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Peeking at a JSON token using a PipeReader in the ASP.NET Core request pipeline
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using System; | |
using System.Buffers; | |
using System.Collections.Generic; | |
using System.IO.Pipelines; | |
using System.Linq; | |
using System.Text.Json; | |
using System.Text.Json.Serialization; | |
using System.Threading.Tasks; | |
using Microsoft.AspNetCore.Builder; | |
using Microsoft.AspNetCore.Hosting; | |
using Microsoft.AspNetCore.Http; | |
using Microsoft.Extensions.DependencyInjection; | |
using Microsoft.Extensions.Hosting; | |
namespace WebApplication409 | |
{ | |
public class Startup | |
{ | |
// This method gets called by the runtime. Use this method to add services to the container. | |
// For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940 | |
public void ConfigureServices(IServiceCollection services) | |
{ | |
} | |
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline. | |
public void Configure(IApplicationBuilder app, IWebHostEnvironment env) | |
{ | |
if (env.IsDevelopment()) | |
{ | |
app.UseDeveloperExceptionPage(); | |
} | |
app.UseRouting(); | |
app.UseEndpoints(endpoints => | |
{ | |
endpoints.MapGet("/", async context => | |
{ | |
await context.Response.WriteAsync("Hello World!"); | |
}); | |
endpoints.MapPost("/json", async context => | |
{ | |
// Allow posting either an array or object | |
var token = await PeekJsonTokenType(context.Request.BodyReader); | |
switch (token) | |
{ | |
case JsonTokenType.StartObject: | |
var person = await JsonSerializer.DeserializeAsync<Person>(context.Request.BodyReader.AsStream()); | |
break; | |
case JsonTokenType.StartArray: | |
var people = await JsonSerializer.DeserializeAsync<Person[]>(context.Request.BodyReader.AsStream()); | |
break; | |
default: | |
break; | |
} | |
}); | |
}); | |
} | |
public class Person | |
{ | |
[JsonPropertyName("name")] | |
public string Name { get; set; } | |
[JsonPropertyName("age")] | |
public int Age { get; set; } | |
} | |
private static async ValueTask<JsonTokenType> PeekJsonTokenType(PipeReader reader) | |
{ | |
// Separate method so that we can use the ref struct | |
static bool DetermineTokenType(in ReadOnlySequence<byte> buffer, out JsonTokenType jsonToken) | |
{ | |
var jsonReader = new Utf8JsonReader(buffer); | |
if (jsonReader.Read()) | |
{ | |
jsonToken = jsonReader.TokenType; | |
return true; | |
} | |
jsonToken = JsonTokenType.None; | |
return false; | |
} | |
while (true) | |
{ | |
var result = await reader.ReadAsync(); | |
var buffer = result.Buffer; | |
if (DetermineTokenType(buffer, out var tokenType)) | |
{ | |
// Don't consume any of the buffer so we can re-parse it with the | |
// serializer | |
reader.AdvanceTo(buffer.Start, buffer.Start); | |
return tokenType; | |
} | |
else | |
{ | |
// We don't have enough to read a token, keep buffering | |
reader.AdvanceTo(buffer.Start, buffer.End); | |
} | |
// If there's no more data coming, then bail | |
if (result.IsCompleted) | |
{ | |
return JsonTokenType.None; | |
} | |
} | |
} | |
} | |
} |
There’s one thing I missed, quitting the loop if the body is complete. Let me add that.
Awesome, will pop that into our code base.
Thanks so much for all your help; means a lot! As I mentioned on the ticket, this really isn't my area of expertise, but I've learnt a lot thanks to you. Appreciate it :)
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Which then begs the question, how to differentiate between invalid input and there not being enough read to read a token. I guess the exception vs Read returning false is that differentiator.