EntityFramework 6.4.4 - 如何将集合投影到实现 IEnumerable<T> 的自定义对象？

Question

无法映射到实现 IEnumerable 的自定义对象？

public class BranchCollection : IEnumerable<int>
{
      public IEnumerable<int> BranchIds { get; set; } = new List<int>();
      public BranchPermission() { }
      
      //Custom Code here
}
public class UserDto {
      public BranchCollection Collection { get; set; }
}

正在尝试使用以下代码投影到 UserDto

dbContext.Users.Select(
  x => new UserDto {
    Collection = new BranchCollection { BranchIds = x.Branches.Select(y => y.BranchId) }
  }
  .ToList();

异常

A type that implements IEnumerable 'BranchCollection' cannot be initialized in a LINQ to Entities query.

堆栈跟踪

at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.CheckInitializerType(Type type) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MemberInitTranslator.TypedTranslate(ExpressionConverter parent, MemberInitExpression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MemberInitTranslator.TypedTranslate(ExpressionConverter parent, MemberInitExpression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SelectTranslator.Translate(ExpressionConverter parent, MethodCallExpression call) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.Convert() at System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable1 forMergeOption) at System.Data.Entity.Core.Objects.ObjectQuery1.<>c__DisplayClass41_0.b__1() at System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess) at System.Data.Entity.Core.Objects.ObjectQuery1.<>c__DisplayClass41_0.b__0() at System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func1 operation) at System.Data.Entity.Core.Objects.ObjectQuery1.GetResults(Nullable1 forMergeOption) at System.Data.Entity.Core.Objects.ObjectQuery1.<System.Collections.Generic.IEnumerable.GetEnumerator>b__31_0() at System.Data.Entity.Internal.LazyEnumerator1.MoveNext() at System.Collections.Generic.List1..ctor(IEnumerable1 collection) at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source)

Answer 1

EF 不能 understand/initiate 那 class。它旨在适应简单 classes 和集合的投影。如果你觉得你必须这样做，那么 double-project:

var userDetails dbContext.Users.Select(x => new 
{
    BranchIds = x.Branches.Select(b => b.BranchId).ToList()
}).ToList()  // This executes the query getting our branch IDs (and include anything from the User we need as well.
.Select(x => new UserDto
{
    Collection = new BranchCollection { BranchIds = x.BranchIds }
}.ToList(); // Project to your custom type from memory.

对于 class 这样的你想要包装 IEnumerable 的地方，我建议将源集合传递到构造函数中以初始化私有成员并使其不可变，公开你的枚举器等。如果项目可以是 added/removed 然后用你的 class 中的方法来适应它，而不是公开一个包含的集合，以后任何人都可以写

userDto.Collection = new BranchCollection();

...并且可能会搞砸。

EntityFramework 6.4.4 - 如何将集合投影到实现 IEnumerable<T> 的自定义对象？

EntityFramework 6.4.4 - How to project collection to a custom object implementing IEnumerable<T>?

c#

entity-framework

entity-framework-6