使用反射在C#中创建动态LINQ语句 [英] Use reflection to make dynamic LINQ statements in C#

问题描述

如果我有类似LINQ的语句

If I have a LINQ statement like

x = Table.SingleOrDefault(o => o.id == 1).o.name;

如何使用反射使用传入的变量替换"id"和"name"?尝试时，我一直在获取未将对象引用设置为对象错误实例的信息.我的尝试是这样的

how can I replace "id" and "name" with passed in variables using reflection? I keep getting object reference not set to instance of an object errors when I try. My attempts are things like

x = (string)Table.SingleOrDefault(o => (int?)o.GetType().GetProperty(idString)
.GetValue(o, null) == 1).GetType().GetField(nameString).GetValue(x);

任何帮助都会很棒.谢谢.

Any help would be great. Thanks.

推荐答案

您应该使用表达式树而不是反射树.它将执行得更好，并且您将可以将其与LINQ to Objects和LINQ to SQL/Entities一起使用.

You should use Expression Trees instead of reflection. It will perform better, and you'll be able to use it with both LINQ to Objects and LINQ to SQL/Entities.

var source = new List<Test> { new Test { Id = 1, Name = "FirsT" }, new Test { Id = 2, Name = "Second" } };
var idName = "Id";
var idValue = 1;

var param = Expression.Parameter(typeof(Test));
var condition =
    Expression.Lambda<Func<Test, bool>>(
        Expression.Equal(
            Expression.Property(param, idName),
            Expression.Constant(idValue, typeof(int))
        ),
        param
    ).Compile(); // for LINQ to SQl/Entities skip Compile() call

var item = source.SingleOrDefault(condition);

然后，您可以使用反射获得Name属性(只需执行一次，因此可以使用反射来实现.

then, you can get Name property using reflection (you'll do it just once, so it's fine to do it using reflection.

var nameName = "Name";
var name = item == null ? null : (string) typeof(Test).GetProperty(nameName).GetValue(item);

Test类定义为

public class Test
{
    public int Id { get; set; }
    public string Name { get; set; }
}

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