问题

我需要能够通过表达式引用对象的属性，我是这样做的：

Expression<Func<MyFirstModel, object>> MyFirstModelIdProperty = x => x.Id;

Intellisense 允许我这样做，但我不确定这是否是正确的声明。

现在，我正在尝试在 LINQ 表达式中实际使用属性引用：

int id = 500;
myFirstModelDataSet.SingleOrDefault(x => MyFirstModelIdProperty.Compile()(x) == id);

显然，这甚至无法编译，但我不确定该怎么做。

最终，我还将不得不支持这种情况：

Expression<Func<MyFirstModel, object>> MyFirstModelIdProperty = x => x.Id;
Expression<Func<MySecondModel, object>> MySecondModelIdProperty = x => x.TheId;

MySecondModel secondModel = new MySecondModel() { TheId = 55 };
myFirstModelDataSet.SingleOrDefault(x => MyFirstModelIdProperty.Compile()(x) == MySecondModelIdProperty.Compile()(secondModel));

最终目标是希望用户能够告诉我的项目如何通过“id”查询 DbSet，而无需明确指定属性（通过字符串）。

Expression<Func<MyFirstModel, object>> MyFirstModelIdProperty = x => x.Id;

int id = 500;
myFirstModelDataSet.SingleOrDefault(x=> MyFirstModelIdProperty == id); //??

Expression<Func<MyFirstModel, object>> MyFirstModelIdProperty = x => x.Id;
Expression<Func<MySecondModel, object>> MySecondModelIdProperty = x => x.TheId;

MySecondModel secondModel = new MySecondModel() { TheId = 55 };
myFirstModelDataSet.SingleOrDefault(x=> MyFirstModelIdProperty == MySecondModelIdProperty); //??

答案1

得分: 2

假设您正在使用`IQueryable`（否则只需编译表达式），最简单的方法是使用[LINQKit][1]：

```csharp
myFirstModelDataSet.AsExpandable().SingleOrDefault(x => MyFirstModelIdProperty.Invoke(x) == (object)id);

否则，您将需要自己执行一些表达式树编译：

int someId = 42;
var equalExp = Expression.Equal(MyFirstModelIdProperty.Body, Expression.Convert(Expression.Constant(someId), typeof(object)));
var expression = Expression.Lambda<Func<Person, bool>>(equalExp, MyFirstModelIdProperty.Parameters);
myFirstModelDataSet.SingleOrDefault(expression);

对于第二个，您需要执行参数替换，可以使用ReplacingExpressionVisitor 轻松完成。大致如下：

var secondOp = new ReplacingExpressionVisitor(MySecondModelIdProperty.Parameters, MyFirstModelIdProperty.Parameters)
        .Visit(MySecondModelIdProperty.Body);

var equalExp = Expression.Equal(MyFirstModelIdProperty.Body, secondOp);

<details>
<summary>英文:</summary>

Assuming your are working with `IQueryable`(otherwise just compile expression) - the easiest option would be to use [LINQKit][1]:

```csharp
myFirstModelDataSet.AsExpandable().SingleOrDefault(x=&gt; MyFirstModelIdProperty.Invoke(x) == (object)id);

Otherwise you will need to perform some expression tree compilation yourself:

int someId = 42;
var equalExp = Expression.Equal(MyFirstModelIdProperty.Body, Expression.Convert(Expression.Constant(someId), typeof(object)));
var expression = Expression.Lambda<Func<Person, bool>>(equalExp, MyFirstModelIdProperty.Parameters);
myFirstModelDataSet.SingleOrDefault(expression);

For the second one you will need to perform parameter replacement which can easily be done with ReplacingExpressionVisitor. Something along these lines:

var secondOp = new ReplacingExpressionVisitor(MySecondModelIdProperty .Parameters, MyFirstModelIdProperty.Parameters)
        .Visit(MySecondModelIdProperty.Body);

var equalExp = Expression.Equal(MyFirstModelIdProperty.Body, secondOp);