JsonSerializer.Deserialize fails

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Your problem is that System.Text.Json is case-sensitive by default, so "id": 9 (all lowercase) is not mapped to the Id property. From the docs:

Case-insensitive property matching

By default, deserialization looks for case-sensitive property name matches between JSON and the target object properties. To change that behavior, set JsonSerializerOptions.PropertyNameCaseInsensitive to true:

Note: The web default is case-insensitive.

var options = new JsonSerializerOptions
{
   PropertyNameCaseInsensitive = true,
};
var weatherForecast = JsonSerializer.Deserialize<WeatherForecast>(jsonString, options);

So you need to do that also:

var u = JsonSerializer.Deserialize<User>(str, new JsonSerializerOptions { PropertyNameCaseInsensitive = true });

Demo fiddle #1 here.

(If the difference is entirely due to camel casing and not more general differences in case, you can instead configure the serializer to use camel case as shown in this answer by t.j..)

You can configure the option on startup in ASP.NET Core 3.0 as shown in How to set json serializer settings in asp.net core 3?:

services.AddControllers().AddJsonOptions(options =>
{
    options.JsonSerializerOptions.PropertyNameCaseInsensitive = true;
});

Alternatively you could apply [JsonPropertyName("id")] to your model:

public class User {
    [JsonPropertyName("id")]
    public int Id { get; set; }
}

Demo fiddle #2 here.

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