SignalR/SignalR · error · ArgumentNullException

serializer

Error message

serializer

What it means

JsonSerializerExtensions.Parse<T>(string) is an extension method on JsonSerializer; C# extension methods are invoked as static calls, so a null receiver does not NRE at the call site — the method's explicit guard is what catches it. It then opens a StringReader and deserializes, requiring a real serializer.

Source

Thrown at src/Microsoft.AspNet.SignalR.Core/Json/JsonSerializerExtensions.cs:29

namespace Microsoft.AspNet.SignalR.Json
{
    /// <summary>
    /// Extensions for <see cref="JsonSerializer"/>.
    /// </summary>
    public static class JsonSerializerExtensions
    {
        /// <summary>
        /// Deserializes the JSON to a .NET object.
        /// </summary>
        /// <param name="serializer">The serializer</param>
        /// <typeparam name="T">The <see cref="System.Type"/> of object being deserialized.</typeparam>
        /// <param name="json">The JSON to deserialize</param>
        /// <returns>The deserialized object from the JSON string.</returns>
        public static T Parse<T>(this JsonSerializer serializer, string json)
        {
            if (serializer == null)
            {
                throw new ArgumentNullException("serializer");
            }

            using (var reader = new StringReader(json))
            {
                return (T)serializer.Deserialize(reader, typeof(T));
            }
        }

        /// <summary>
        /// Deserializes the JSON to a .NET object.
        /// </summary>
        /// <param name="serializer">The serializer</param>
        /// <typeparam name="T">The <see cref="System.Type"/> of object being deserialized.</typeparam>
        /// <param name="jsonBuffer">The JSON buffer to deserialize</param>
        /// <param name="encoding">The encoding to use.</param>
        /// <returns>The deserialized object from the JSON string.</returns>
        public static T Parse<T>(this JsonSerializer serializer, ArraySegment<byte> jsonBuffer, Encoding encoding)
        {

View on GitHub (pinned to 693053b89a)

Solutions

1. Ensure the JsonSerializer is constructed/resolved before calling Parse.
2. If resolving from the dependency resolver, verify resolver.Resolve<JsonSerializer>() is non-null.
3. Initialize the serializer field eagerly (e.g. in the constructor) rather than lazily without a null check.

Example fix

// before
JsonSerializer _serializer; // never assigned
var msg = _serializer.Parse<Message>(json); // throws ArgumentNullException

// after
_serializer = new JsonSerializer();
var msg = _serializer.Parse<Message>(json);

if (serializer == null)
    throw new InvalidOperationException("JsonSerializer was not initialized.");
var value = serializer.Parse<T>(json);

When it happens

Trigger: Calling serializer.Parse<T>(json) where the `serializer` variable is null. Because it is extension-method syntax, the null slips past the compiler.

Common situations: A resolver returning a null JsonSerializer; a service field that was never initialized; a JsonSerializer obtained via a factory that returned null under an error condition.

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AI-assisted analysis of SignalR/SignalR@693053b89a (2026-08-13). Data as JSON: /api/errors/cd1229a9c4b1687a. Report an issue: GitHub.