C# 两个iQueryTables上的LINQ to Entities.Concat（）抛出NullReferencesException

我有以下抽象概念：

public interface IRepository<TEntity>
{
    IQueryable<TEntity> Entities { get; }
}

我想注意选择中的所有属性都是按正确顺序设置的

执行

.Entities

查询时，我得到一个

NullReferenceException

，它是从

EntitiyFramework.dll

抛出的，具有以下顶部堆栈跟踪：

   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.VisitSetOp(SetOp op, Node n, AliasGenerator alias, Func`3 setOpExpressionBuilder)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.Visit(UnionAllOp op, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.UnionAllOp.Accept[TResultType](BasicOpVisitorOfT`1 v, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.BasicOpVisitorOfT`1.VisitNode(Node n)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.VisitAsRelOp(Node inputNode)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.Visit(FilterOp op, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.FilterOp.Accept[TResultType](BasicOpVisitorOfT`1 v, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.BasicOpVisitorOfT`1.VisitNode(Node n)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.Visit(ConstrainedSortOp op, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.ConstrainedSortOp.Accept[TResultType](BasicOpVisitorOfT`1 v, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.BasicOpVisitorOfT`1.VisitNode(Node n)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.VisitAsRelOp(Node inputNode)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.BuildProjection(Node relOpNode, IEnumerable`1 projectionVars)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.Visit(PhysicalProjectOp op, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.PhysicalProjectOp.Accept[TResultType](BasicOpVisitorOfT`1 v, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.BasicOpVisitorOfT`1.VisitNode(Node n)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator..ctor(Command itree, Node toConvert)
   at System.Data.Entity.Core.Query.PlanCompiler.ProviderCommandInfoUtils.Create(Command command, Node node)
   at System.Data.Entity.Core.Query.PlanCompiler.CodeGen.Process(List`1& childCommands, ColumnMap& resultColumnMap, Int32& columnCount)
   at System.Data.Entity.Core.Query.PlanCompiler.PlanCompiler.Compile(List`1& providerCommands, ColumnMap& resultColumnMap, Int32& columnCount, Set`1& entitySets)
   at System.Data.Entity.Core.Query.PlanCompiler.PlanCompiler.Compile(DbCommandTree ctree, List`1& providerCommands, ColumnMap& resultColumnMap, Int32& columnCount, Set`1& entitySets)
   at System.Data.Entity.Core.EntityClient.Internal.EntityCommandDefinition..ctor(DbProviderFactory storeProviderFactory, DbCommandTree commandTree, DbInterceptionContext interceptionContext, IDbDependencyResolver resolver, BridgeDataReaderFactory bridgeDataReaderFactory, ColumnMapFactory columnMapFactory)
   at System.Data.Entity.Core.EntityClient.Internal.EntityProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree, DbInterceptionContext interceptionContext)
   at System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree, DbInterceptionContext interceptionContext)
   at System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.CreateCommandDefinition(ObjectContext context, DbQueryCommandTree tree)
   at System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.Prepare(ObjectContext context, DbQueryCommandTree tree, Type elementType, MergeOption mergeOption, Boolean streaming, Span span, IEnumerable`1 compiledQueryParameters, AliasGenerator aliasGenerator)
   at System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass7.<GetResults>b__6()
   at System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func`1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass7.<GetResults>b__5()
   at System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func`1 operation)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<System.Collections.Generic.IEnumerable<T>.GetEnumerator>b__0()
   at System.Data.Entity.Internal.LazyEnumerator`1.MoveNext()
   at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source)
   at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__2[TResult](IEnumerable`1 sequence)
   at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.ExecuteSingle[TResult](IEnumerable`1 query, Expression queryRoot)
   at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute[TResult](Expression expression)
   at System.Data.Entity.Internal.Linq.DbQueryProvider.Execute[TResult](Expression expression)

---

我有一个强烈的印象，这可能是Entity Framework 6.1.3中的一个bug，但我不确定。我想知道这是否真的是一个bug，或者我正在做一些愚蠢的事情，经过3个小时的调查后我无法弄清楚。

