score:205

Accepted answer

Easiest approach:

myList = myList.ConvertAll(d => d.ToLower());

Not too much different than your example code. ForEach loops the original list whereas ConvertAll creates a new one which you need to reassign.

score:-1

var _reps = new List(); // with variant data

_reps.ConvertAll<string>(new Converter<string,string>(delegate(string str){str = str.ToLower(); return str;})).Contains("invisible"))

score:3

[TestMethod]
public void LinqStringTest()
{
    List<string> myList = new List<string> { "aBc", "HELLO", "GoodBye" };
    myList = (from s in myList select s.ToLower()).ToList();
    Assert.AreEqual(myList[0], "abc");
    Assert.AreEqual(myList[1], "hello");
    Assert.AreEqual(myList[2], "goodbye");
}

score:4

ForEach uses Action<T>, which means that you could affect x if it were not immutable. Since x is a string, it is immutable, so nothing you do to it in the lambda will change its properties. Kyralessa's solution is your best option unless you want to implement your own extension method that allows you to return a replacement value.

score:53

That's because ToLower returns a lowercase string rather than converting the original string. So you'd want something like this:

List<string> lowerCase = myList.Select(x => x.ToLower()).ToList();

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