CombineQueryFilers

MyDbContext.cs

public class MyDbContext : DbContext
{
    private ITenantIdLocator tenantIdLocator;

    public MyDbContext(ITenantIdLocator tenantIdLocator)
    {
        if (tenantIdLocator == null) throw new ArgumentNullException(nameof(tenantIdLocator));
        this.tenantId = tenantIdLocator.GetTenantId();
    }
    
    // Leave these as fields, because else the filters won't work correctly.
    // If you want to change them, make a method which changes the value.
    private bool filtersDisabled = false;
    private bool tenantFilterEnabled  = true;
    private bool softDeleteFilterEnabled = true;
    private int tenantId = 0;

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        ApplyQueryFilters(modelBuilder);
    }

    private void ApplyQueryFilters(ModelBuilder modelBuilder)
    {
        var clrTypes = modelBuilder.Model.GetEntityTypes().Select(et => et.ClrType).ToList();
        
        var baseFilter = (Expression<Func<IEntity, bool>>) (_ => filtersDisabled);
        var tenantFilter = (Expression<Func<IMultiTenantEntity, bool>>) (e => !tenantFilterEnabled || e.TenantId == tenantId);
        var softDeleteFilter = (Expression<Func<ISoftDeletableEntity, bool>>) (e => !softDeleteFilterEnabled || e.IsDeleted == false);

        foreach (var type in clrTypes)
        {
            var filters = new List<LambdaExpression>();

            if (typeof(IMultiTenantEntity).IsAssignableFrom(type))
                filters.Add(tenantFilter);
            if (typeof(ISoftDeletableEntity).IsAssignableFrom(type))
                filters.Add(softDeleteFilter);

            var queryFilter = CombineQueryFilters(type, baseFilter, filters);
            modelBuilder.Entity(type).HasQueryFilter(queryFilter);
        }
    }

    // EFCore currently has 2 limitations:
    //
    // - In Expression<Func<TEntity, bool>>, TEntity has to be the final entity type and cannot
    //   be, for example, the interface type. To work around it, we change the type in the expression
    //   with ReplacingExpressionVisitor. See: https://github.com/aspnet/EntityFrameworkCore/issues/10257
    //
    // - Only 1 HasQueryFilter() call is supported per entity type. The last one will overwrite each call that
    //   came before it. To work around this, we combine the multiple query filters in a single expression.
    //   See: https://github.com/aspnet/EntityFrameworkCore/issues/10275
    private LambdaExpression CombineQueryFilters(Type entityType, LambdaExpression baseFilter, IEnumerable<LambdaExpression> andAlsoExpressions)
    {
        var newParam = Expression.Parameter(entityType);

        var andAlsoExprBase = (Expression<Func<IEntity, bool>>) (_ => true);
        var andAlsoExpr = ReplacingExpressionVisitor.Replace(andAlsoExprBase.Parameters.Single(), newParam, andAlsoExprBase.Body);
        foreach (var expressionBase in andAlsoExpressions)
        {
            var expression = ReplacingExpressionVisitor.Replace(expressionBase.Parameters.Single(), newParam, expressionBase.Body);
            andAlsoExpr = Expression.AndAlso(andAlsoExpr, expression);
        }

        var baseExp = ReplacingExpressionVisitor.Replace(baseFilter.Parameters.Single(), newParam, baseFilter.Body);
        var exp = Expression.OrElse(baseExp, andAlsoExpr);

        return Expression.Lambda(exp, newParam);
    }
}

@nphmuller oh well yes. :D