C#. How to return char?

I need to write a function that returns all characters that occur 2 or more times in the text. There isn't any problem when I use it without function (for example on button click). That's how I do it:

for (int i = 0; i < alph.Length; i++) // alph is my text(string)
        {
            int count = allText.Split(alphCh[i]).Length - 1; 
            if (count >= 2)                   
                listView2.Items.Add(alphCh[i].ToString());                
        }

That's how I write function:

public char[] chars2(string text)
    {
        char[] allChar = text.ToCharArray();
        string allText = text.ToString();
        string allTextL = text.ToLower();
        string alph = "abcdefghijklmnopqrstuvwxyz";
        char[] alphCh = alph.ToCharArray();  
        char[] result = new char[0];
        int allcount = 0;

        for (int i = 0; i < alph.Length; i++)
        {
            int count = allText.Split(alphCh[i]).Length - 1;
            if (count >= 2)
            {
                allcount++;
                result = new char[allcount];
                for (int j = 0; j < allcount; j++)
                {
        开发者_如何学Python            result[j] = alphCh[i];
                    return result;
                }
            }               
        }  
        return result;            
    }

But function returns just first character that occurs 2 or more times in the text. For example I write abcbca - func returns a, and I want to func returns a, b, c, to write it into ListView for example. What do I do wrong? Please, I need your help so much. Thanx.

If you have C# 3.0 or newer you can use LINQ:

char[] result = text
    .GroupBy(x => x)
    .Where(g => g.Count() > 1)
    .Select(g => g.Key)
    .ToArray();

Linq can simplify. Does this meet the requirements?

"aabbccpoiu".ToCharArray()
    .GroupBy(c => c)
    .Where(g => g.Count() > 1)
    .Select(g => g.Key)

It returns a, b and c.

Return a List instead, and use it inside the funciton as you do with the listview; just use List<char> rVal = new List<char>(); and then rVal.Add(character) if its not already added

Also remove the return inside the loop:

Just a suggestion=

public List<char> getMoreThanTwice(string text) {

char[] characters = text.toCharArray();
Dictionary<char, int> chars = new Dictionary<char, int>();
List<char> morethantwice = new List<char>();

for (int i=0;i<characters.Length;i++) {
  if (chars.containsKey(characters[i])) {
   chars[characters[i]] = chars[characters[i]] + 1;
  }else{
   chars.Add(characters[i], 1);
   }
}

foreach (KeyValuePair keypair in chars) {
   if (keypair.Value >= 2)  {
     morethantwice.Add(keypair.Key);
   }
 }

return morethantwice;
}

The first return result; will return directly when it has found the first result.