使用 c# 使用子项进行过滤和排序的动态 Lambda 表达式

Question

你好，我按照 https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/expression-trees/how-to-use-expression-trees-to-build-dynamic-queries 中的指南在我的关系 class 中创建过滤器和排序，我需要使用子列 "Channel"

进行过滤和排序

这是我的基地class

public class MeterTransaction : EviModelBase
{        
    public int TariffDuration { get; set; }
    public decimal? TariffPackageKwh { get; set; }
    public decimal? TariffPackagePrice { get; set; }
    public decimal? TariffRatePerKwh { get; set; }
    public decimal? TariffRateMinFee { get; set; }

    // Meter CreditBalance after transaction
    public decimal CreditBalance { get; set; }
    public DateTimeOffset? CreditExpiration { get; set; }
    [Timestamp]
    public byte[] RowVersion { get; set; }
    public Meter Meter { get; set; }
}

这是仪表 Class

public class Meter : EviModelBase
{
    public MeterVendor Vendor { get; set; }
    public string Model { get; set; }
    public string SerialNumber { get; set; }
    public string Channel { get; set; }

}

这是过滤和排序数据的代码：

public static IQueryable<T> OrderByNew<T>(this IQueryable<T> source, SortModel sortModel)
            {
                    ParameterExpression p = Expression.Parameter(typeof(T), "p");

                    // Construct the nested properties
                    string[] nestedProps = sortModel.ColId.Split('.');
                    Expression mbr = p;
                    for (int i = 0; i < nestedProps.Length; i++)
                            mbr = Expression.PropertyOrField(mbr, nestedProps[i]);

                    LambdaExpression pred = Expression.Lambda(
                            Expression.Equal(
                                    mbr,
                                    Expression.Constant("EVI0000101")
                                    ),
                                    p
                    );

                    var method = string.Equals(sortModel.Sort, "desc", StringComparison.OrdinalIgnoreCase) ?
                            "OrderByDescending" :
                            "OrderBy";
                    var whereCallExpression = Expression.Call(typeof(Queryable), "where", new Type[] { source.ElementType }, source.Expression, pred);
                    var orderByExpression = Expression.Lambda(mbr, p);

                    MethodCallExpression orderByCallExpression = Expression.Call(
                            typeof(Queryable),
                            method,
                            new Type[] { source.ElementType},
                            whereCallExpression,
                            Expression.Quote(orderByExpression));
                    // ***** End OrderBy *****

                    return source.Provider.CreateQuery<T>(orderByCallExpression);
            }
    }

实际上 "whereCallExpression" 正在正常工作并无误地过滤我想要的数据，但排序逻辑出错 "no generic method 'OrderBy' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments."

我怎样才能做到这一点？

问候

Answer 1

您忽略了一个事实，即与 Where 相比，OrderBy 方法有 2 个通用类型参数（TSource 和 TKey）。因此，您需要在生成呼叫时同时提供它们：

var orderByCallExpression = Expression.Call(
    typeof(Queryable),
    method,
    new Type[] { source.ElementType, orderByExpression.Body.Type }, // <--
    whereCallExpression,
    Expression.Quote(orderByExpression));

使用 c# 使用子项进行过滤和排序的动态 Lambda 表达式

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