Question

我想创建一个动态lambda表达式进行过滤。

我搜索了一点，但找不到有用的东西用于儿童收藏。那么如何创建这样的表达式？

Expression<Func<ORDERS, bool>> filter1 = c => c.ORDER_DETAILS.Any(x => x.PRODUCTS.HEADING.Contains("foo"));

PS：我问了类似的问题，但我没有得到正确答案。如果我没错的话，我决定从这个方向走。

有关我的问题的更多信息：（How to filter child collection with linq dynamic）

我正在尝试过滤用户请求的结果。比如你有   订单和订单详情和产品是儿童收藏。

当用户想要按产品过滤时，我会收到错误   “ICollection1”中没有属性或字段“产品”

我正在写这样的查询。

var orders = _uow.Repository（）       .Query（）       。哪里（ “PRODUCTS.HEADING.ToLower（）。包含（\” 福\ “）”）       .INCLUDE（ “ORDER_DETAILS”）       .INCLUDE（ “ORDER_DETAILS.PRODUCTS”）       .ToList（）;所以不可能像这样过滤子集合吗？或者任何方式来过滤？

Answer 1

Yes, you can. One approach I've used uses an object which is the same as your return type as a search filter. So if you want to search for the customer name of "Bill", then you set Order.Customer.Name to Bill. Passing that object to a method then applies all applicable searches.

To do this, start by defining a list of searchable fields:

Field<Order>[] Fields;

Fill these by declaring new fields:

var newField = new Field<Order>(o => o.Customer.Name, true, "Customer Name");

The "true" parameter means that it will act as the sort field for the results.

The Field object contains enough information to generate expressions later. It will look like this:

public class Field<T>
{
    public Field(Expression<Func<T, object>> field, bool sortField = false, string displayName = null)
    {
        //get & validate member
        MemberExp = field.Body is UnaryExpression ? ((UnaryExpression)field.Body).Operand as MemberExpression
                                                  : (MemberExpression)field.Body;

        Field = MemberExp?.Member;
        if (Field == null) throw new ArgumentException("Field expression is not a member.");

        //set field type
        switch (Field.MemberType)
        {
            case MemberTypes.Property:
                PropertyInfo p = (PropertyInfo)Field;
                FieldType = p.PropertyType;
                break;
            case MemberTypes.Field:
                FieldInfo f = (FieldInfo)Field;
                FieldType = f.FieldType;
                break;
            default:
                throw new Exception("Unsupported member type detected.");
        }

        //store input values
        FieldExpression = field;
        SortField = sortField;
        DisplayName = displayName ?? Field.Name;
    }

    public bool SortField { get; set; }
    public string DisplayName { get; private set; }
    public MemberExpression MemberExp { get; private set; }
    public Expression<Func<T, object>> FieldExpression { get; private set; }
    public Func<T, object> GetValue => FieldExpression.Compile();
    public Type FieldType { get; set; }

    /// <summary>
    /// Gets the full field name, i.e o => o.Customer.CustomerName returns "Customer.CustomerName"
    /// </summary>
    public string UnqualifiedFieldName
    {
        get
        {
            var stringExp = MemberExp.ToString();
            var paramEnd = stringExp.IndexOf('.') + 1;
            return  stringExp.Substring(paramEnd);
        }
    }
}

Once you have defined all the searchable fields, you will call a method to fetch the results of your search based on the search filters (T) you've collected from the user:

//get the results in ascending order, 10 items per page, first page
var results = GetSearchResults(searchFilters, "ASC", 10, 1);

The method will require you to have a queryable collection of data. I'll assume that you have some method such as context.GetCollection() that retrieves your data. The GetSearchResults method will look like this:

//Returns a filtered dataset based on provided search filters
//searchFilters is an object T which contains the search filters entered.
private List<T> GetSearchResults(T searchFilters, string sortDir = "ASC", int pageSize, int currentPage)
{
    IQueryable<T> searchResults = context.GetCollection(); //get your data context here

    var filterExpressions = new List<Expression<Func<T, bool>>>();

    //Add filters
    foreach (var field in Fields)
    {
        //try to get the search value, ignoring null exceptions because it's much harder
        //to check for null objects at multiple levels. Instead the exception tells us there's
        //no search value
        string searchValue = null;
        try 
        {
            searchValue = field.GetValue(searchFilters)?.ToString(); 
        }
        catch (NullReferenceException) { }
        if (string.IsNullOrWhiteSpace(searchValue)) continue;

        //shared expression setup
        ParameterExpression param = field.FieldExpression.Parameters.First();
        Expression left = field.FieldExpression.Body;
        ConstantExpression right = Expression.Constant(searchValue);
        Expression body = null;

        //create expression for strings so we can use "contains" instead of "equals"           
        if (field.FieldType == typeof(string))
        {
            //build the expression body
            MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });                    
            body = Expression.Call(left, method, right);
        }
        else
        {   //handle expression for all other types      
            body = Expression.Equal(left, right);
        }

        //finish expression
        Expression<Func<T, bool>> lambda = Expression.Lambda<Func<T, bool>>(body, param);
        filterExpressions.Add(lambda);
    }

    //apply the expressions
    searchResults = filterExpressions.Aggregate(searchResults, (current, expression) => current.Where(expression));

    //get sort field
    Field<T> sortField = Fields.FirstOrDefault(f => f.SortField);
    searchResults = searchResults.OrderBy($"{sortField.UnqualifiedFieldName} {sortDir}");                                                                         

    // Get the search results
    int count = searchResults.Count();
    int maxPage = count / pageSize;
    if (maxPage * pageSize < count) maxPage++;
    if (currentPage > maxPage) currentPage = maxPage;
    int skip = Math.Max(0, (filters.page - 1) * pageSize);
    int display = Math.Max(0, Math.Min(count - skip, pageSize));
    return searchResults.Skip(skip).Take(display).ToList();
}

This method uses your Field[] array to build expressions for your criteria and apply them to the dataset.

I hope that helps! Let me know if you have any questions.

如何创建像Expression <func <t，bool =“”>

1 个答案: