In general I would like to NOT contaminate my ViewObjects folder with POCO's then writing REST API in ASP.NET Core, that is: I want the HTTP endpoint signature to take a dynamic argument :

• What I want : public JsonResult DoSomething([FromBody] dynamic model)
• What I don't want : public JsonResult DoSomething([FromBody] SomeClass model)

I then want to access the model parameters like this :

var value1 = (string)model["key1"];

, which indeed was all possible in .NET Core < 3.

However, since .NET Core 3, Microsoft set MVC Controllers to default use the new System.Text.Json API that while allegedly (and I trust it) should be way faster, is not able to parse the dynamic arriving at the endpoint.

Since .NET Core 3 you will get the following error then trying to parse a dynamic JSON type, eg.:

• var value1 = (string)model["key1"];

"Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: 'The best overloaded method match for 'System.Text.Json.JsonElement.this[int]' has some invalid arguments'"

Solution

(Found here at StackOverflow)

1. Install the Microsoft.AspNetCore.Mvc.NewtonsoftJson Nuget package
2. Reconfigure your MVC controllers to use Json.NET by having the following code in the ConfigureServices section of your ASP.NET Core 3 Startup class :
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   services.AddControllers()
       .AddNewtonsoftJson(options =>
       {
           options.SerializerSettings.ContractResolver = new DefaultcotractResolver();
       });