So, you've just upgraded your ABP Framework application to a newer version (like v9.2.0+) and suddenly, your application can't read data from its MongoDB database. You're seeing strange deserialization errors, especially related to Guid types. What's going on?

You've likely run into a classic compatibility issue with the MongoDB .NET driver.

The Problem: Legacy vs. Standard GUIDs

Here's the short version:

• Old MongoDB Drivers (used in older ABP versions) stored Guid values in a format called CSharpLegacy.
• New MongoDB Drivers (v3.0+), now default to a universal Standard format.

When your newly upgraded app tries to read old data, the new driver expects the Standard format but finds CSharpLegacy. The byte orders don't match, and... boom. Deserialization fails.

The ABP Framework team has an excellent official guide covering this topic in detail. We highly recommend reading their MongoDB Driver 2 to 3 Migration Guide for a full understanding.

Our tip below serves as a fast, application-level fix if you need to get your system back online quickly without performing a full data migration.

The Quick Fix: Tell the Driver to Use the Old Format

Instead of changing your data, you can simply tell the new driver to continue using the old CSharpLegacy format for all Guid and Guid? properties. This provides immediate backward compatibility without touching your database.

It’s a simple, two-step process.

Step 1: Create a Custom Convention

First, create this class in your .MongoDb project. It tells the serializer how to handle Guid types.

using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Conventions;
using MongoDB.Bson.Serialization.Serializers;
using System;

public class LegacyGuidConvention : ConventionBase, IMemberMapConvention
{
    public void Apply(BsonMemberMap memberMap)
    {
        if (memberMap.MemberType == typeof(Guid))
        {
            memberMap.SetSerializer(new GuidSerializer(GuidRepresentation.CSharpLegacy));
        }
        else if (memberMap.MemberType == typeof(Guid?))
        {
            var guidSerializer = new GuidSerializer(GuidRepresentation.CSharpLegacy);
            var nullableGuidSerializer = new NullableSerializer<Guid>(guidSerializer);
            memberMap.SetSerializer(nullableGuidSerializer);
        }
    }
}

Step 2: Register the Convention at Startup

Now, register this convention in your YourProjectMongoDbModule.cs file. Add this code to the top of the ConfigureServices method. This ensures your rule is applied globally as soon as the application starts.

using Volo.Abp.Modularity;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Conventions;

public class YourProjectMongoDbModule : AbpModule
{
    public override void ConfigureServices(ServiceConfigurationContext context)
    {
        // Fix Start
        var conventionPack = new ConventionPack { new LegacyGuidConvention() };
        ConventionRegistry.Register(
            "LegacyGuidConvention",
            conventionPack,
            t => true); // Apply to all types
        // Fix End

        // ... Your existing ConfigureServices code
    }
}

An Alternative to Full Data Migration

It's important to note that the method described here is an application-level fix. It's a fantastic alternative to performing a full data migration, which involves writing scripts to convert every legacy GUID in your database.

If you are interested in the more permanent, data-centric approach, the ABP.IO community has a detailed guide on Migrating MongoDB GUIDs from Legacy to Standard Format.

Our quick fix is ideal for getting a system back online fast or when a database migration is too complex. The full migration is better for long-term standards compliance. Choose the path that best fits your project's needs!

That's It!

Restart your application, and the errors should be gone. Your app can now correctly read its old Guid data, and it will continue to write new data in the same legacy format, ensuring consistency.

This approach is a lifesaver for existing projects, saving you from a risky and time-consuming data migration. For brand-new projects, you might consider starting with the Standard representation, but for everything else, this is a clean and effective fix. Happy coding!