            Debug.WriteLine("C# HTTP trigger function processed a request.");

            string name = req.Query["name"];

            string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
            dynamic data = JsonConvert.DeserializeObject(requestBody);
            name = name ?? data?.name;

            retu
 name != null
                ? (ActionResult)new OkObjectResult($"Hello, {name}")
                : new BadRequestObjectResult("Please pass a name on the query string or in the request body");
        }

        public Task OnExceptionAsync(FunctionExceptionContext exceptionContext, CancellationToken cancellationToken)
        {
            Debug.WriteLine($"Exception raised by the application {exceptionContext.Exception.ToString()}");
            retu
 Task.CompletedTask;
        }

        public Task OnExecutedAsync(FunctionExecutedContext executedContext, CancellationToken cancellationToken)
        {
            Debug.WriteLine($"This part should be executed at last");
            retu
 Task.CompletedTask;
        }

        public Task OnExecutingAsync(FunctionExecutingContext executingContext, CancellationToken cancellationToken)
        {
            Debug.WriteLine($”This part should be executed at before function code");
            retu
 Task.CompletedTask;
        }
    }
}

The template code has got the following
modifications (just to know):

1. The function class must be
   changed from static to instance
2. You should implement function
   filters at class level and override the respective method OnExceptionAsync,
   OnExecutedAsync and OnExecutingAsync.

Once, the changes to the code is done. Run it locally, may be in your favourite IDE to test how it works. Happy Lea
ing!!