3. Custom Attributes

WebApiClient provides many built-in attributes, including interface-level, method-level, and parameter-level attributes. They respectively implement the IApiActionAttribute, IApiActionFilterAttribute, IApiParameterAttribute, IApiParameterable, and IApiReturnAttribute interfaces.

3.1 Customizing IApiParameterAttribute

For example, if the server requires submission in x-www-form-urlencoded format, and the interface design is not ideal, the current requirement is to submit a JSON text fieldX={X} and fieldY={Y}. Here is how we design the corresponding interface:

[HttpHost("/upload")]
ITask<bool> UploadAsync(
    [FormField][AliasAs("fieldX")] string xJson,
    [FormField][AliasAs("fieldY")] string yJson);

Clearly, the parameter type string for our interface is too broad and lacks constraints. We would like it to be like this:

[HttpHost("/upload")]
ITask<bool> UploadAsync([FormFieldJson] X fieldX, [FormFieldJson] Y fieldY);

The [FormFieldJson] attribute serializes the parameter value into JSON and uses it as a field content in the form.

[AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false)]
class FormFieldJson: Attribute, IApiParameterAttribute
{
    public async Task BeforeRequestAsync(ApiActionContext context, ApiParameterDescriptor parameter)
    {
      var options = context.HttpApiConfig.FormatOptions;
      var json = context.HttpApiConfig.JsonFormatter.Serialize(parameter.Value, options);
      var fieldName = parameter.Name;
      await context.RequestMessage.AddFormFieldAsync(fieldName, json);
    }
}