DesignTimeQueryMiddleware.cs

using Microsoft.AspNetCore.Http.Extensions;

namespace XmlDataSourceProxy;

/// <summary>
/// Snoops for the special query-string parameter <c>_query</c> that, when
/// present, switches the proxy into "design-time" mode: the response will be
/// a generated SSRS &lt;Query&gt; document with type-inferred fields,
/// instead of the actual data. The parameter is stripped from the request
/// before YARP forwards it upstream, so it never reaches the API.
///
/// Truthy values: "1", "true", "yes", "on" (case-insensitive). Bare
/// presence (e.g. "?_query") also counts.
/// </summary>
public class DesignTimeQueryMiddleware
{
    public const string FlagItemKey = "XmlDataSourceProxy.GenerateQuery";

    private const string TriggerName = "_query";

    private readonly RequestDelegate _next;

    public DesignTimeQueryMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        if (context.Request.Query.TryGetValue(TriggerName, out var rawValue) &&
            IsTruthy(rawValue.ToString()))
        {
            context.Items[FlagItemKey] = true;

            // Rebuild the query string without the trigger param so YARP
            // (and the upstream) never see it.
            var qb = new QueryBuilder();
            foreach (var kvp in context.Request.Query)
            {
                if (string.Equals(kvp.Key, TriggerName, StringComparison.OrdinalIgnoreCase)) continue;
                foreach (var value in kvp.Value)
                {
                    qb.Add(kvp.Key, value ?? string.Empty);
                }
            }
            context.Request.QueryString = qb.ToQueryString();
        }

        await _next(context);
    }

    private static bool IsTruthy(string? value)
    {
        if (string.IsNullOrEmpty(value)) return true; // bare ?_query
        return value.Equals("1", StringComparison.OrdinalIgnoreCase)
            || value.Equals("true", StringComparison.OrdinalIgnoreCase)
            || value.Equals("yes", StringComparison.OrdinalIgnoreCase)
            || value.Equals("on", StringComparison.OrdinalIgnoreCase);
    }
}