MySQL - 与实体框架有关的问题

Question

此查询是工作。

string value = null; 
var temp1 = (from d in db.Device 
      where         
       (value == null ? true : d.Status == 1) 
       select d).ToList(); 

但是这个查询不起作用。

string value = null; 
var temp1 = (from d in db.Device 
      where         
       (value == null ? true : d.Status == 1) 
       select d).OrderBy(i => i.Title).ToList(); 

Device模型有Status和Title性能。

错误：Object reference not set to an instance of an object.

MySql.Data版本：6.9.9

Connector/Net：6.9.9

MySql.Data.Entity.EF6 

只需添加排序依据，并得到了错误。你能告诉我为什么吗？

堆栈跟踪：

at MySql.Data.Entity.CaseFragment.Accept(SqlFragmentVisitor visitor) at MySql.Data.Entity.BinaryFragment.Accept(SqlFragmentVisitor visitor) at MySql.Data.Entity.SqlGenerator.FuseSelectWithInnerSelect(SelectStatement outer, SelectStatement inner) at MySql.Data.Entity.SqlGenerator.TryFusingSelect(InputFragment f) 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.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 System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source) 

Answer 1

它看起来像你正试图重写一个包罗万象的查询作为LINQ。您不需要使用LINQ在所有写一个包罗万象的查询，你可以简单地忽略的条件，如果你不想要它，例如：

string value=someOptionalParameter; 
var query = (value==null)? db.Device : db.Where(d=>d.Status == 1);  
var temp1 = query.OrderBy(d=>d.Title).ToList(); 

在这种情况下，value不完全可以使用。它只是用作一面旗帜。如果您没有要使用它，例如，有选择地通过Category场过滤器，你可以以同样的方式使用它在where子句中：

string value=someOptionalParameter; 
var query = (value==null)? db.Device : db.Where(d=>d.Category == value);  
var temp1 = query.OrderBy(d=>d.Title).ToList(); 

在一般情况下，你可以在“建设”的查询位一点点。一个IQueryable< T>直到你试图枚举它和/或它转换成一个列表不被翻译成SQL，数组等等。你可以动态地从用户的选择创建一个这样的查询：

var query = dbContext.MyEntity; 
if (someField != null) 
{ 
    query = query.Where(d=>d.SomeField = someField); 
} 

if (someOtherField != null) 
{ 
    query = query.Where(d=>d.SomeOtherField = someField); 
} 

if (mustOrder != null) 
{ 
    query = query.OrderBy(d=>d.OrderField); 
} 

//Execute the final query 
return query.ToList(); 

Answer 2

根据堆栈跟踪，这是处理? :运算符部分（CaseFragment）时的MySQL连接器错误。

，通过消除了? :结构

(value == null ? true : d.Status == 1) 

具有同等

(value == null || d.Status == 1)