c# modifying structs in a List<T>

Short question: How can I modify individual items in a List? (or more precisely, members of a struct stored in a List?)

Full explanation:

First, the struct definitions used below:

public struct itemInfo
{
    ...(Strings, Chars, boring)...
    public String nameStr;
    ...(you get the idea, nothing fancy)...
    public String subNum;   //BTW this is the element I'm trying to sort on
}

public struct slotInfo
{
    public Char catID;
    public String sortName;
    public Bitmap mainIcon;
    public IList<itemInfo> subItems;
}

public struct catInfo
{
    public Char catID;
    public String catDesc;
    public IList<slotInfo> items;
    public int numItems;
}

catInfo[] gAllCats = new catInfo[31];

gAllCats is populated on load, and so on down the line as the program runs.

The issue arises when I want to sort the itemInfo objects in the subItems array. I'm using LINQ to do this (because there doesn't seem to be any other reasonable way to sort lists of a non-builtin type). So here's what I have:

foreach (slotInfo sInf in gAllCats[c].items)
{
    var sortedSubItems =
        from itemInfo iInf in sInf.subItems
        orderby iInf.subNum ascending
        select iInf;
    IList<itemInfo> sortedSubTemp = new List<itemInfo();
    foreach (itemInfo iInf in sortedSubItems)
    {
        sortedSubTemp.Add(iInf);
    }
    sInf.subItems.Clear();
    sInf.subItems = sortedSubTemp;   // ERROR: see below
}

The error is, "Cannot modify members of 'sInf' because it is a 'foreach iteration variable'".

a, this restriction makes no sense; isn't that a primary use of the foreach construct?

b, (also out of spite) what does Clear() do if not modify the list? (BTW, the List does get cleared, according to the debugger, if I remove the last line and run it.)

c# arrays foreach

Edit Report

The problem you are having in your foreach is that structs are value types, and as a result, the loop iteration variable isn't actually a reference to the struct in the list, but rather a copy of the struct.

My guess would be the compiler is forbidding you change it because it most likely would not do what you expect it to anyway.

subItems.Clear() is less of a problem, because altho the field may be a copy of the element in the list, it is also a reference to the list (shallow copy).

The simplest solution would probably be to change from a struct to a class for this. Or use a completely different approach with a for (int ix = 0; ix < ...; ix++), etc.

answered 2009-07-01T05:18:29.070

c# modifying structs in a List<T>

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