C#•2y ago

System.NullReferenceException in an if statement

I keep getting System.NullReferenceException directing me to the last if statement in the following code:

public override IEnumerable<Node> GetReachableNodes(TransformComponent xform, EntityQuery<NodeContainerComponent> nodeQuery, EntityQuery<TransformComponent> xformQuery,
        MapGridComponent? grid, IEntityManager entMan)
    {
        if (grid == null)
            yield break;

        entMan.System<SpreaderSystem>().GetNeighbors(xform.Owner, Name, out _, out _, out var neighbors);

        foreach (var neighbor in neighbors)
        {
            if (!nodeQuery.TryGetComponent(neighbor, out var nodeContainer) ||
                !_nodeContainer.TryGetNode<SpreaderNode>(nodeContainer, Name, out var neighborNode))
            {
                continue;
            }

            yield return neighborNode;
        }
    }

public override IEnumerable<Node> GetReachableNodes(TransformComponent xform, EntityQuery<NodeContainerComponent> nodeQuery, EntityQuery<TransformComponent> xformQuery,
        MapGridComponent? grid, IEntityManager entMan)
    {
        if (grid == null)
            yield break;

        entMan.System<SpreaderSystem>().GetNeighbors(xform.Owner, Name, out _, out _, out var neighbors);

        foreach (var neighbor in neighbors)
        {
            if (!nodeQuery.TryGetComponent(neighbor, out var nodeContainer) ||
                !_nodeContainer.TryGetNode<SpreaderNode>(nodeContainer, Name, out var neighborNode))
            {
                continue;
            }

            yield return neighborNode;
        }
    }

The problem is in _nodeContainer.TryGetNode.... After I moved TryGetNode from one place to another and added a new parameter, that error started appearing. Why does it always appear if all TryGetNode's parameters are nullable:

        public bool TryGetNode<T>(NodeContainerComponent? component, string? identifier, [NotNullWhen(true)] out T? node) where T : Node
        {
            if (identifier == null || component == null)
            {
                node = null;
                return false;
            }

            if (component.Nodes.TryGetValue(identifier, out var n) && n is T t)
            {
                node = t;
                return true;
            }

            node = null;
            return false;
        }

        public bool TryGetNode<T>(NodeContainerComponent? component, string? identifier, [NotNullWhen(true)] out T? node) where T : Node
        {
            if (identifier == null || component == null)
            {
                node = null;
                return false;
            }

            if (component.Nodes.TryGetValue(identifier, out var n) && n is T t)
            {
                node = t;
                return true;
            }

            node = null;
            return false;
        }

26 Replies

mtreit•2y ago

_nodeContainer could be null?

faintOP•2y ago

Unsure what you mean, since nodeContainer is a method and it returns bool value

mtreit•2y ago

_nodeContainer is a variable Likely a field

faintOP•2y ago

oh, I see what you mean lemme explain

faintOP•2y ago

here is what it looks like in code

faintOP•2y ago

I'm not that good at C# to explain it better

mtreit•2y ago

So it's null The default value for all reference types is null

faintOP•2y ago

why is it so

mtreit•2y ago

Because variables for reference types are essentially a pointer. Null means they aren't pointing to anything yet.

Angius•2y ago

That ! is a hint, it tells the compiler "shut up about null, I know what I'm doing"

faintOP•2y ago

I just don't know why it doesn't work only with my system if that way of getting a system is used everywhere in the solution I might wanna do another way that also works

Angius•2y ago

What is that [Dependency] attribute? Is it some field-based dependency injection framework at work?

mtreit•2y ago

Fixing this might be as simple as:

private readonly NodeContainerSystem _nodeContainer = new();

private readonly NodeContainerSystem _nodeContainer = new();

But yeah I have no idea what that Dependency attribute is

faintOP•2y ago

I shift+clicked it and here is the code

using System;

namespace Robust.Shared.IoC
{
    /// <summary>
    /// Specifies that the field this is applied to is a dependency,
    /// which will be resolved by <see cref="IoCManager" /> when the containing class is instantiated.
    /// </summary>
    /// <remarks>
    /// <para>
    /// The dependency is resolved as if <see cref="IoCManager.Resolve{T}()" /> were to be called,
    /// but it avoids circular references and init order issues due to internal code in the <see cref="IoCManager" />.
    /// </para>
    /// <para>
    /// The dependency can be injected into read only fields without issues,
    /// and as a matter of fact it is recommended to use read only fields.
    /// </para>
    /// <para>
    /// If you would like to run code after the dependencies have been injected, use <see cref="IPostInjectInit" />
    /// </para>
    /// </remarks>
    [AttributeUsage(AttributeTargets.Field)]
    public sealed class DependencyAttribute : Attribute
    {
    }
}

using System;

namespace Robust.Shared.IoC
{
    /// <summary>
    /// Specifies that the field this is applied to is a dependency,
    /// which will be resolved by <see cref="IoCManager" /> when the containing class is instantiated.
    /// </summary>
    /// <remarks>
    /// <para>
    /// The dependency is resolved as if <see cref="IoCManager.Resolve{T}()" /> were to be called,
    /// but it avoids circular references and init order issues due to internal code in the <see cref="IoCManager" />.
    /// </para>
    /// <para>
    /// The dependency can be injected into read only fields without issues,
    /// and as a matter of fact it is recommended to use read only fields.
    /// </para>
    /// <para>
    /// If you would like to run code after the dependencies have been injected, use <see cref="IPostInjectInit" />
    /// </para>
    /// </remarks>
    [AttributeUsage(AttributeTargets.Field)]
    public sealed class DependencyAttribute : Attribute
    {
    }
}

Angius•2y ago

Yeah, it does seem to be some weird DI framework

faintOP•2y ago

It’s RobustToolbox a game engine for SS14

Angius•2y ago

In that case, is the class you're injecting the dependency into registered in the IoC container?

faintOP•2y ago

can’t answer since I don’t know much about IoC and stuff sorry

Angius•2y ago

Time to hit the docs, then And see how IoC works in this toolbox thing

faintOP•2y ago

yeah, was going to visit a IoC docs page after I finish the thing Im working currently on lol

mtreit•2y ago

You could try not using dependency injection just to get it working

faintOP•2y ago

Currently trying another thing of getting a system for my class the main reason I asked it here is cause the error didn’t seem clear to me like uh I didn’t get what exactly is null also I just think that Dependency thing doesn’t work in classes that inherit Node. Cause I can surely say it works fine for _EntitySystem_s it finally works! Appreciate your help @mtreit @ZZZZZZZZZZZZZZZZZZZZZZZZZ

Angius•2y ago

Nice

faintOP•2y ago

Just had to use so called entitymanager to get the system though now I’m really curious how IoC works thanks once again!

Angius•2y ago

IoC/DI generally works by creating instances of what you think you'll need at some point, storing them in a container, and then injecting those instances where they're needed Basically, there's only one place where the things get created, so it's easy to swap them later if need be For example, instead of having to swap every var uploader = new ImgurUploader() for var uploader = new CatboxUploader(), you only swap services.AddSingleton<IUploader, ImgurUploader>() for services.AddSingleton<IUploader, CatboxUploader>() in one place And everything that requests an IUploader as a dependency will now get the new one

faintOP•2y ago

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System.NullReferenceException in an if statement