Can all 'for' loops be replaced with a LINQ statement?

Question

Is it possible to write the following \'foreach\' as a LINQ statement, and I guess the more general question can any for loop be replaced by a LINQ statement.

I\'m not i

Answer 1

In fact, your code does something which is fundamentally very functional, namely it reduces a list of strings to a single string by concatenating the list items. The only imperative thing about the code is the use of a StringBuilder.

The functional code makes this much easier, actually, because it doesn’t require a special case like your code does. Better still, .NET already has this particular operation implemented, and probably more efficient than your code¹⁾:

return String.Join(", ", ListOfResources.Select(s => s.Id.ToString()).ToArray());

(Yes, the call to ToArray() is annoying but Join is a very old method and predates LINQ.)

Of course, a “better” version of Join could be used like this:

return ListOfResources.Select(s => s.Id).Join(", ");

The implementation is rather straightforward – but once again, using the StringBuilder (for performance) makes it imperative.

public static String Join(this IEnumerable items, String delimiter) {
    if (items == null)
        throw new ArgumentNullException("items");
    if (delimiter == null)
        throw new ArgumentNullException("delimiter");

    var strings = items.Select(item => item.ToString()).ToList();
    if (strings.Count == 0)
        return string.Empty;

    int length = strings.Sum(str => str.Length) +
                 delimiter.Length * (strings.Count - 1);
    var result = new StringBuilder(length);

    bool first = true;

    foreach (string str in strings) {
        if (first)
            first = false;
        else
            result.Append(delimiter);
        result.Append(str);
    }

    return result.ToString();
}

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¹⁾ Without having looked at the implementation in the reflector, I’d guess that String.Join makes a first pass over the strings to determine the overall length. This can be used to initialize the StringBuilder accordingly, thus saving expensive copy operations later on.

EDIT by SLaks: Here is the reference source for the relevant part of String.Join from .Net 3.5:

string jointString = FastAllocateString( jointLength );
fixed (char * pointerToJointString = &jointString.m_firstChar) {
    UnSafeCharBuffer charBuffer = new UnSafeCharBuffer( pointerToJointString, jointLength); 

    // Append the first string first and then append each following string prefixed by the separator. 
    charBuffer.AppendString( value[startIndex] ); 
    for (int stringToJoinIndex = startIndex + 1; stringToJoinIndex <= endIndex; stringToJoinIndex++) {
        charBuffer.AppendString( separator ); 
        charBuffer.AppendString( value[stringToJoinIndex] );
    }
    BCLDebug.Assert(*(pointerToJointString + charBuffer.Length) == '\0', "String must be null-terminated!");
}