Sample showing an EF Core QueryFilter use case of soft delete, tenant id and toggling one or both on/off.

MyContext.Filters.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using Microsoft.EntityFrameworkCore;
using Remotion.Linq.Parsing.ExpressionVisitors;

namespace MyApp
{
    // Dynamic query filters are currently pretty limited in EF Core 2.0.
    // They require the filter value to be a field of the DbContext class.
    // Hence the partial class, to split up the filter logic in at least a
    // seperate file.
    // See: https://github.com/aspnet/EntityFrameworkCore/issues/10274
    public abstract partial class MyContext
    {
        private bool filtersDisabled = false;

        private int tenantId;
        private bool tenantIdEnabled = true;

        private bool softDeleteEnabled = true;

        // Called by OnModelCreating()
        private void ApplyQueryFilters(ModelBuilder modelBuilder)
        {
            var clrTypes = modelBuilder.Model.GetEntityTypes().Select(et => et.ClrType).ToList();

            var baseFilter = (Expression<Func<IEntity, bool>>) (e => filtersDisabled);
            var tenantFilter = (Expression<Func<ITenantEntity, bool>>) (e => !tenantIdEnabled || e.tenantId == tenantId);
            var softDeleteFilter = (Expression<Func<ISoftDeletableEntity, bool>>) (e => !softDeleteEnabled || e.IsDeleted == false);

            // Apply base + tenant + softdelete
            var bothTypes = clrTypes
                .Where(t => typeof(ITenantEntity).IsAssignableFrom(t) &&
                            typeof(ISoftDeletableEntity).IsAssignableFrom(t))
                .ToList();
            for (var i = bothTypes.Count - 1; i >= 0; i--)
            {
                var type = bothTypes[i];
                var filter = CombineQueryFilters(type, baseFilter, new LambdaExpression[] { tenantFilter, softDeleteFilter });
                modelBuilder.Entity(type).HasQueryFilter(filter);
                clrTypes.Remove(type); // Remove, or query filter will be overridden by code below.
            }

            // Apply base + tenant
            var administrationTypes = clrTypes.Where(t => typeof(ITenantEntity).IsAssignableFrom(t));
            foreach (var type in administrationTypes)
            {
                var filter = CombineQueryFilters(type, baseFilter, new LambdaExpression[] { tenantFilter });
                modelBuilder.Entity(type).HasQueryFilter(filter);
            }

            // Apply base + softdelete
            var softDeleteTypes = clrTypes.Where(t => typeof(ISoftDeletableEntity).IsAssignableFrom(t));
            foreach (var type in softDeleteTypes)
            {
                var filter = CombineQueryFilters(type, baseFilter, new LambdaExpression[] { softDeleteFilter });
                modelBuilder.Entity(type).HasQueryFilter(filter);
            }
        }

        // Called by SaveChanges(Async)()
        private void ApplySaveFilters()
        {
            // TODO
        }
        
        // TODO: 
        // - Method to toggle filter bool fields.
        // - Method to set tenantId field.

        // EFCore currently has 2 limitations:
        //
        // - In Expression<Func<TEntity, bool>>, TEntity has to be to 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. 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>>) (e => 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);
        }
    }
}

var administrationFilter = (Expression<Func<ITenantEntity, bool>>) (e => tenantIdEnabled && e.TenantId == tenantId);
var softDeleteFilter = (Expression<Func<ISoftDeletableEntity, bool>>) (e => softDeleteEnabled && e.IsDeleted == false);

this code is correct :

var administrationFilter = (Expression<Func<ITenantEntity, bool>>) (e => !tenantIdEnabled || (tenantIdEnabled && e.TenantId == tenantId));
var softDeleteFilter = (Expression<Func<ISoftDeletableEntity, bool>>) (e => !softDeleteEnabled || (softDeleteEnabled && e.IsDeleted == false));

@akinix Thanks! I didn't see your message (got no notification), but I added the fixes. There were some other bugs, like the global filter toggle, that didn't work correctly either, which I also fixed.