Strange null reference exception in LINQ query with EF

Question

I have the following three related entity classes: ``` public class ContextInstance { public Int64 Id { get; set; } public virtual List 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: ``` modelBuilder.Conventions.Remove(); modelBuilder.Conventions.Remove(); modelBuilder.Entity() .HasMany(ci => ci.ContextParamValues) .WithRequired(cpv => cpv.ContextInstance) .HasForeignKey(…

Accepted Answer

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.

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.

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)
```

EDIT (removed all the middle steps, check history if you're interested):

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?

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();
}
```

Strange null reference exception in LINQ query with EF

Solution

Problem