在LINQ查询与EF奇怪的空引用异常 [英] Strange null reference exception in LINQ query with EF
问题描述
我有以下三个相关的实体类：
public class ContextInstance
{
public Int64 Id { get; set; }
public virtual List<ContextParamValue> ContextParamValues { get; set; }
}
public class ContextParamValue
{
public Int64 Id { get; set; }
public virtual Int64 ContextParamId { get; set; }
public virtual ContextParam ContextParam { get; set; }
public virtual ContextInstance ContextInstance { get; set; }
public virtual Int64 ContextInstanceId { get; set; }
public string Value { get; set; }
}
public class ContextParam
{
public Int64 Id { get; set; }
[Required]
public string Name { get; set; }
[DefaultValue("")]
public string Description { get; set; }
}
我已经建立了流畅的关系如下：
modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>();
modelBuilder.Conventions.Remove<ManyToManyCascadeDeleteConvention>();
modelBuilder.Entity<ContextInstance>()
.HasMany(ci => ci.ContextParamValues)
.WithRequired(cpv => cpv.ContextInstance)
.HasForeignKey(cpv => cpv.ContextInstanceId)
.WillCascadeOnDelete(true);
我有以下的帮手类中，ParamValueToList方法，它是间歇扔的空引用异常的：
I have the following "Helper" class, the ParamValueToList method of which is intermittently throwing a null-reference exception:
public class RuntimeHelper : IDisposable
{
DocumentDbContext db;
ConfigurationHelper ch;
private RuntimeHelper()
{
}
public RuntimeHelper(DocumentDbContext context)
{
db = context;
ch = new ConfigurationHelper(context);
}
public List<ContextParamValue> ParamValuesToList(string[] ParamNames, string[] ParamValues)
{
Trace.TraceInformation("-- ParamValuesToList invoked --");
if (ParamNames != null && ParamNames.Length != ParamValues.Length)
throw new System.ArgumentException("ParamNames and ParamValues may not differ in length.");
Dictionary<string, string> d = new Dictionary<string, string>();
for (int i = 0; i < ParamNames.Length; i++)
{
string pName = ParamNames[i];
string pValue = ParamValues[i];
d.Add(pName, pValue);
Trace.TraceInformation("ParamValuesToList Key: " + pName + "; Value: " + pValue + ";");
}
Trace.TraceInformation("Value of db:" + db.ContextParamValues.ToString());
var cpvList = db.ContextParamValues
.Include(x => x.ContextParam)
.ToArray<ContextParamValue>();
List<ContextParamValue> lst = cpvList
.Where(pv => d.Contains(new KeyValuePair<string, string>(pv.ContextParam.Name, pv.Value)))
//.Where(pv => true == true)
.ToList<ContextParamValue>();
Trace.TraceInformation("-- ParamValuesToList executed --");
return lst;
}
public List<ContextInstance> GetContextInstances(List<ContextParamValue> ContextParamValues, bool AsNoTracking = false)
{
if (!AsNoTracking)
return db.ContextInstances
.Include(x => x.ContextClass)
.Include(x => x.ContextParamValues.Select(p => p.ContextParam))
.Include(x => x.Documents)
.AsEnumerable<ContextInstance>() // <-- Allows boolean method as part of LINQ query
.Where(ci => IsSubset(ci.ContextParamValues, ContextParamValues))
.ToList<ContextInstance>();
else
return db.ContextInstances
.Include(x => x.ContextClass)
.Include(x => x.ContextParamValues.Select(p => p.ContextParam))
.Include(x => x.Documents)
.AsNoTracking()
.AsEnumerable<ContextInstance>()// <-- Allows boolean method as part of LINQ query
.Where(ci => IsSubset(ci.ContextParamValues, ContextParamValues))
.ToList<ContextInstance>();
}
public List<ContextInstance> GetContextInstances(string[] ParamNames, string[] ParamValues, bool AsNoTracking = false)
{
return GetContextInstances(ParamValuesToList(ParamNames, ParamValues), AsNoTracking);
}
}
这是扔上述方法的具体声明误差
The specific statement from the above method that is throwing the error is
List<ContextParamValue> lst = cpvList
.Where(pv => d.Contains(new KeyValuePair<string, string>(pv.ContextParam.Name, pv.Value)))
.ToList<ContextParamValue>();
的空引用例外的不的在以下条件下抛出
The null reference exception is NOT thrown under the following condition:
- 存在着，对于一个给定ContextInstance，只有1 ContextParamValue
- 示例，ContextParamValue.ContextParam.Name =客户端Id和ContextParamValue1.Value =1
的空引用异常的是在下列条件下引发
The null reference exception is thrown under the following condition:
- 存在着，对于给定ContextInstance，二 - 或 - 更ContextParamValues
- 例，ContextParamValue1.ContextParam.Name =客户端Id和ContextParamValue1.Value =1加ContextParamValue2.ContextParam.Name =MotivationId和ContextParamValue2.Value =1
我可以证实有关问题的helper方法如下：
- d为不为空也不包含空值
- cpvList不是null，不为空
- ContextParam不会在所有情况下的父ContextParamValue实体加载（只加载第一个ContextParamValue实例，但随后的情况下仅一个空值加载）。
- 有数据库中没有空ContextParam项...所有ContextParamValues具有一个ContextParam条目。
任何keyvaluepairs时出现错误
- d is not null nor does it contain any keyvaluepairs with null values
- cpvList is not null and not empty when the error occurs.
