Question

Ok.. I have a really awkward problem that I believe is related with how C# handles value types vs reference types but I’m just not sure what exactly the bug is.

public partial class LogicSimulationViewerForm : Form
    {

    private Dictionary<string, PointStruct> pointValues;


private void SearchPoint(string code)
    {
        ReadDefaultPointValuesResponse result = ddcdao.ReadDefaultPoint(points);
        pointValues = new Dictionary<string, PointStruct>();

        for (int j = 0; j < result.pointidentifier.Length; j++)
        {
            if (!pointValues.ContainsKey(result.pointidentifier[j]))
            {
                PointStruct ps = new PointStruct();
                ps.name = "Random String"; 
                ps.pointidentifier = result.pointidentifier[j];
                ps.outofservice = result.outofservice[j];

                pointValues.Add(result.pointidentifier[j], ps);
                ...

pointValues is stored as a private field in a class. Now in the same class but in a different function, if I try to do the following:

PointStruct ps = pointValues[s];
MessageBox.Show(ps.name);
MessageBox.Show(ps.pointidentifier);
MessageBox.Show(ps.outofservice);

The ps.pointidentifier and ps.outofservice is displayed correctly but ps.name is always returned as null no matter what I do. How can I fix this issue?

Edit: Upon request, I am adding more code to further illustrate the problem:

public struct PointStruct
{
    public string pointidentifier;
    public string affect;
    public string outofservice;
    public string priorityarray;
    public string pointtype;
    public string alarmstate;
    public string correctvalue;
    public string presentvalue;
    public string name;
    public string test;
}

Answer 1

As long as there is no voodoo (explicit field layouts, property indirection, etc), there is absolutely no reason why a field should wipe itself, regardless of whether it is a class or a struct.

If it was a class, we could perhaps put that down to a careless update somewhere else, i.e.

var foo = pointValues[key];
// snip 2000 lines
foo.name = newValue; // which happens to be null

which would of course update the same fundamental object as the one referenced by the dictionary. But that doesn’t apply to a struct, since the copies are separate (unless updating directly in an array).

The only way I can see of causing that, given that you state that pointValues.Add(...) is only used in one place, is that you are overwriting it elsewhere via the indexer:

pointValues[key] = newValueWithANullName;

All that said, though; unless you have some very specific reasons, there is very little purpose for PointStruct to be a struct. That looks to me like it should be a class. It is very “fat” for a struct. Also; in most cases, structs should be immutable.