I'm in Net core 3.1 and I'm trying to use AddNewtonsoftJson()
I'm getting:
"IMvcBuilder' does not contain a definition for 'AddNewtonsoftJson' and no accessible extension method 'AddNewtonsoftJson' accepting a first argument of type 'IMvcBuilder' could be found (are you missing a using directive or an assembly reference?)"
This is what I write in my ConfigureService:
services.AddControllers().AddNewtonsoftJson();
I already have installed NewtonSoft in my startup project.
You need also to install Microsoft.AspNetCore.Mvc.NewtonsoftJson which contains corresponding extension methods, as written in the docs .
The package Microsoft.AspNetCore.Mvc.NewtonsoftJson is no longer included in Microsoft.AspNetCore.App framework (aka. meta package in version 2.x) because the new System.Text.Json is now mature enough to replace Newtonsoft.Json as the built-in json (de)serializer.
Therefore consider replacing all uses of Newtonsoft.Json with System.Text.Json for performance and futural compatibility.
Only if it is not possible, you could install the Microsoft.AspNetCore.Mvc.NewtonsoftJson package manually, while a direct reference to Newtonsoft.Json is no longer need, for Microsoft.AspNetCore.Mvc.NewtonsoftJson already references Newtonsoft.Json.