Set Value before save

EF Core value generation on add with custom ValueGenerator

Generates values for properties when an entity is added to a context.

could be utilized to assign TenantId to the new entities. Inside the Next method you could obtain the TenantId from the context (or some service).

Taking your sample, the value generator could be a nested class inside your DbContext like this:

class TenantIdValueGenerator : ValueGenerator<int>
{
    public override bool GeneratesTemporaryValues => false;
    public override int Next(EntityEntry entry) => GetTenantId(entry.Context);
    int GetTenantId(DbContext context) => ((YourDbContext)context)._tenantProvider.GetTenantId();
}

The all you need is to assign the generator to TenantId property using some of the HasValueGenerator fluent API.

The only problem is that by design the value generators are called only if the property does not have explicitly set value (for int property - if the value is 0).

So the better approach it to abstract (and fully control) the TenantId property by removing it from entity models and replacing it with shadow property.

Hence my suggestion is, remove the TenantId from entity classes and call the following method inside your OnModelCreating for each entity that needs TenantId column:

void ConfigureTenant<TEntity>(ModelBuilder modelBuilder) where TEntity : class
{
    modelBuilder.Entity<TEntity>(builder =>
    {
        builder.Property<int>("TenantId")
            .HasValueGenerator<TenantIdValueGenerator>();

        builder.HasQueryFilter(e => EF.Property<int>(e, "TenantId") == _tenantProvider.GetTenantId());
    });
}