[英]Strange null reference exception in LINQ query with EF
I have the following three related entity classes: 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; }
}
I have set up fluent relationships as follows: 我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);
I have the following "Helper" class, the ParamValueToList method of which is intermittently throwing a null-reference exception : 我有以下“Helper”类,其ParamValueToList方法间歇性地抛出空引用异常 :
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);
}
}
The specific statement from the above method that is throwing the error is 上面抛出错误的方法的具体说法是
List<ContextParamValue> lst = cpvList
.Where(pv => d.Contains(new KeyValuePair<string, string>(pv.ContextParam.Name, pv.Value)))
.ToList<ContextParamValue>();
The null reference exception is NOT thrown under the following condition: 在以下条件下不引发空引用异常:
The null reference exception is thrown under the following condition: 空引用异常在以下条件下抛出:
I can confirm the following about the helper method in question: 我可以确认以下关于有问题的帮助方法:
The following trace and stacktrace information is generated during runtime: 运行时期间会生成以下跟踪和堆栈 跟踪信息:
Application: 2014-05-16T19:00:20 PID[4800] Error System.NullReferenceException: Object reference not set to an instance of an object. 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。 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\\D 应用程序:在DocumentManagement.Helpers.RuntimeHelper。<> c__DisplayClass28.b__27(ContextParamValue pv)中的c:\\ Users \\ xxx \\ Dropbox \\ xxx \\ Active Projects \\ xxx \\ DocumentManagement \\ Helpers \\ DocsHelper_RT.cs:第229行应用程序: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)在c:\\ Users \\ xxx \\ Dropbox \\ xxx \\ Active Projects \\ xxx \\ DocumentManagement \\ Helpers \\ DocsHelper_RT.cs:第228行应用程序:在DocumentManagement.Helpers.RuntimeHelper。 GetContextInstances(String [] ParamNames,String [] ParamValues,Boolean AsNoTracking)在c:\\ Users \\ xxx \\ Dropbox \\ xxx \\ Active Projects \\ xxx \\ DocumentManagement \\ Helpers \\ DocsHelper_RT.cs:第262行应用程序:at xxx.Controllers.ClientController c:\\ Users \\ xxx \\ D中的.LoadStep2(Int64 ClientId,String Error) ropbox\\xxx\\Active Projects\\xxx\\xxx\\Views\\Client\\ClientController.cs:line 198 ropbox \\ xxx \\ Active Projects \\ xxx \\ xxx \\ Views \\ Client \\ ClientController.cs:第198行
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pv.ContextParam为null,则代码抛出异常的唯一方法是,因为这是您解除引用可能导致空指针异常的内容的唯一位置。
This would happen if you have ContextParamValues records without a corresponding ContextParam record, thus ContextParam would be null. 如果您具有ContextParamValues记录而没有相应的ContextParam记录,则会发生这种情况,因此ContextParam将为null。 Since we can't see your data model, you will have to check for that. 由于我们无法看到您的数据模型,因此您必须检查它。
Add this line of code and check in the debugger to see if it's true: 添加这行代码并检查调试器以查看它是否为真:
bool containsNulls = db.ContextParamValues
.Include(x => x.ContextParam)
.Any(x => x.ContextParam == null)
EDIT (removed all the middle steps, check history if you're interested): 编辑(删除所有中间步骤,如果您感兴趣,请查看历史记录):
Well, this doesn't actually answer the question, but it would solve your problem. 嗯,这实际上并没有回答这个问题,但它可以解决你的问题。 Let's just rewrite your code to be simpler and more efficient. 让我们重写您的代码,使其更简单,更高效。 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? 如果我正确地阅读了您的代码,那么您要做的就是返回与所提供的名称/值对相关联的ContextInstances,正确吗?
Why not just do this (add includes as you see fit): 为什么不这样做(根据需要添加包括):
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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