- ContextParam does not load for parent ContextParamValue entities in all cases (it only loads for the first ContextParamValue instance but for subsequent instances only a null value is loaded).
- There are no null ContextParam entries in database... All ContextParamValues has one ContextParam entry.
的下面的跟踪的和的堆栈跟踪的运行过程中产生的信息：
The following trace and stacktrace information is generated during runtime:
应用：2014-05-16T19：00：20 PID [4800]错误
System.NullReferenceException：对象不设置到对象的实例
。应用：在
DocumentManagement.Helpers.RuntimeHelper<> c__DisplayClass28.b__27（ContextParamValue
PV）在C：\Users\xxx\Dropbox\xxx\Active
类项目\xxx\DocumentManagement\Helpers\DocsHelper_RT.cs：线229
应用：在System.Linq.Enumerable.WhereArrayIterator1.MoveNext（）
应用：在System.Collections.Generic.List1。 .ctor（IEnumerable1
集合）应用：在
System.Linq.Enumerable.ToList [TSource]（IEnumerable1源）
应用：在
DocumentManagement.Helpers.RuntimeHelper.ParamValuesToList（的String []
ParamNames，字符串[] ParamValues）在C：\Users\xxx\Dropbox\xxx\Active
Projects\xxx\DocumentManagement\Helpers\DocsHelper_RT。 CS：行228
应用：在
C
DocumentManagement.Helpers.RuntimeHelper.GetContextInstances（字符串[]
ParamNames，字符串[] ParamValues，布尔AsNoTracking）：\Users\\ \\xxx\Dropbox\xxx\Active
Projects\xxx\DocumentManagement\Helpers\DocsHelper_RT.cs：行262
应用：在xxx.Controllers.ClientController.LoadStep2（Int64的
客户端ID，字符串错误）在C：\Users\xxx\Dropbox\xxx\Active
Projects\xxx\xxx\Views\Client\ClientController.cs：行198
Application: 2014-05-16T19:00:20 PID[4800] Error System.NullReferenceException: Object reference not set to an instance of an object. Application: at DocumentManagement.Helpers.RuntimeHelper.<>c__DisplayClass28.b__27(ContextParamValue pv) in c:\Users\xxx\Dropbox\xxx\Active Projects\xxx\DocumentManagement\Helpers\DocsHelper_RT.cs:line 229 Application: at System.Linq.Enumerable.WhereArrayIterator1.MoveNext() Application: at System.Collections.Generic.List1..ctor(IEnumerable1 collection) Application: at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source) Application: at DocumentManagement.Helpers.RuntimeHelper.ParamValuesToList(String[] ParamNames, String[] ParamValues) in c:\Users\xxx\Dropbox\xxx\Active Projects\xxx\DocumentManagement\Helpers\DocsHelper_RT.cs:line 228 Application: at DocumentManagement.Helpers.RuntimeHelper.GetContextInstances(String[] ParamNames, String[] ParamValues, Boolean AsNoTracking) in c:\Users\xxx\Dropbox\xxx\Active Projects\xxx\DocumentManagement\Helpers\DocsHelper_RT.cs:line 262 Application: at xxx.Controllers.ClientController.LoadStep2(Int64 ClientId, String Error) in c:\Users\xxx\Dropbox\xxx\Active Projects\xxx\xxx\Views\Client\ClientController.cs:line 198
推荐答案
这是您的代码会抛出一个例外是，如果pv.ContextParam是空的唯一方法，因为这是唯一的地方，你被提领的东西，可能会导致空指针异常。
The only way that your code would throw an exception would be if pv.ContextParam were null, because that is the only place you are dereferencing something that might cause a null pointer exception.
如果您有ContextParamValues记录没有相应的记录ContextParam这会发生，从而ContextParam将是空。由于我们无法看到你的数据模型，你将不得不以检查
This would happen if you have ContextParamValues records without a corresponding ContextParam record, thus ContextParam would be null. Since we can't see your data model, you will have to check for that.
添加这行代码，并检查调试程序，看它是否是真的：
Add this line of code and check in the debugger to see if it's true:
bool containsNulls = db.ContextParamValues
.Include(x => x.ContextParam)
.Any(x => x.ContextParam == null)
编辑（删除了所有的中间环节，检查历史如果你有兴趣）：
EDIT (removed all the middle steps, check history if you're interested):
那么，这实际上并没有回答这个问题，但它会解决你的问题。让我们只重写代码更简单，更高效。如果我看了你的代码的权利，所有你希望做的是恢复已经与所提供的名称/值对相关ContextValueParams的ContextInstances，是否正确？
Well, this doesn't actually answer the question, but it would solve your problem. Let's just rewrite your code to be simpler and more efficient. If I read your code right, all you're looking to do is return the ContextInstances that have associated ContextValueParams with the provided name/value pairs, correct?
为什么不干脆做到这一点（包括添加你认为合适的）：
Why not just do this (add includes as you see fit):
public List<ContextInstance> GetContextInstances(
string[] ParamNames, string[] ParamValues, bool AsNoTracking = false)
{
var p = ParamNames.Zip(ParamValues, (a,b) => a+b);
var ctx = db.ContextInstances
.Where(x => p.All(y => x.ContextParamValues
.Select(z => z.ContextParam.Name + z.Value).Contains(y)));
return (AsNoTracking ? ctx.AsNoTracking() : ctx).ToList();
}
这篇关于在LINQ查询与EF奇怪的空引用异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
