String to char to strings

First off, the buffer is a char array. Arrays have a fixed size and to expand them you need to create a new one. To overcome this cumbersome work, there is a StringBuilder class that does this automatically.

Secondly, if you keep these 'Alpha', 'Beta', ... strings in if statements you will have a very long piece of code. You can replace this by using a dictionary, or create it from a single string or text file.

To put this into practice:

class MyClass
{
    static Dictionary<char, string> _map = new Dictionary<char, string>();

    static MyClass()
    {
        _map.Add('a', "Alpha");
        _map.Add('b', "Beta");
        // etc
    }

    static string WordMap(string data)
    {
        var output = new StringBuilder();
        foreach (char c in data)
        {
            if (_map.ContainsKey(c))
            {
                output.Append(_map[c]);
                output.Append(' ');
            }
        }
        return output.ToString();
    }
}

Solution without a dictionary:

static string WordMap(string data)
{
    const string words = "Alpha Beta Gamma Delta ...";

    string[] wordMap = words.Split(' ');

    var output = new StringBuilder();
    foreach (char c in data)
    {
        int index = c - 'a';
        if (index >= 0 && index < wordMap.Length)
        {
            output.Append(wordMap[index]);
            output.Append(' ');
        }
    }
    return output.ToString();
}

With LINQ and String.Join it's short and readable. Since you want to have a new word for special chards you need a word-map. I would use a Dictionary<char, string>:

static Dictionary<char, string> wordMap = new Dictionary<char, string>()
{
    {'a', "Alpha"}, {'b', "Beta"},{'c', "Gamma"}, {'d', "Delta"}
};

static string WordMap(string value)
{
    var strings = value
        .Select(c => 
        {
            string word; 
            if(!wordMap.TryGetValue(c, out word))
                word = c.ToString();
            return word;
        });
    return string.Join("", strings);
}

Test:

string newString = WordMap("abcdeghj");  // AlphaBetaGammaDeltaeghj

Tips: You don't have to create character array from string, you can easily access single characters in string by indexer:

char some = "123"[2];

When you use "" then you create string not char therefore you should use '' to create character for comparison:

if (some == 'a') Console.WriteLine("character is a, see how you compare chars!!!");

A good solution ...

    string[] words = { "Alpha", "Beta", "C_word", "D_Word" }; // ....

    string myPhrase = "aBC";

    myPhrase.Replace(" ", string.Empty).ToList().ForEach(a =>
    { 
        int asciiCode = (int)a;
        /// ToUpper()
        int index = asciiCode >= 97 ? asciiCode - 32 : asciiCode;
        Console.WriteLine(words[index - 65]); // ascii --> 65-a , 66-b ...
    });

One more variation of answer which contains not found option.

 static Dictionary<char, string> Mapping =
     new Dictionary<char, string>() 
         { { 'a', "alpha" }, { 'b', "beta" }, { 'c', "gamma" }, { 'd', "zeta" } };

    static void Main(string[] args)
    {            
        string test = "abcx";
        Console.WriteLine(string.Join(" ", test.Select(t => GetMapping(t))));  
        //output alpha beta gamma not found         
        Console.ReadKey();
    }

    static string GetMapping(char key)
    {
        if (Mapping.ContainsKey(key))
            return Mapping.First(a => a.Key == key).Value;
        else
            return "not found";            
    }      

Just declare a second variable where you build up your result. And I think you have some syntax problems, you need to have "==" in a condition, otherwise it's an assignment.

static string WordMap(string value)
{
      string result = string.Empty;
      char[] buffer = value.ToCharArray();
      for (int i = 0; i < buffer.Length; i++)
      {
          if (letter == "a")
          {
              result += ("Alpha");
          }
          //and so on            

       }

       return result;           
 }

But I would only do this that way, if this is "just for fun" code, as it will not be very fast. Building up the result like I did is slow, a better way would be

result = string.Concat(result, "(Alpha)");

And an even faster way is using a StringBuilder (s. documentation for that), which offers you fast and convenient methods to deal with bigger strings. Only downfall here is, that you need to know a little bit, how big the result will be in characters, as you need to provide a starting dimension. And here you should not start with simply 1, or 100. Each time, when the StringBuilder is full, it creates a new bigger instance and copies the values, so multiple instances of that will fill your memory, which can cause an out of memory exception, when dealing with some ten thousands of characters.

But as said, for just for fun code, all of that does not matter... And of course, you need to be aware, that if you do it like that, your result will be in one straight line, no breaks. If you want line breaks add "\n" at the end of the strings. Or add anything elese you need.

Regards,

Markus