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为什么循环中的实体框架核心调用抛出InvalidOperationException?

entity-framework-core postgresql database

Neutrino · 技术社区 · 7 年前

// Arrange.
using(var context = new BasicContext())
{
   Author[] authors = new[]
   {
      new Author { Name = "Bob"},
      new Author { Name = "Fred" }
   };
   context.Authors.AddRange(authors);

   Book[] books = new[]
   {
      new Book { Title = "Book 1" },
      new Book { Title = "Book 2" }
   };

   context.SaveChanges();            
}

// Act.
bool exception = false;

using(var context = new BasicContext())
{
   foreach(Author a in context.Authors)
   {
      try
      {
         string title = context.Books.First().Title;
      }
      catch(Exception)
      {
         exception = true;
      }
   }
}

// Assert.
Assert.False(exception);

使用Postgress的实体连接器(Npgsql)引发的异常为:

Npgsql.NpgsqlOperationInProgressException : A command is already in progress: SELECT "a"."AuthorId", "a"."Name"
FROM "Authors" AS "a"
Stack Trace:
   at Npgsql.NpgsqlConnector.StartUserAction(ConnectorState newState, NpgsqlCommand command)
   at Npgsql.NpgsqlCommand.<ExecuteDbDataReader>d__92.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Runtime.CompilerServices.ValueTaskAwaiter`1.GetResult()
   at Npgsql.NpgsqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
   at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.Execute(IRelationalConnection connection, String execute
Method, IReadOnlyDictionary`2 parameterValues, Boolean closeConnection)
   at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.ExecuteReader(IRelationalConnection connection, IReadOnlyDictionary`2 parameterValues)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable.Enumerator.BufferlessMoveNext(Boolean buffer)
   at Microsoft.EntityFrameworkCore.Query.QueryMethodProvider.<_ShapedQuery>d__3`1.MoveNext()
   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
   at lambda_method(Closure , QueryContext )
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass20_0`1.<CompileQueryCore>b__0(QueryContext qc
)
   at System.Linq.Queryable.First[TSource](IQueryable`1 source)
   at BasicTesting.BasicTests.TestEnumerate() in C:\_Home\Development\Workspaces\DotnetCoreTesting\EntityTesting3\BasicTesting\BasicTests.cs:line 147If 'foreach(Author a in context.Authors)' is replaced with 'foreach(Author a in context.Authors.ToArray())' no exception occurs.

这不是我所期望的。

当foreach循环到达上下文时。应评估作者。当子序列的书籍。“First”表达式导致另一个数据库操作—不应存在正在进行的现有操作。

1 回复 | 直到 7 年前

bricelam 7 年前

一些提供商不支持多个开放读卡器。避免这种情况的最好方法是在查询书籍之前读完所有作者。正在添加 .ToList() 这是最简单(但不一定是最好)的方法。

foreach (Author a in context.Authors.ToList())
{
    // ... context.Books ...
}