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259
ViewsWhy are Middleware Components Called Twice in a .Net Core Pipeline?
If I create a blank ASP.net Web Core Application, then replace the Configure() method in Startup.cs with the following method, each Use() and Run() operation is called twice.
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
app.Use(async (context, next) =>
{
// Do work that doesn't write to the Response.
await next.Invoke();
// Do logging or other work that doesn't write to the Response.
int i = 0;
});
app.Use(async (context, next) =>
{
// Do work that doesn't write to the Response.
await next.Invoke();
// Do logging or other work that doesn't write to the Response.
int j = 0;
});
app.Run(async context =>
{
await context.Response.WriteAsync("Hello, World!");
});
}
So the order of operations is:
- In the first app.Use(), await next.Invoke() is called
- In the second app.Use(), wait next.Invoke() is called
- App.Run() is called
Then, it comes back through the pipeline..
- In the second app.Use(), int j = 0 is called
- In the first app.Use(), int i = 0 is called
All of this is expected as it passes through the Middleware and back. But then, each of the above steps is repeated again.
Why is each middleware component called twice?
1 answers
Answer question0
The reason it is called twice is that your browser is making two requests to your app. One for the app root path '/' the other one for /favicon.ico (some browsers like chrome makes favicon request by default, screenshot below). You can check your browser dev tools.
You can also verify that by putting a debug log inside app.Use to see the request paths. e.g.
app.Use(async (context, next) =>
{
Debug.WriteLine(context.Request.Path.Value);
// Do work that doesn't write to the Response.
await next.Invoke();
// Do logging or other work that doesn't write to the Response.
int i = 0;
});
So your middleware setup runs twice. Once for root '/' and the second time for '/favicon.ico' path