Encoding and decoding of Json is largely a transparent task, regardless of the serialization library you use - typically (and recommended!) System.Text.Json.

For example, assume you have the following Person type:

public sealed class Person
{
  public string Name { get; set; }
  public byte Age { get; set; }
}

If we wanted to serialize this, we would do it like so:

var person = new Person { Name = "James Bond", Age = 40 };
// Configure options for serialization
var options = new JsonSerializerOptions
{
	WriteIndented = true
};
// Serialize
var json = JsonSerializer.Serialize(person, options);
// Print to console
Console.WriteLine(json);

This would print the following:

{
  "Name": "James Bond",
  "Age": 40
}

Note here that the Age, an integer, is printed as a native number, 40.

All straightforward enough.

Sometimes, however, you are the consumer of Json that you did not encode.

Generally, it will look like what we printed above.

However, there are times the Json is encoded like this:

{
  "Name": "James Bond",
  "Age": "40"
}

Here, the number is encoded as a string.

We can try and deserialize this like so:

var apiJson =
"""
{
  "Name": "James Bond",
  "Age": "40"
}
""";
// Deserialize to an object
person = JsonSerializer.Deserialize<Person>(apiJson);

This code, unfortunately, will throw an exception:

The JSON value could not be converted to System.Byte. Path: $.Age | LineNumber: 2 | BytePositionInLine: 14.

This is because by default, System.Text.Json will make no attempt to figure out that the number is encoded as a string.

You will have to explicitly tell it to do that, using JsonSerializerOptions, like so:

var apiJson =
"""
{
  "Name": "James Bond",
  "Age": "40"
}
""";
// Configure options for deserialization
var serializationOptions = new JsonSerializerOptions
{
	NumberHandling = System.Text.Json.Serialization.JsonNumberHandling.AllowReadingFromString
};
// Deserialize to an object
person = JsonSerializer.Deserialize<Person>(apiJson, serializationOptions);

We are setting the NumberHandling property to AllowReadingFromString.

This will print the following:

James Bond is 40 years old

TLDR

We can read numbers encoded as strings in Json using JsonSerializerOptions with NumberHandling set to JsonNumberHandling.AllowReadingFromString

The code is in my GitHub.

Happy hacking!