Question

Trying to reverse a string but getting error in Aggregate function

private string Reverse(string strValue)
        {

            char[] chArray = strValue.ToCharArray();
            var reverse = chArray.Reverse();

            var res = reverse.Aggregate((a,b)=>a+b);

            return res.ToString();
        }

Cannot implicitly convert type ‘int’ to ‘char’. An explicit conversion exists (are you missing a cast?)
So what is the mistake?

Answer 1

Exactly as it says: the result of a + b is an int, but you want it to be a char. To get it to compile you can just use:

var res = reverse.Aggregate((a,b)=>(char) (a+b));

… but I don’t think that will do what you want it to.

I suggest you don’t use LINQ for this to start with:

private string Reverse(string strValue)
{
    char[] chArray = strValue.ToCharArray();
    Array.Reverse(chArray);
    return new string(chArray);
}

Note that this doesn’t work properly with things like combining characters, surrogate pairs etc.

If you really wanted to use LINQ, you could do it with:

private string Reverse(string strValue)
{
    return new string(strValue.Reverse().ToArray());
}

There’s no need to call Aggregate. If you really want to make it slow and use string concatenation with Aggregate (it’ll be O(n²)) you can do it like this:

private string Reverse(string strValue)
{
    return strValue.Reverse().Aggregate("", (str, c) => str + c);
}

Here’s we’ve provided a string seed value, so the parameter str will be a string and + will be string concatenation, not character addition.

You can make it more efficient using StringBuilder:

private string Reverse(string strValue)
{
    return strValue.Reverse()
                   .Aggregate(new StringBuilder(), (sb, c) => sb.Append(c))
                   .ToString();
}

… but I’m still not sure it’s worth it…