JsonConverters apply to types, not to fields or properties.
Instead of adding an attribute to a property that uses an existing type, consider creating a new type and writing a convert for that instead.
public struct MyCustomType
{
...
}
public class MyClass
{
public MyCustomType {get; set;}
}
Besides - in what other way would you ever want to serialize a raw integer? If the integer represents something, then create a struct or class for that something.
See also: "ValueObject" (Domain Driven Design fundamental concept)
Based on your comment below, an alternate approach would be to forget about JsonConverters and simply expose a secondary property:
public class MyClass
{
[JsonIgnore]
public Int64 MyInt {get; set;}
[JsonProperty("MyInt")]
public Int64 MyEncryptedInt
{
get { return Encrypt(MyInt); }
set { MyInt = Decrypt(value); }
}
}