Lambda表达式可访问对象的属性，该属性是c#中另一个对象的属性 [英] Lambda expression to access a property of an object that is property of another object in c#

问题描述

我有这两节课:

public class Contratos
{
//...
    public int EntidadeFinanceiraId { get; set; }
   [Column("Nome")]
    public EntidadesFinanceiras entidadeFinanceira { get; set; }
//...
}

public class EntidadesFinanceiras
{
    [Key]
    public int ID { get; set; }
    public string Nome { get; set; }
//...
}

，并希望根据Contratos.entidadeFinanceira.Nome来动态过滤"Contratos列表".这是一种方法的一部分，该方法可根据用户选择的属性来过滤列表.

and want to dinamically filter a List of Contratos based on Contratos.entidadeFinanceira.Nome. This is part of a Method that filters the list based on a property selected by the user.

    public IQueryable<Models.Contratos> applyLambdaFilter(string val, string col, string oper, IQueryable<Models.Contratos> contratosList)
    {
        if (!string.IsNullOrWhiteSpace(val))
        {
            string typeName;
            string columnName;
            Type propType;
            string[] propName = col.Split(new[] { '.' });
            if (propName.Count() > 1)
            {
                typeName = "GAcordos.Models." + propName[0]; //entidadeFinanceira
                columnName = propName[1]; //Nome
                propType = Type.GetType("GAcordos.Models.Contratos").GetProperty(propName[0]).PropertyType.GetProperty(columnName).PropertyType; //String
            }
            else
            {
                typeName = "GAcordos.Models.Contratos";
                columnName = propName[0]; //Other Contratos property
                propType = Type.GetType(typeName).GetProperty(columnName).PropertyType;
            }
            if (propType != null)
            {
                var fixedItem = Comparators.getFixedItemWithType(val, propType);
                var param = Expression.Parameter(typeof(GAcordos.Models.Contratos), "x");
                var body = Expression.Equal(Expression.PropertyOrField(param, col.ToString()), fixedItem, false, Type.GetType("GAcordos.Helpers.Comparators").GetMethod(oper, new Type[] { propType, propType }));
                var lambda = Expression.Lambda<Func<GAcordos.Models.Contratos, bool>>(body, param);
                contratosList = contratosList.Where(lambda.Compile()).AsQueryable();
            }
        }
        return contratosList;
    }

该方法执行时会引发异常'entidadeFinanceira.Nome'不是'GAcordos.Models.Contratos'类型的成员

When the Method executes it throws an exception 'entidadeFinanceira.Nome' is not a member of type 'GAcordos.Models.Contratos' on the line

var body = Expression.Equal(Expression.PropertyOrField(param, col.ToString()), fixedItem, false, Type.GetType("GAcordos.Helpers.Comparators").GetMethod(oper, new Type[] { propType, propType }));

但是如果我直接写表达式:

But if I write the expression directly:

contratosList = contratosList.Where(x => x.entidadeFinanceira.Nome == val);

它工作正常.

那么，如何构建lambda表达式x => x.property.property == constVal?

So, how can I build the lambda expression x => x.property.property == constVal?

推荐答案

简单来说，您需要使用PropertyOrField的两种用法.

Simply, you need two uses of PropertyOrField.

手动构建的，x => x.Foo.Bar == constVal是:

var param = Expression.Parameter(typeof(ObjectType), "x");
var lambda = Expression.Lambda<Func<ObjectType, bool>>(
    Expression.Equal(
        Expression.PropertyOrField(
           Expression.PropertyOrField(param, "Foo"), "Bar"
        ), Expression.Constant(constVal, constValType)
    ), param);

(请注意，在constVal为null的情况下包含constValType是很重要的；这样可以避免很多意外的问题)

(note that it is important to include constValType in case constVal is null; this avoids a lot of unexpected problems)

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