LINQ：Expression.Call（typeof（Queryable），“选择”

Question

我正在尝试创建一个小型“automapper-esq”实用程序，它将采用 LinqToSql 实体并将其映射到“投影类”。

到目前为止，我有这样的东西：

class Entity
{
    public int ID { get; set; }
    public string WantedProperty { get; set; }
    public string UnWantedPropertyData { get; set; }
    ...More Unwanted Properties...
    public IEnumerable<ChildEntity> ChildEntities { get; set; }
}

class EntityProjection
{
    public int ID { get; set; }
    public string WantedProperty { get; set; }
    public IEnumerable<ChildEntityProjection> ChildEntities { get; set; }
}

class ChildEntityProjection
{
    public int ID { get; set; }
    public string WantedProperty { get; set; }
    public string UnWantedPropertyData { get; set; }
    ...More Unwanted Properties...
}


var results = context.Table.Select(ProjectionHelper.BuildProjection<Entity,EntityProjection>());

其中 BuildProjection 返回：

Expression<Func<TSource, TResult>>

本质上创建了一个像这样的 lambda：

A => new EntityProjection() { ID = A.ID, WantedProperty = A.WantedProperty }

现在棘手的部分......我也希望能够投影“父”实体的关联属性。本质上我需要的是得到这样的东西：

A => new EntityProjection() {
  ID = A.ID,
  WantedProperty = A.WantedProperty,
  ChildEntities = A.ChildEntities.Select(B => new ChildEntityProjection {
    ID = B.ID,
    WantedProperty = B.WantedProperty
  }
}

我已经得到了这部分：

A => new EntityProjection() {
  ID = A.ID,
  WantedProperty = A.WantedProperty,
  ChildEntities = System.Collections.Generic.List1[ChildEntity].Select(B => new ChildEntityProjection {
    ID = B.ID,
    WantedProperty = B.WantedProperty
  }
}

通过这样做：

IQueryable<ChildEntity> list = new List<ChildEtity>().AsQueryable();
Expression _selectExpression = Expression.Call(
  typeof(Queryable),
  "Select",
  new Type[] { typeof(ChildEntity), typeof(ChildEntityProjection) },
  Expression.Constant(list),
  _nestedLambda);

这是我目前陷入困境的地方......在尝试替换 Expression.Constant 时我有点困惑(list) 与表示属性的实际数据类型的其他表达式，以便“System.Collections.Generic.List1[ChildEntity].Select(B=>...”将替换为“A.ChildEntities.Select( B=>...”

有什么想法吗？

Answer 1

我一直在寻找如何使用表达式（正确的术语？）来做到这一点，并且我最终确实弄清楚了。

我必须将其更改

IQueryable<ChildEntity> list = new List<ChildEtity>().AsQueryable(); 
Expression _selectExpression = Expression.Call( 
  typeof(Queryable), 
  "Select", 
  new Type[] { typeof(ChildEntity), typeof(ChildEntityProjection) }, 
  Expression.Constant(list), 
  _nestedLambda); 

为：

MethodInfo selectMethod = null;
foreach (MethodInfo m in typeof(Enumerable).GetMethods().Where(m => m.Name == "Select"))
  foreach (ParameterInfo p in m.GetParameters().Where(p => p.Name.Equals("selector")))
    if (p.ParameterType.GetGenericArguments().Count() == 2)
      selectMethod = (MethodInfo)p.Member;

var _selectExpression = Expression.Call(
  null,
  selectMethod.MakeGenericMethod(new Type[] { typeof(ChildEntity), typeof(ChildEntityProjection) }),
  new Expression[] { _myPropertyExpression, _myFuncExpression });

希望这可以帮助其他人......

Answer 2

这是建议的查询。您可能想更改您的 IEnumerable<>在 EntityProjection to a List<> 中以避免延迟加载（如果重要的话......）。无论如何，我希望这会有所帮助。

EntityProjection projection = context.Table
    .Where(entity => entity.ID == 123)
    .Select(entity => new EntityProjection()
    {
        ID = entity.ID,
        WantedProperty = entity.WantedProperty,
        ChildEntities = entity.ChildEntities
            .Select(child => new ChildEntityProjection()
            {
                ID = child.EmployeeID,
                WantedProperty = child.WantedProperty
            })
            .ToList()
    })
    .SingleOrDefault();