Question

Why Cant I do something like this?

If I have a List<String> myList populated with items, I want to be able to act on each member in a conditional way like so:

myList.ForEach(a => { if (a.Trim().Length == 0) a = '0.0'; }) 

But this will not compile. Im guessing its something to do with missing a return value?

Im trying to prepare a list of strings for conversion to doubles, and I want the empty items to show ‘0.0’ so I can just convert the whole list in one go.

Answer 1

ForEach is not mutable, it doesn’t change the data structure in any way.

You can either:

1. Handle the loop yourself, with an index
2. Produce a new list, using .Select and .ToList (provided you’re using .NET 3.5)

Example of #1:

for (Int32 index = 0; index < myList.Count; index++)     if (myList[index].Trim().Length == 0)         myList[index] = '0.0'; 

With .NET 3.5 and Linq:

myList = (from a in myList           select (a.Trim().Length == 0) ? '0.0' : a).ToList(); 

With .NET 3.5, not using the Linq-syntax, but using the Linq-code:

myList = myList.Select(a => (a.Trim().Length == 0) ? '0.0' : a).ToList(); 

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Edit: If you want to produce a new list of doubles, you can also do that in one go using Linq:

List<Double> myDoubleList =     (from a in myList      select (a.Trim().Length == 0 ? '0' : a) into d      select Double.Parse(d)).ToList(); 

Note that using ‘0.0’ instead of just ‘0’ relies on the decimal point being the full stop character. On my system it isn’t, so I replaced it with just ‘0’, but a more appropriate way would be to change the call to Double.Parse to take an explicit numeric formatting, like this:

List<Double> myDoubleList =     (from a in myList      select (a.Trim().Length == 0 ? '0.0' : a) into d      select Double.Parse(d, CultureInfo.InvariantCulture)).ToList();