参数类型不匹配 Transforming anonymous Expression<Func<T,U>> to non anonymous Expression<Func<T,U>>

Question

假设我们有三个这样的类：

public class ParentType {
    private ParentType() {}

    public int Id { get; protected set; }
    public SubType Sub { get; protected set; }
}

public class SubType{
    private SubType(){}

    public int Id { get; protected set; }
    public ICollection<ColSubType> ColSubs{get; protected set;}
}

public class ColSubType{
    private ColSubType(){}

    public int Id { get; protected set; }
    public SubType SubType { get; set; }
}

我有一个这样的匿名表达式：

x => new
{
   x.Id,
   Sub = new
   {
      x.Sub.Id,
      ColSubs = x.Sub.ColSubs.Select(u=> new {
             u.Id
      }).ToList()
   }
}

我需要将其转换为非匿名表达式，如下所示：

x => new ParentType()
{
   Id = x.Id,
   Sub = new SubType()
   {
      Id = x.Sub.Id,
      ColSubs = x.Sub.ColSubs.Select(u=> new ColSubs(){
             Id = u.Id
      }).ToList()
   }
}

感谢@IvanStoev 对这个问题的回答：我可以转换简单的表达式，但是当我添加 x.Sub.ColSubs.Select(...) 时，出现以下错误：

System.ArgumentException: Argument types do not match

Answer 1

以下是您需要添加到递归 Transform 方法的代码，该方法恰好处理该场景，即检测 Select 方法，转换 selector 参数，修改 Select 的 TResult 泛型类型参数并用新的 selector 调用它，最后调用 ToList 以防目标不是 IEnumerable<T>:

if (source.Type != type && source is MethodCallExpression call && call.Method.IsStatic
    && call.Method.DeclaringType == typeof(Enumerable) && call.Method.Name == nameof(Enumerable.Select))
{
    var sourceEnumerable = call.Arguments[0];
    var sourceSelector = (LambdaExpression)call.Arguments[1];
    var sourceElementType = sourceSelector.Parameters[0].Type;
    var targetElementType = type.GetGenericArguments()[0];
    var targetSelector = Expression.Lambda(
        Transform(sourceSelector.Body, targetElementType),
        sourceSelector.Parameters);
    var targetMethod = call.Method.GetGenericMethodDefinition()
        .MakeGenericMethod(sourceElementType, targetElementType);
    var result = Expression.Call(targetMethod, sourceEnumerable, targetSelector);
    if (type.IsAssignableFrom(result.Type)) return result;
    return Expression.Call(
        typeof(Enumerable), nameof(Enumerable.ToList), new[] { targetElementType },
        result);
}

参数类型不匹配 Transforming anonymous Expression<Func<T,U>> to non anonymous Expression<Func<T,U>>

Argument types do not match Transforming anonymous Expression<Func<T,U>> to non anonymous Expression<Func<T,U>>

c#

linq

lambda

expression

visitor-pattern