自定义 LINQ Where 子句实际上不适用 Where

Question

I have a custom Filter Expression where a user can send a dictionary to my api and I will create a custom where clause. I am no expert in writing custom LINQ so if there is a better way then that would be great as well.

All of this works (No errors) but it doesn't apply my filtering thus returning all it can find)

My Repository call where I append my custom filter (Look at the "FilterByString" area). _entities is a DbSet

// This is in my contructor
private readonly DbSet<T2> _entities;
_entities = _context.Set<T2>();

public async Task<BasePagedResponse<T1>> PagedList(Guid companyId, int pageNumber, int pageSize, Dictionary<string, string> filterParameters, Expression<Func<T2, bool>> predicate, Func<IQueryable<T2>, IIncludableQueryable<T2, object>> including = null)
    {
        try
        {
            var modelList = await _entities
                                    .AsQueryable()
                                    .Where(predicate)
                                    .CustomInclude(including)
                                    .FilterByString(filterParameters)
                                    .Skip((pageNumber - 1) * pageSize)
                                    .Take(pageSize).ToListAsync();

            var mappedList = _mapper.Map<List<T1>>(modelList);
            var total = await GetTotalRecords(companyId);
            var pagedResult = new PagedResult<T1>(mappedList, total, pageNumber, pageSize);
            
            return new BasePagedResponse<T1>(pagedResult, true, null);
        }
        catch (SqlException ex)
        {
            return new BasePagedResponse<T1>(null, false, new[] { new Error(GlobalVariables.error_list, $"Error listing models-({typeof(T1).Name}). {ex.Message}") });
        }
    }

My static SQL Helper that takes the source query and appends my custom Where Clause

 public static IQueryable<T> FilterByString<T>(this IQueryable<T> source, Dictionary<string, string> filterParameters = null)
    {
        if (filterParameters != null)
        {
            foreach (var keyValuePair in filterParameters)
            {
                source.Where(Filter<T>(keyValuePair.Key, keyValuePair.Value));
            }
        }

        return source;
    }

My custom "Filter" (Looks like a lot but it just looks for types of string, bool or int to filter on)

private static Expression<Func<T, bool>> Filter<T>(string propertyName, string queryText)
    {
        var parameter = Expression.Parameter(typeof(T), "entity");
        var getter = Expression.Property(parameter, propertyName);
        if (getter.Type == typeof(string))
        {
            var stringContainsMethod = typeof(string).GetMethod("Contains", new[] { typeof(string) });
            var containsCall = Expression.Call(getter, stringContainsMethod,
                Expression.Constant(queryText, typeof(string)));
            return Expression.Lambda<Func<T, bool>>(containsCall, parameter);
        }
        if (getter.Type == typeof(int))
        {
            var stringEqualsMethod = typeof(int).GetMethod("Equals", new[] { typeof(int) });
            var equalsCall = Expression.Call(getter, stringEqualsMethod,
                Expression.Constant(value: Int32.Parse(queryText), typeof(Int32)));
            return Expression.Lambda<Func<T, bool>>(equalsCall, parameter);
        }
        if (getter.Type == typeof(bool))
        {
            var stringEqualsMethod = typeof(bool).GetMethod("Equals", new[] { typeof(bool) });
            var equalsCall = Expression.Call(getter, stringEqualsMethod,
                Expression.Constant(value: bool.Parse(queryText), typeof(bool)));
            return Expression.Lambda<Func<T, bool>>(equalsCall, parameter);
        }

        throw new Exception("No proper type defined for Filter Parameter. Supports only String, Int and Boolean");
    }

Answer 1

It is a decorator API, where .Where(...) returns a query that is composed to include the predicate, but which does not change the original query; you need to retain the result:

foreach (var keyValuePair in filterParameters)
{
    source = source.Where(Filter<T>(keyValuePair.Key, keyValuePair.Value));
    // ^^^^^
}