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Serialization on an OnAssemblyResolve handler may cause recursion

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This article helps you resolve the problem where the serialization will cause the assembly resolver handler to be called recursively.

Original product version:   .NET Framework 3.5 Service Pack 1, 4.5
Original KB number:   2756498

Symptoms

Assume that you have a class marked with the XmlSerializerAssembly attribute as in the following example:

[Serializable, XmlRoot(ElementName="Bindings", IsNullable=false), XmlSerializerAssembly(AssemblyName="Contoso.ObjectLoaders.Bindings")]
public class Bindings
{
    (...)
}

The generated assembly is signed and placed in Global Assembly Cache (GAC) instead of in the bin folder. You also implement a handler for AppDomain.AssemblyResolve event and you serialize an instance of the class in the assembly resolver handler method as follows:

private static Assembly ResolveAssemblies(object sender, ResolveEventArgs args)
{
    // Some code here
    var serializer = new XmlSerializer(typeof(Bindings));
    // Some more code here
}

In this scenario, the serialization will cause the assembly resolver handler to be called recursively, which may result in a stack overflow.

Cause

This behavior is by design. Assemblies with the XmlSerializerAssembly attribute are loaded with Assembly.LoadWithPartialName(string), which will call the OnResolveAssembly handler again.

Workaround 1: Remove XmlSerializerAssembly attribute

This method will prevent the assembly from being loaded by Assembly.LoadWithPartialName(string).

Workaround 2: Write a more robust Assembly Resolve handler

It's the recommended workaround and will resolve any issues related to recursion. The following example is a simple code template for this workaround. The main idea is to add the already resolved assemblies into a generic list (the concurrent bag was chosen because its thread safe) and return if the assembly is already in the process of being resolved.

static ConcurrentBag<string> listOfAssemblies = new ConcurrentBag<string>();
private static Assembly ResolveAssemblies(object sender, ResolveEventArgs args)
{
    if (listOfAssemblies.Contains(args.Name))
    {
        // Already resolving this assembly, return now
        return null;
    }
    try
    {
        listOfAssemblies.Add(args.Name);
        // Add your handler code here
    }
    finally
    {
        // Assembly was handled, remove from list
        listOfAssemblies.Remove(args.Name);
    }
}

References