Alternating between multiple models with the same DbContext type

The model built in OnModelCreating can use a property on the context to change how the model is built. For example, suppose you wanted to configure an entity differently based on some property:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    if (UseIntProperty)
    {
        modelBuilder.Entity<ConfigurableEntity>().Ignore(e => e.StringProperty);
    }
    else
    {
        modelBuilder.Entity<ConfigurableEntity>().Ignore(e => e.IntProperty);
    }
}

Unfortunately, this code wouldn't work as-is, since EF builds the model and runs OnModelCreating only once, caching the result for performance reasons. However, you can hook into the model caching mechanism to make EF aware of the property producing different models.

IModelCacheKeyFactory

EF uses the IModelCacheKeyFactory to generate cache keys for models; by default, EF assumes that for any given context type the model will be the same, so the default implementation of this service returns a key that just contains the context type. To produce different models from the same context type, you need to replace the IModelCacheKeyFactory service with the correct implementation; the generated key will be compared to other model keys using the Equals method, taking into account all the variables that affect the model.

The following implementation takes the UseIntProperty into account when producing a model cache key:

public class DynamicModelCacheKeyFactory : IModelCacheKeyFactory
{
    public object Create(DbContext context)
        => context is DynamicContext dynamicContext
            ? (context.GetType(), dynamicContext.UseIntProperty)
            : (object)context.GetType();
}

Starting with EF Core 6 you have to implement the overload of the Create method that also handles design-time model caching. As in the following example:

public class DynamicModelCacheKeyFactoryDesignTimeSupport : IModelCacheKeyFactory
{
    public object Create(DbContext context, bool designTime)
        => context is DynamicContext dynamicContext
            ? (context.GetType(), dynamicContext.UseIntProperty, designTime)
            : (object)context.GetType();

    public object Create(DbContext context)
        => Create(context, false);
}

Finally, register your new IModelCacheKeyFactory in your context's OnConfiguring:

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    => optionsBuilder
        .UseInMemoryDatabase("DynamicContext")
        .ReplaceService<IModelCacheKeyFactory, DynamicModelCacheKeyFactory>();

See the full sample project for more context.