### Question

Can I dynamically ignore a property from a sealed class using [`System.Text.Json.JsonSerializer`][4]?



### Example Code

**Example class from another library**:

```cs
public sealed class FrozenClass
{
    // [JsonIgnore] <- cannot apply because I don't own this class
    public int InternalId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}
```

**Default Serialization**:

```cs
var person = new FrozenClass() { InternalId = 3, FirstName = "Dorothy", LastName = "Vaughan" };
var jsonString = System.Text.Json.JsonSerializer.Serialize(person);
```

**Actual Outcome**:

```js
{ "InternalId": 3, "FirstName": "Dorothy", "LastName": "Vaughan" }
```

**Expected Outcome**:

```js
{ "FirstName": "Dorothy", "LastName": "Vaughan" }
```


### Alternatives that don't work

There are two other questions on how to [add JsonIgnore at RunTime][1] & [add dynamic property name for serialization][2], but both are targeted at [Newtonsoft's Json.NET][3] - and often reference [this extension method][5]


There are several ways to modify the way a class is serialized with the native `JsonSerializer`, but all seem to rely on modifying the underlying class:

* You can [exclude properties from serialization][6] by adding the [`[JsonIgnore]`][7] attribute to the original class
* You can [register a JsonConverter on a property][8] by adding the [`[JsonConverter]`][9] attribute to the original class

However, in my case, the class is from another library and cannot be extended

### Workarounds

A possible workaround is to create another exportable class and setup a mapper between them

```cs
public class MyFrozenClass
{
    public MyFrozenClass(FrozenClass frozen)
    {
        this.FirstName = frozen.FirstName;
        this.LastName = frozen.LastName;
    }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}
```
```cs
var jsonString = System.Text.Json.JsonSerializer.Serialize(new MyFrozenClass(person));
```

[1]: https://stackoverflow.com/q/25157511/1366033
[2]: https://stackoverflow.com/q/50565358/1366033
[3]: https://www.newtonsoft.com/json
[4]: https://learn.microsoft.com/en-us/dotnet/api/system.text.json.jsonserializer
[5]: https://blog.rsuter.com/advanced-newtonsoft-json-dynamically-rename-or-ignore-properties-without-changing-the-serialized-class/
[6]: https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-how-to#exclude-properties-from-serialization
[7]: https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonignoreattribute
[8]: https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to#registration-sample---jsonconverter-on-a-property
[9]: https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonconverter-1