Support conditionally (e.g. via a global flag) excluding properties in schema generation

[ph3rin]

I have a use-case where we decide to include/exclude certain properties from the schema based on a global flag.

My current workaround is to use a custom ISchemaProcessor to strip properties. However, that requires iterating through all properties of the ContextualType, generating a name for the property, and then remove it from the property dictionary:

var flag = true;
var settings = new SystemTextJsonSchemaGeneratorSettings();
if (flag)
{
    settings.SchemaProcessors.Add(new CustomIgnoreProcessor());
}

class MyClass
{
    public int Foo { get; set; }
    [CustomIgnore]
    public bool Bar { get; set; }
}

class CustomIgnoreAttribute : Attribute
{
}

class CustomIgnoreProcessor : ISchemaProcessor
{
    public void Process(SchemaProcessorContext context)
    {
        if (!context.Schema.IsObject) return;
        
        foreach (var property in context.ContextualType.Properties)
        {
            if (property.GetAttribute<CustomIgnoreAttribute>(true) == null)
            {
                continue;
            }
                
            var name = context
                .Generator.Settings.ReflectionService
                .GetPropertyName(property, context.Generator.Settings);
            context.Schema.Properties.Remove(name);
        }
    }
}

In the example above, CustomIgnore will only take effect if flag is being set to true.

Maybe there is a more elegant way. Currently the awkwardness mainly stems from not being able to get a JsonSchemaProperties's associated ContextualPropertyInfo. Maybe we can include it in the schema (as a nullable property because schemas aren't necessarily created from types).