Lambda表达式访问对象的属性,该对象是c#中另一个对象的属性

Question

我有这两个班:

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; }
//...
}

并希望以Contratos.entidadeFinanceira.Nome为基础过滤Contratos列表.这是根据用户选择的属性过滤列表的方法的一部分.

    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;
    }

当方法执行时抛出异常 'entidadeFinanceira.Nome'不是该行的类型'GAcordos.Models.Contratos'的成员

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

但如果我直接写表达式:

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

它工作正常.

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

Answer 1

简单地说,你需要两种用途PropertyOrField.

手动构建,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);

(注意,包括很重要constValType的情况constVal是null,这避免了很多意想不到的问题)