dotnet/yarp · error · ArgumentException

'{nameof(key)}' cannot be null or empty.

Error message

'{nameof(key)}' cannot be null or empty.

What it means

Thrown by the QueryParameterRouteTransform constructor when `key` is null or empty. The `key` is the query-parameter name to set/append; the base QueryParameterTransform also validates it, but this subclass re-checks to give an explicit ArgumentException before delegating to the base. A blank key produces an invalid query string.

Source

Thrown at src/ReverseProxy/Transforms/QueryParameterFromRouteTransform.cs:15

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;

namespace Yarp.ReverseProxy.Transforms;

public class QueryParameterRouteTransform : QueryParameterTransform
{
    public QueryParameterRouteTransform(QueryStringTransformMode mode, string key, string routeValueKey)
        : base(mode, key)
    {
        if (string.IsNullOrEmpty(key))
        {
            throw new ArgumentException($"'{nameof(key)}' cannot be null or empty.", nameof(key));
        }

        if (string.IsNullOrEmpty(routeValueKey))
        {
            throw new ArgumentException($"'{nameof(routeValueKey)}' cannot be null or empty.", nameof(routeValueKey));
        }

        RouteValueKey = routeValueKey;
    }

    internal string RouteValueKey { get; }

    /// <inheritdoc/>
    protected override string? GetValue(RequestTransformContext context)
    {
        var routeValues = context.HttpContext.Request.RouteValues;
        if (!routeValues.TryGetValue(RouteValueKey, out var value))
        {

View on GitHub (pinned to bd11867bee)

Solutions

1. Provide a non-empty query-parameter name for `key`.
2. If sourced from config, validate the key is present before constructing.
3. Ensure the transform config block has all required keys.

Example fix

// before
new QueryParameterRouteTransform(QueryStringTransformMode.Append, "", "userId");
// after
new QueryParameterRouteTransform(QueryStringTransformMode.Append, "uid", "userId");

ArgumentException.ThrowIfNullOrEmpty(key);

static bool IsValidQueryKey(string? key) => !string.IsNullOrEmpty(key);

When it happens

Trigger: Constructing `new QueryParameterRouteTransform(mode, key, routeValueKey)` with `key` null/empty, or via transform config that omits the query key.

Common situations: Config sets `Set`/`Append` query transform from route but leaves `key` blank. Programmatically building the transform from a config value that defaulted to empty.

Related errors

• '{nameof(routeValueKey)}' cannot be null or empty.
• '{nameof(key)}' cannot be null or empty.
• 'key' cannot be null or empty.
• 'key' cannot be null or empty.
• '{nameof(fromMethod)}' cannot be null or empty.

AI-assisted analysis of dotnet/yarp@bd11867bee (2026-08-13). Data as JSON: /api/errors/1e7f01b5b3404feb. Report an issue: GitHub.