这肯定是一个bug，因为你没有做错什么，而且例外是对用户非常不友好

有一个解决方法，但需要额外的编码。诀窍是使用中间投影到“平坦”数据类（因为bug在某种程度上与嵌套的

公司

投影相关），然后

Concat

，最后将所需的投影应用到连接的结果上。所有这些都不会影响最终的SQL查询，它应该是简单的

UNION-All

下面是它的外观：

public class CustomerRepository : IRepository<Customer>
{
    private readonly MyDbContext dbContext;

    public CustomerRepository(MyDbContext dbContext)
    {
        this.dbContext = dbContext;
    }

    public IQueryable<Customer> Entities => InternalCustomersData.Concat(ExternalCustomersData).Select(CustomerSelector);

    private IQueryable<Customer> InternalCustomers => InternalCustomersData.Select(CustomerSelector);

    private IQueryable<Customer> ExternalCustomers => ExternalCustomersData.Select(CustomerSelector);

    private IQueryable<CustomerData> InternalCustomersData =>
        from customer in dbContext.InternalCustomers
        select new CustomerData
        {
            Id = customer.Id,
            Name = customer.Name,       
            CompanyId = 1,
            CompanyName = "Company",
        };

    private IQueryable<CustomerData> ExternalCustomersData =>
        from customer in dbContext.ExternalCustomers
        select new CustomerData
        {
            Id = customer.Id,
            Name = customer.Name,       
            CompanyId = customer.Company.Id,
            CompanyName = customer.Company.Name,
        };

    private static readonly Expression<Func<CustomerData, Customer>> CustomerSelector = data => new Customer
    {
        Id = data.Id,
        Name = data.Name,
        Company = new Company
        {
            Id = data.CompanyId,
            Name = data.CompanyName,
        }
    };

    private class CustomerData
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int CompanyId { get; set; }
        public string CompanyName { get; set; }
    }
}

公共类CustomerRepository:IRepository
{
私有只读MyDbContext；
公共CustomerRepository（MyDbContext）
{
this.dbContext=dbContext；
}
public IQueryable Entities=>InternalCustomerData.Concat（ExternalCustomerData）.Select（CustomerSelector）；
private IQueryable InternalCustomers=>InternalCustomerData.Select（CustomerSelector）；
private IQueryable ExternalCustomers=>ExternalCustomerData.Select（CustomerSelector）；
私有IQueryable InternalCustomerData=>
来自dbContext.InternalCustomers中的customer
选择新CustomerData
{
Id=customer.Id，
Name=customer.Name，
CompanyId=1，
CompanyName=“公司”，
};
private IQueryable ExternalCustomerData=>
来自dbContext.ExternalCustomers中的客户
选择新CustomerData
{
Id=customer.Id，
Name=customer.Name，
CompanyId=customer.Company.Id，
CompanyName=customer.Company.Name，
};
私有静态只读表达式CustomerSelect=data=>new Customer
{
Id=data.Id，
Name=data.Name，
公司=新公司
{
Id=data.CompanyId，
Name=data.CompanyName，
}
};
私有类CustomerData
{
公共int Id{get；set；}
公共字符串名称{get；set；}
public int CompanyId{get；set；}
公共字符串CompanyName{get；set；}
}
}

---

烦人，但有效。

@DanielLorenz除非表中有重复项需要删除，否则无效。我怀疑根本没有复制品，所以没有必要特意去检查它们。@DanielLorenz不，不会的。这段代码永远不会运行，它只是用来构建转换成SQL的表达式，SQL在取消引用时传播空值，而不是抛出空值。这正是@Servy在这两个问题上所说的。我已经尝试了

.Union（）

，它的行为方式与

.Concat（）

相同（如中抛出了相同的异常），但这里的想法应该只是一个串联。请幽默我，然后做。包括公司的外部客户。Concat可能最终在内存中而不是在sql server上运行。您必须看到SQL在网络上运行，才能看到它具体在做什么。除非SQL甚至不能跨越网络，否则它肯定是一个bug，如果不是关键的，不要浪费时间。解决方法是为

Company.Id

提供非常量值，例如为

InternalCustomers

使用旧式属性get，并使用

var internalCompanyId=1；在db.InternalCustomers中从客户返回选择新客户{Id=customer.Id，Name=customer.Name，Company=new Company{Id=internalCompanyId，Name=“Company”，}，}

