How do I turn this Swashbuckle .NET 6 operation filter into a generic filter that can handle multiple class types in C# ASP.NET Core?

Question

I have the following code for operation filter that helps me serialize arrays then sends it to a FromForm binder, how I turned it generic so I can get several class types and pass it over as T type to the following class so I wrote this:

public class ArraySettingsOperationFilter<T> where T : IOperationFilter, new()
{
    public void Apply(OpenApiOperation operation, OperationFilterContext context)
    {
        if (operation.RequestBody != null && operation.RequestBody.Content.TryGetValue("multipart/form-data", out var openApiMediaType))
        {
            var options = new JsonSerializerOptions { WriteIndented = true };
            var array = new OpenApiArray
         {
        new OpenApiString(JsonSerializer.Serialize(new T(),options)),
         };
            openApiMediaType.Schema.Properties["Competences"].Example = array;
        }
    }
}

on startup class i'm getting every class that is marked by an interface and pass them over to operationfilter within Swagger generator settings:

var types = typeof(IMarkerInterface)
            .Assembly.GetTypes()
            .Where(t => t.IsSubclassOf(typeof(IMarkerInterface)) && !t.IsAbstract)
            .Select(t => Activator.CreateInstance(t) as IMarkerInterface).ToList();

then:

services.AddSwaggerGen(c =>
{
    foreach (var type in types)
    {
    c.OperationFilter<ArraySettingsOperationFilter<type>>(); //however here error says type is a variable but used like a type.
    }      
});

Anyone has a clue how to fix this issue? Or a workaround?

Answer 1

Well, c.OperationFilter<T> is just a generic extension method that adds FilterDescriptors to OperationFilterDescriptors list. So you don't have to rely on that generic method. A few modifications like the below will get you there:

First change your generic ArraySettingsOperationFilter to a class that accepts a type in the constructor

public class ArraySettingsOperationFilter : IOperationFilter
{
    private readonly Type type;

    public ArraySettingsOperationFilter(Type type)
    { 
        // you can check this type implements the interface here
        this.type = type;
    }

    public void Apply(OpenApiOperation operation, OperationFilterContext context)
    {
        if (operation.RequestBody != null && operation.RequestBody.Content.TryGetValue("multipart/form-data", out var openApiMediaType))
        {
            var options = new JsonSerializerOptions { WriteIndented = true };
            var array = new OpenApiArray
         {
        new OpenApiString(JsonSerializer.Serialize(Activator.CreateInstance(this.type),options)),
         };
            openApiMediaType.Schema.Properties["Competences"].Example = array;
        }
    }
}

Make FilterDescriptors from the marked classes

 var filterDescriptors = typeof(IMarkerInterface)
        .Assembly.GetTypes()
        .Where(t => t.IsSubclassOf(typeof(IMarkerInterface)) && !t.IsAbstract)
        .Select(r => new FilterDescriptor { Type = typeof(ArraySettingsOperationFilter), Arguments = new object[] { r } })
        .ToArray();
// Arguments array will be used for instantiating ArraySettingsOperationFilter

Configure Swaggergen

   builder.Services.AddSwaggerGen(c => c.OperationFilterDescriptors.AddRange(filterDescriptors));

Edit

You may skip instantiating ArraySettingsOperationFilter if your marked types do the same thing.

   var filterDescriptors = typeof(IMarkerInterface)
       .Assembly.GetTypes()
       .Where(t => t.IsSubclassOf(typeof(IMarkerInterface)) && !t.IsAbstract)
       .Select(r => new FilterDescriptor { Type = r } )
       .ToArray();