C# &引用;对象引用未设置为对象的实例";-但什么都不是空的?
是的,你可能认为;“天哪,再来一个?” 是的,再来一个 “对象引用未设置为对象的实例。” 我最近一直在使用EF6,在开发了一段时间后,我发现需要进行更多的优化。很多东西都被返工了,没有问题,但我似乎不明白这一点 在我的应用程序中,我一直在使用这段伪代码从数据库中获取项目C# &引用;对象引用未设置为对象的实例";-但什么都不是空的?,c#,entity-framework-6,dbcontext,C#,Entity Framework 6,Dbcontext,是的,你可能认为;“天哪,再来一个?” 是的,再来一个 “对象引用未设置为对象的实例。” 我最近一直在使用EF6,在开发了一段时间后,我发现需要进行更多的优化。很多东西都被返工了,没有问题,但我似乎不明白这一点 在我的应用程序中,我一直在使用这段伪代码从数据库中获取项目 DbContext context = new DbContext(); public IEnumerable<string> GetExistingNames(IEnumerable<string>
DbContext context = new DbContext();
public IEnumerable<string> GetExistingNames(IEnumerable<string> names)
{
foreach(string name in names)
{
string existingName = context.Names.Where(n => n.Name == name).FirstOrDefault();
if(existingName == null) continue;
yield return existingName;
}
}
据我所知,这应该翻译成SELECT。。。从名字里。。。在(…)
中。但是,我的应用程序在foreach(查询中的var name)
上一点击name
,就会崩溃,抛出令人恐惧的NullReferenceException
。
但是,如果(query==null),它会传递,这意味着查询不是null。在这一点上,我感到困惑。它怎么可能不为null,但仍然抛出此错误
如果我尝试用这种方法访问它,我不确定是否执行了查询。因此,我尝试使用ToList()
从查询中创建一个列表,但在创建列表时会引发相同的异常
似乎每次我调用query
,它都会给我一个NullReferenceException
。但是,如果(query==null)
,它仍然传递。所以我的问题是,
为什么它通过了测试,但却无法访问?我是否曲解了IQueryable
?如果我确实误解了它,应该如何正确地理解它?
编辑
我在发布之前已经调试过了。我确信这一点
名称
不为空
上下文
不为空
调用函数的代码:
//A wrapper for the DbContext. This is only used for some methods
//which require the DbContext
DbContextWrapper wrapper = new DbContextWrapper();
public void ProcessNames(List<string> inputNames)
{
//...
foreach(string existingName in wrapper.GetExistingNames(inputNames))
{
//Do something with the names
}
//...
}
然而,我明白了这一点
System.Data.Entity.Infrastructure.DbQuery<MyDbContext.Names>
System.Data.Entity.Infrastructure.DbQuery
作为实际查询
堆栈跟踪
at MySql.Data.Entity.SqlGenerator.Visit(DbPropertyExpression expression)
at MySql.Data.Entity.SqlGenerator.Visit(DbInExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbInExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SqlGenerator.VisitBinaryExpression(DbExpression left, DbExpression right, String op)
at MySql.Data.Entity.SqlGenerator.Visit(DbAndExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbAndExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SelectGenerator.Visit(DbFilterExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbFilterExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SqlGenerator.VisitInputExpression(DbExpression e, String name, TypeUsage type)
at MySql.Data.Entity.SelectGenerator.VisitInputExpressionEnsureSelect(DbExpression e, String name, TypeUsage type)
at MySql.Data.Entity.SelectGenerator.Visit(DbProjectExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbProjectExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SelectGenerator.GenerateSQL(DbCommandTree tree)
at MySql.Data.MySqlClient.MySqlProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree)
at System.Data.Entity.Core.Common.DbProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree, DbInterceptionContext interceptionContext)
at System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree, DbInterceptionContext interceptionContext)
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.CreateCommandDefinition(DbProviderFactory storeProviderFactory, DbCommandTree commandTree, DbInterceptionContext interceptionContext, IDbDependencyResolver resolver)
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.Infrastructure.DefaultExecutionStrategy.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 MyNameSpace.DbContextWrapper.<GetExistingNames>d__1b.MoveNext() in c:~omitted~\DbContextWrapper.cs:line 70
at MyNameSpace.NameProcessor.ProcessNames(List<string> inputNames) in c:~omitted~\NameProcessor.cs:line 60
在MySql.Data.Entity.SqlGenerator.Visit(DbPropertyExpression表达式)
在MySql.Data.Entity.SqlGenerator.Visit(DbInExpression表达式)
位于System.Data.Entity.Core.Common.CommandTrees.DbInExpression.Accept[TResultType](DbExpressionVisitor`1访问者)
在MySql.Data.Entity.SqlGenerator.VisitBinaryExpression(DbExpression左、DbExpression右、String op)
在MySql.Data.Entity.SqlGenerator.Visit(DbAndExpression表达式)
位于System.Data.Entity.Core.Common.CommandTrees.DbAndExpression.Accept[TResultType](DbExpressionVisitor`1访问者)
在MySql.Data.Entity.SelectGenerator.Visit(DbFilterExpression表达式)
位于System.Data.Entity.Core.Common.CommandTrees.DbFilterExpression.Accept[TResultType](DbExpressionVisitor`1访问者)
位于MySql.Data.Entity.SqlGenerator.VisitInputExpression(dbe表达式,字符串名称,TypeUsage类型)
在MySql.Data.Entity.SelectGenerator.VisitInputExpressionEnsureSelect(DBE表达式,字符串名称,类型用法类型)
在MySql.Data.Entity.SelectGenerator.Visit(DbProjectExpression)中
位于System.Data.Entity.Core.Common.CommandTrees.DbProjectExpression.Accept[TResultType](DbExpressionVisitor`1访问者)
在MySql.Data.Entity.SelectGenerator.GenerateSQL(DbCommandTree树)中
位于MySql.Data.MySqlClient.MySqlProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest,DbCommandTree commandTree)
位于System.Data.Entity.Core.Common.DbProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest、DbCommandTree commandTree、DbInterceptionContext interceptionContext)
位于System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree,DbInterceptionContext interceptionContext)
