How to ignore a property in runtime deserialization ASP.NET Core app

I have class for JSON deserialization in my ASP.NET Core 6.0 application:

public class DocumentInfo
{
    [JsonProperty("FileName", NullValueHandling = NullValueHandling.Ignore)]
    [JsonPropertyName("FileName")]
    public string FileName { get; set; }

    [JsonPropertyName("Template")]
    [JsonProperty("FileName", Required = Required.DisallowNull, NullValueHandling = NullValueHandling.Ignore)]
    public string Template { get; set; }

    [JsonProperty("VersionBody", NullValueHandling = NullValueHandling.Ignore)]
    [JsonPropertyName("VersionBody")]
    public VersionBody VersionBody { get; set; }

    [Newtonsoft.Json.JsonIgnore]
    public string FileExtension => Path.GetExtension(this.FileName);
}

Controller API method:

[HttpPost(template:"generate"), Consumes("application/json"), Produces("application/json")]
public FileStreamResult? Generate([FromBody] DocumentInfo request,
        CancellationToken cancellationToken = default)

Sometime the template property may not be present in serialized query. Then the controller should ignore it. But now I get an error during deserialization that the field is required but missing.

I tried

[DataMember(IsRequired = false)] 

but that didn't help me.

How to avoid error at runtime when deserializing?

> Sometime the template property may not be present in serialized query.
> Then the controller should ignore it. But now I get an error during
> deserialization that the field is required but missing. How to avoid
> error at runtime when deserializing?

Actually, the first thing you need to consider while ignoring serialization is nullability. In other words, the ignoring property should be null. Because, if you set WhenWritingDefault or NullValueHandling.Ignore to a non nullable property model binder will not ignore that as it will be treated as required. Thus, evaluation became self-contradictory.

In addition, to handle runtime exception during null property serialization we need to explicitely define JsonSerializerOptions as WhenWritingNull in program.cs file.

Let's have a look in practice:

Model:

<!-- language: c# -->
public class DocumentInfo
{
[JsonProperty("FileName", NullValueHandling = NullValueHandling.Ignore)]
[JsonPropertyName("FileName")]
public string FileName { get; set; }

        [JsonPropertyName("Template")]
        [JsonProperty("Template", NullValueHandling = NullValueHandling.Ignore)]
        public string? Template { get; set; }
        [Newtonsoft.Json.JsonIgnore]
        public string FileExtension => Path.GetExtension(this.FileName);
    }

Note: I made Template property as nullable by putting string?

Program.cs:

<!-- language: c# -->
builder.Services.AddControllers().AddJsonOptions(options =>
{

    options.JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;

});

Output:

Note: If you would like to know more details how to Ignore all null-value properties during runtime you could check our official document here