ASP.NET OpenApi SwaggerUI default version

ApiVersionRouteParameterOperationFilter.cs

// inspired by https://stackoverflow.com/questions/69899771/add-default-value-to-swagger-path-parameters
// this filter adds the latest controller or action version to the swagger ui parameters
// so when [Route("api/v{version:apiVersion}/[controller]")] 
// in swaggerUI the version will be an additional required parameter to fill in.
// This filter will add a default value for the latest 

internal class ApiVersionRouteParameterOperationFilter : IOperationFilter
{
	internal static IEnumerable<T> GetControllerAndActionAttributes<T>( OperationFilterContext context ) where T : Attribute
	{
		var controllerAttributes = context.MethodInfo.DeclaringType.GetTypeInfo().GetCustomAttributes<T>();
		var actionAttributes = context.MethodInfo.GetCustomAttributes<T>();

		return controllerAttributes.Concat(actionAttributes);
	}

	public void Apply( OpenApiOperation operation, OperationFilterContext context )
	{
		var parameter = operation.Parameters.FirstOrDefault(p => 
			p.In == ParameterLocation.Path && p.Name.Equals(VERSION_ROUTE_NAME));
		if (parameter is null)
			return;

		var apiattrs = GetControllerAndActionAttributes<ApiVersionAttribute>(context);
		parameter.Schema.Default = new OpenApiString(apiattrs.Max(a => a.Versions.Single()).ToString());
		parameter.Description += "\nexplicit versions:" + string.Join(';',apiattrs.Select(a => a.Versions.Single()));
	}
}