如何在c#表达式中从属性调用属性

本文关键字：从属性 调用 属性 表达式 | 更新日期: 2023-09-27 18:10:53

我有3个班级

    AutoYearMake{
       int Year { get; set; }
       string Make { get; set; }
            }
    AutoModel{
       IAutoYearMake AutoYearMakeParent { get; set; }
       string Model { get; set; }
            } 
    AutoTrim{
        IAutoModel AutoModelParent { get; set; }
        string Trim { get; set; }
            }

我需要创建一个查询数据库。我怎样才能动态地得到这样一个表达式:

   Expression<Func<AutoTrim, bool>> expression =  expression = t => t.AutoModelParent.AutoYearMakeParent.Year == year.Value
                              && t.AutoModelParent.AutoYearMakeParent.Make 
                              == make && t.AutoModelParent.Model == model;

这是我的代码。它不工作。

        ParameterExpression parameter = Expression.Parameter(typeof (AutoTrim), "a");
        MemberExpression yearProp = Expression.Property(parameter, "AutoModelParent.AutoYearMakeParent.Year");
        MemberExpression makeProp= Expression.Property(parameter, "AutoModelParent.AutoYearMakeParent.Make");
        MemberExpression modelProp= Expression.Property(parameter, "AutoModelParent.Model");
        Expression right = Expression.Constant(2014);
        Expression e1 = Expression.Equal(yearProp, right);
        right = Expression.Constant("make");
        Expression e2 = Expression.Equal(makeProp, right);
        right = Expression.Constant("model");
        Expression e3 = Expression.Equal(modelProp, right);
        Expression predicateBody = Expression.AndAlso(e1, e2);
        Expression final = Expression.AndAlso(e1, e2);

如何解决这个问题?我试着用Expression.Call。

如何在c#表达式中从属性调用属性

Expression.Property()方法的第一个参数是应该访问属性的对象。如果你想从参数t访问属性AutoModelParent，你可以使用:

Expression.Property(parameter, "AutoModelParent")

所以，如果你想访问参数t的AutoModelParent属性的Model属性你可以输入:

Expression.Property(Expression.Property(parameter, "AutoModelParent"), "Model")

考虑这个解决方案:

private static MemberExpression GetPropertyPathAccessor(Expression parameter, string path)
{
    return (MemberExpression) path.Split('.').Aggregate(parameter, Expression.Property);
}

或者如果你不喜欢一行字

private static MemberExpression GetPropertyPathAccessor(Expression parameter, string path)
{
    Expression current = parameter;
    foreach (var propertyName in path.Split('.'))
    {
        current = Expression.Property(current, propertyName);
    }
    return (MemberExpression)current;
}

那么你可以使用:

MemberExpression yearProp = GetPropertyPathAccessor(parameter, "AutoModelParent.AutoYearMakeParent.Year");

这个怎么样?

在expFunc中有了Func<>之后，你可以马上使用它。

var argParam = Expression.Parameter(typeof(AutoTrim), "s");
var expFunc = Expression.Lambda<Func<AutoTrim, bool>>(
        Expression.AndAlso(
            Expression.AndAlso(
                Expression.Equal(
                    Expression.Property(Expression.Property(Expression.Property(argParam, "AutoModelParent"), "AutoYearMakeParent"), "Year"), 
                    Expression.Constant(year.Value)),
                Expression.Equal(
                    Expression.Property(Expression.Property(Expression.Property(argParam, "AutoModelParent"), "AutoYearMakeParent"), "Make"),
                    Expression.Constant(make))
            ),
            Expression.Equal(
                Expression.Property(Expression.Property(argParam, "AutoModelParent"), "Model"), 
                Expression.Constant(model))
        ),
        argParam
    ).Compile();

当然你也需要这些(只是示例值):

int? year = 2000;
string make = "BMW";
string model = "6";
相关文章：