c# - 使用 EF 的 LINQ 查询中出现奇怪的空引用异常

Question

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; }
}

我已经建立了如下流畅的关系:

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 方法间歇性地抛出一个空引用异常:

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);
        }
}

上述方法中抛出错误的具体语句是

List<ContextParamValue> lst = cpvList
                 .Where(pv => d.Contains(new KeyValuePair<string, string>(pv.ContextParam.Name, pv.Value)))
                 .ToList<ContextParamValue>();

在以下情况下不会抛出空引用异常:

• 对于给定的 ContextInstance，仅存在 1 个 ContextParamValue
• 示例，ContextParamValue.ContextParam.Name = "ClientId"和 ContextParamValue1.Value = "1"

空引用异常是在以下情况下抛出:

• 对于给定的 ContextInstance，存在两个或多个 ContextParamValues
• 例如，ContextParamValue1.ContextParam.Name =“ClientId”和 ContextParamValue1.Value =“1”加上 ContextParamValue2.ContextParam.Name =“MotivationId”和 ContextParamValue2.Value =“1”。

我可以确认以下有关相关辅助方法的信息:

• d 不为空，也不包含任何具有空值的键值对
• 错误发生时cpvList不为null且不为空。
• 并非在所有情况下都为父 ContextParamValue 实体加载 ContextParam(它仅为第一个 ContextParamValue 实例加载，但对于后续实例仅加载空值)。
• 数据库中没有空的 ContextParam 条目...所有 ContextParamValues 都有一个 ContextParam 条目。

以下trace 和stacktrace 信息是在运行时生成的:

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

Answer 1

如果 pv.ContextParam 为 null，您的代码将抛出异常的唯一方法是，因为这是您取消引用可能导致空指针异常的唯一地方。

如果您有 ContextParamValues 记录而没有相应的 ContextParam 记录，则会发生这种情况，因此 ContextParam 将为空。由于我们看不到您的数据模型，因此您必须进行检查。

添加这行代码并在调试器中检查它是否正确:

bool containsNulls = db.ContextParamValues
    .Include(x => x.ContextParam)
    .Any(x => x.ContextParam == null)

编辑(删除了所有中间步骤，如果您有兴趣，请查看历史记录):

好吧，这实际上并没有回答问题，但它会解决您的问题。让我们重写您的代码，使其更简单、更高效。如果我没看错你的代码，你要做的就是返回关联了 ContextValueParams 和提供的名称/值对的 Co​​ntextInstances，对吗？

为什么不直接这样做(添加您认为合适的包含):

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();
}