对于

string

属性，不需要这样做，只需要对第一个值类型属性执行此操作。

   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.VisitSetOp(SetOp op, Node n, AliasGenerator alias, Func`3 setOpExpressionBuilder)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.Visit(UnionAllOp op, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.UnionAllOp.Accept[TResultType](BasicOpVisitorOfT`1 v, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.BasicOpVisitorOfT`1.VisitNode(Node n)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.VisitAsRelOp(Node inputNode)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.Visit(FilterOp op, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.FilterOp.Accept[TResultType](BasicOpVisitorOfT`1 v, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.BasicOpVisitorOfT`1.VisitNode(Node n)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.Visit(ConstrainedSortOp op, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.ConstrainedSortOp.Accept[TResultType](BasicOpVisitorOfT`1 v, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.BasicOpVisitorOfT`1.VisitNode(Node n)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.VisitAsRelOp(Node inputNode)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.BuildProjection(Node relOpNode, IEnumerable`1 projectionVars)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator.Visit(PhysicalProjectOp op, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.PhysicalProjectOp.Accept[TResultType](BasicOpVisitorOfT`1 v, Node n)
   at System.Data.Entity.Core.Query.InternalTrees.BasicOpVisitorOfT`1.VisitNode(Node n)
   at System.Data.Entity.Core.Query.PlanCompiler.CTreeGenerator..ctor(Command itree, Node toConvert)
   at System.Data.Entity.Core.Query.PlanCompiler.ProviderCommandInfoUtils.Create(Command command, Node node)
   at System.Data.Entity.Core.Query.PlanCompiler.CodeGen.Process(List`1& childCommands, ColumnMap& resultColumnMap, Int32& columnCount)
   at System.Data.Entity.Core.Query.PlanCompiler.PlanCompiler.Compile(List`1& providerCommands, ColumnMap& resultColumnMap, Int32& columnCount, Set`1& entitySets)
   at System.Data.Entity.Core.Query.PlanCompiler.PlanCompiler.Compile(DbCommandTree ctree, List`1& providerCommands, ColumnMap& resultColumnMap, Int32& columnCount, Set`1& entitySets)
   at System.Data.Entity.Core.EntityClient.Internal.EntityCommandDefinition..ctor(DbProviderFactory storeProviderFactory, DbCommandTree commandTree, DbInterceptionContext interceptionContext, IDbDependencyResolver resolver, BridgeDataReaderFactory bridgeDataReaderFactory, ColumnMapFactory columnMapFactory)
   at System.Data.Entity.Core.EntityClient.Internal.EntityProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree, DbInterceptionContext interceptionContext)
   at System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree, DbInterceptionContext interceptionContext)
   at System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.CreateCommandDefinition(ObjectContext context, DbQueryCommandTree tree)
   at System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.Prepare(ObjectContext context, DbQueryCommandTree tree, Type elementType, MergeOption mergeOption, Boolean streaming, Span span, IEnumerable`1 compiledQueryParameters, AliasGenerator aliasGenerator)
   at System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass7.<GetResults>b__6()
   at System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func`1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass7.<GetResults>b__5()
   at System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func`1 operation)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<System.Collections.Generic.IEnumerable<T>.GetEnumerator>b__0()
   at System.Data.Entity.Internal.LazyEnumerator`1.MoveNext()
   at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source)
   at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__2[TResult](IEnumerable`1 sequence)
   at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.ExecuteSingle[TResult](IEnumerable`1 query, Expression queryRoot)
   at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute[TResult](Expression expression)
   at System.Data.Entity.Internal.Linq.DbQueryProvider.Execute[TResult](Expression expression)

Company = new Company
{
    Id = 0, //random number
    Name = "Hello",
},

public class CustomerRepository : IRepository<Customer>
{
    private readonly MyDbContext dbContext;

    public CustomerRepository(MyDbContext dbContext)
    {
        this.dbContext = dbContext;
    }

    public IQueryable<Customer> Entities => InternalCustomersData.Concat(ExternalCustomersData).Select(CustomerSelector);

    private IQueryable<Customer> InternalCustomers => InternalCustomersData.Select(CustomerSelector);

    private IQueryable<Customer> ExternalCustomers => ExternalCustomersData.Select(CustomerSelector);

    private IQueryable<CustomerData> InternalCustomersData =>
        from customer in dbContext.InternalCustomers
        select new CustomerData
        {
            Id = customer.Id,
            Name = customer.Name,       
            CompanyId = 1,
            CompanyName = "Company",
        };

    private IQueryable<CustomerData> ExternalCustomersData =>
        from customer in dbContext.ExternalCustomers
        select new CustomerData
        {
            Id = customer.Id,
            Name = customer.Name,       
            CompanyId = customer.Company.Id,
            CompanyName = customer.Company.Name,
        };

    private static readonly Expression<Func<CustomerData, Customer>> CustomerSelector = data => new Customer
    {
        Id = data.Id,
        Name = data.Name,
        Company = new Company
        {
            Id = data.CompanyId,
            Name = data.CompanyName,
        }
    };

    private class CustomerData
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int CompanyId { get; set; }
        public string CompanyName { get; set; }
    }
}