位于System.Data.Entity.Core.EntityClient.Internal.EntityCommandDefinition..ctor(DbProviderFactory storeProviderFactory、DbCommandTree commandTree、DbInterceptionContext interceptionContext、IDBDDependencyResolver解析器、BridgeDataReaderFactory BridgeDataReaderFactory、ColumnMapFactory ColumnMapFactory)
位于System.Data.Entity.Core.EntityClient.Internal.EntityProviderServices.CreateCommandDefinition(DbProviderFactory storeProviderFactory、DbCommandTree commandTree、DbInterceptionContext interceptionContext、IDBDDependencyResolver解析程序)
位于System.Data.Entity.Core.EntityClient.Internal.EntityProviderServices.CreatedDBCommandDefinition(DbProviderManifest providerManifest、DbCommandTree commandTree、DbInterceptionContext interceptionContext)
位于System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree,DbInterceptionContext interceptionContext)
位于System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.CreateCommandDefinition(ObjectContext上下文,DbQueryCommandTree树)
在System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.Prepare(ObjectContext上下文、DbQueryCommandTree树、类型elementType、MergeOption MergeOption、布尔流、跨度、IEnumerable`1 compiledQueryParameters、别名生成器AliasGenerator)
位于System.Data.Entity.Core.Objects.elink.elinkQueryState.GetExecutionPlan(可为null的`1 forMergeOption)
在System.Data.Entity.Core.Objects.ObjectQuery`1.c_u DisplayClass7.b__u6()
在System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func`1 Func,IDBEcutionStrategy executionStrategy,Boolean startLocalTransaction,Boolean releaseConnectionOnSuccess)
在System.Data.Entity.Core.Objects.ObjectQuery`1.c__DisplayClass7.b__5()中
在System.Data.Entity.Infrastructure.DefaultExecutionStrategy.Execute[TResult](Func`1操作)中
位于System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(可为null`1 forMergeOption)
位于System.Data.Entity.Core.Objects.ObjectQuery`1.b_u 0()
位于System.Data.Entity.Internal.LazyEnumerator`1.MoveNext()
在c:~省略~\DbContextWrapper.cs:第70行中的MyNameSpace.DbContextWrapper.d_u1b.MoveNext()处
在c:~省略~\NameProcessor.cs:第60行中的MyNameSpace.NameProcessor.ProcessNames(列出inputNames)处
您对查询的null检查永远不会失败,因为它将返回IQueryable对象(即
System.Data.Entity.Infrastructure.DbQuery<MyDbContext.Names>
at MySql.Data.Entity.SqlGenerator.Visit(DbPropertyExpression expression)
at MySql.Data.Entity.SqlGenerator.Visit(DbInExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbInExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SqlGenerator.VisitBinaryExpression(DbExpression left, DbExpression right, String op)
at MySql.Data.Entity.SqlGenerator.Visit(DbAndExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbAndExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SelectGenerator.Visit(DbFilterExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbFilterExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SqlGenerator.VisitInputExpression(DbExpression e, String name, TypeUsage type)
at MySql.Data.Entity.SelectGenerator.VisitInputExpressionEnsureSelect(DbExpression e, String name, TypeUsage type)
at MySql.Data.Entity.SelectGenerator.Visit(DbProjectExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbProjectExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SelectGenerator.GenerateSQL(DbCommandTree tree)
at MySql.Data.MySqlClient.MySqlProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree)
at System.Data.Entity.Core.Common.DbProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree, DbInterceptionContext interceptionContext)
at System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree, DbInterceptionContext interceptionContext)
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.CreateCommandDefinition(DbProviderFactory storeProviderFactory, DbCommandTree commandTree, DbInterceptionContext interceptionContext, IDbDependencyResolver resolver)
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.Infrastructure.DefaultExecutionStrategy.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 MyNameSpace.DbContextWrapper.<GetExistingNames>d__1b.MoveNext() in c:~omitted~\DbContextWrapper.cs:line 70
at MyNameSpace.NameProcessor.ProcessNames(List<string> inputNames) in c:~omitted~\NameProcessor.cs:line 60
public IEnumerable<string> GetExistingNames(ICollection<string> names)
{
IQueryable<Names> query = Names.Where(n => names.Contains(n.Name));
if (query == null) yield break;
foreach (var name in query)
{
yield return name.Name;
}
}
public IEnumerable<string> GetExistingNames(List<string> names)
IQueryable<Names> query = Names.Where(n => names.ToList().Contains(n.Name));
namespace HelperExtensionMethods
{
public static class ExtensionMethods
{
public static string UpdateNullString(this string testNullstring)
{
if (TestNullstring == null)
return "";
return Testullstring;
}
}
}
using HelperExtesionMethods
DbContext context = new DbContext();
public IEnumerable<string> GetExistingNames(ICollection<string> names)
{
IQueryable<Names> query = context.Names.Where(n => names.UpdateNullString().Contains(n.Name.UpdateNullString()));
if(query == null) yield break;
foreach(var name in query)
{
yield return name.Name;
}
}