Feature Request: const fields

[tomasbruckner]

If you have a const field

public const string Bar = "foo";

It would be great if it generated a not nullable enum with only one value ("foo"). Now it is ignored.

When OpenAPI.NET starts supporting version 3.1, it could be generated as a constant value OAI/OpenAPI-Specification#1313

Also if there is some workaround on how to rewrite the string field/property to an enum with one value now, it would be appreciated if you shared this knowledge👍

[tomasbruckner]

Workaround is to create an attribute. Something like

[AttributeUsage(AttributeTargets.Property)]
public class SwaggerConstTypeAttribute : Attribute
{
    public SwaggerConstTypeAttribute(string enumValue)
    {
        Value = enumValue;
    }

    public string Value { get; }
}

Annotate your desired class

public class Foo
{
    [SwaggerConstType("const-value")]
    public string Type { get; set; }
}

Create SchemaFilter

public class SwaggerConstTypeFilter : ISchemaFilter
{
    public void Apply(OpenApiSchema model, SchemaFilterContext context)
    {
        foreach (var property in context.Type.GetProperties())
        {
            var schemaPropertyName = PropertyName(property);
            // This check ensures that properties that are not in the schema are not added.
            // This includes properties marked with [IgnoreDataMember] or [JsonIgnore].
            if (model.Properties?.ContainsKey(schemaPropertyName) != true)
            {
                continue;
            }

            if (property.CustomAttributes.Any(attr => attr.AttributeType == typeof(SwaggerConstTypeAttribute)))
            {
                var constAttribute = property.GetCustomAttribute<SwaggerConstTypeAttribute>();
                model.Properties[schemaPropertyName].Enum = new List<IOpenApiAny>
                {
                    new OpenApiString(constAttribute.Value)
                };
                model.Properties[schemaPropertyName].Nullable = false;
                continue;
            }
        }
    }

   // if you are using camelCasePropertyName
    private static string PropertyName(MemberInfo property)
    {
        return new CamelCasePropertyNamesContractResolver().GetResolvedPropertyName(property.Name);
    }
}

And register the filter in

services.AddSwaggerGen(
    c =>
    {
        ...
        c.SchemaFilter<SwaggerConstTypeFilter>();
        ...
    }
);