带有EF的LINQ查询中的奇怪的空引用异常 [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; }
}
我已经建立了流利的关系，如下所示：
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Helper类，其ParamValueToList方法间歇性地抛出
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);
}
}
上述方法中的具体声明正在抛出错误是
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>();
空引用异常是在以下条件下抛出的 NOT ：
The null reference exception is NOT thrown under the following condition:
- 对于给定的ContextInstance，只存在1个ContextParamValue
- 示例，ContextParamValue.ContextParam.Name =ClientId和ContextParamValue1.Value =1
空引用异常>是在以下条件下抛出：
The null reference exception is thrown under the following condition:
- 对于给定的ContextInstance，更多ContextParamValues
- 示例，ContextParamValue1.ContextParam.Name =ClientId和ContextParamValue1.Value =1PLUS ContextParamValue2.ContextParam.Name =MotivationId和ContextParamValue2.Value =1 。
我可以确认以下有关帮助方法的问题：
- d不为空，也不包含任何具有空值的键值
- cpvList在所有情况下，ContextParam不为父ContextParamValue实体加载ContextParam（它仅为第一个ContextParamValue实例加载，但后续实例仅加载一个空值，因此不为空，并且在发生错误时不为空。
- 被加载）
- 数据库中没有空的ContextParam条目...所有ContextParamValues都有一个ContextParam条目。
- 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.
以下跟踪和 stacktrace 信息在运行时生成：
The following trace and stacktrace information is generated during runtime:
应用程序：2014-05-16T19：00：20 PID [4800]错误
System.NullReferenceException：对象引用未设置为对象的实例
。 c：\Users\xxx\Dropbox\xxx\Active
项目中的
DocumentManagement.Helpers.RuntimeHelper。 \xxx\DocumentManagement\Helpers\DocsHelper_RT.cs：行229
应用程序：在System.Linq.Enumerable.WhereArrayIterator1.MoveNext（）
应用程序：在System.Collections.Generic.List1。应用程序：
System.Linq.Enumerable.ToList [TSource]（IEnumerable1源）
应用程序：
DocumentManagement.Helpers.RuntimeHelper.ParamValuesToList（字符串[]
ParamNames，String [] ParamValues）在c：\Users\xxx\Dropbox\xxx\Active
Projects \xxx\DocumentManagement\Helpers\DocsHelper_RT。 cs：line 228
应用程序：at
DocumentManagement.Helpers.RuntimeHelper.GetContextInstances（String []
中的ParamNames，String [] ParamValues，Boolean AsNoTracking） c：\Users\xxx\Dropbox\xxx\Active
Projects\xxx\DocumentManagement\Helpers\DocsHelper_RT.cs：line 262
应用程序：在xxx.Controllers。 C：\Users\xxx\Dropbox\xxx\Active
中的ClientController.LoadStep2（Int64
ClientId，String Error）Projects \xxx\xxx\Views\Client\\ \\ClientController.cs：第198行
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
你的代码抛出异常的唯一方法是pv.ContextParam为null，因为这是唯一可以引用可能导致空指针异常的地方。
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.
如果您具有没有相应ContextParam记录的ContextParamValues记录，则会发生这种情况我们ContextParam将为null。因为我们看不到你的数据模型，所以你必须检查它。
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.
添加这一行代码并检查调试器，看看它是否正确： / p>
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（删除所有中间步骤，检查历史如果你有兴趣）：
EDIT (removed all the middle steps, check history if you're interested):
嗯，这不是真的回答这个问题，但它会解决你的问题。让我们重写你的代码，使其更简单，更高效。如果我正确阅读你的代码，所有你想要做的是返回ContextValueParams相关联的ContextInstances与提供的名称/值对，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?
为什么不只是这样做（添加包括你认为合适）：</ p>
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();
}
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