Dynamicly add diacritics to characters

I'm not aware of an inbuilt solution for this. Normally such a thing would be done by the IME as Griff says.

You probably need to create a lookup table, which is logically a two-dimensional map and which you probably would implement as Dictionary<char, Dictionary<char,char>>. Populate it in a static constructor like so:

Dictionary<char, Dictionary<char,char>> lookupTable = new Dictionary<char, Dictionary<char,char>>;
static MyClass(){
 lookupTable['a'] = new Dictionary<char,char>();
 lookupTable['a']['"'] = 'ä';
 lookupTable['a']['o'] = 'å';
 // ... etc

I'm not sure if there is even a standard set of diacritical marks from the ASCII set, and there's not much point using the ones in Unicode since if the user can enter those, they can enter the character they want directly. Those you need for ANSI (Western Europe) are grave (è), acute (é), umlaut (ä), circumflex (â), ring (å), slash (ø), caron (š), tilde (ñ) and cedila (ç); sensible characters to use to mark them would be `, ', ", ^, o, v, ~ and c. In the ANSI set there are also eth and thorn, which are not really diacritics of a normal character; oe and ae diphthongs; and the German ß, to think about.

OriginalGriff is right there is no algorithm able to deal with all cases. The only option is to deal with all the possibilities one by one.

You could use an enum to list all diacritics, and dictionaries for each supported letter. For example:

enum Diacritic
{
    //"é"
    Acute,
    //"è"
    Grave,
    //"ê"
    Circonflexe,
    ...
}

//supported letters for e
static Dictionary<Diacritic, char> letterE;
//supported letters for a
static Dictionary<Diacritic, char> letterA;
//supported letters
static Dictionary<char, Dictionary<Diacritic, char>> letters;

//call this function to initialize dictionaries
//or put its code it a static constructor
static void Init()
{
    //all possibilities for "e"
    letterE = new Dictionary<Diacritic, char>();
    letterE[Diacritic.Acute] = 'é';
    letterE[Diacritic.Grave] = 'è';
    letterE[Diacritic.Circonflexe] = 'ê';
    ...

    //all possibilities for "a"
    letterA = new Dictionary<Diacritic, char>();
    letterA[Diacritic.Grave] = 'à';
    letterA[Diacritic.Circonflexe] = 'â';
    ...

    //supported letters
    letters = new Dictionary<char, Dictionary<Diacritic, char>>();
    letters.Add('a', letterA);
    letters.Add('e', letterE);
    ...
}

//will throw a KeyNotFoundException if the requested character doesn't exist
static char AddDiacritics(char letter, Diacritic diacritic)
{
    return letters[letter][diacritic];
}

This code is not very elegant but I am not sure if a more elegant solution exists for that problem...

As Original Griff said there is no built in support. As others said can do using a lookup table. Got this from net, a handy class

public class DiacritMerger
{
    static readonly Dictionary<char, char> _lookup = new Dictionary<char, char>
                     {
                         {'\'', '\u0301'},
                         {'"', '\u0308'},
                         {'^', '\u0302'}
                     };
    public static string AddDiacritics(string asciiBase, string diacrits)
    {
        var combined = asciiBase.Zip(diacrits, (ascii, diacrit) => DiacritVersion(diacrit, ascii));
        return new string(combined.ToArray());
    }
    private static char DiacritVersion(char diacrit, char character)
    {
        char combine;
        return _lookup.TryGetValue(diacrit, out combine) ? new string(new[] { character, combine }).Normalize()[0] : character;
    }
}

Then can use like

MessageBox.Show(DiacritMerger.AddDiacritics("u", "^"));
MessageBox.Show(DiacritMerger.AddDiacritics("e", "\""));

Although this is not really a solution to the problem, it might be usefull to know and might possibly even help build the table used by other solutions.

A string can be normalized to different form. In my case, I want to do the opposite (remove diacritics), and I have used:

string stFormD = fileName.Normalize(NormalizationForm.FormD);
for (int ich = 0; ich < stFormD.Length; ich++)
{
    char currentChar = stFormD[ich];

    System.Globalization.UnicodeCategory uc =
        System.Globalization.CharUnicodeInfo.GetUnicodeCategory(currentChar);

    if (uc != System.Globalization.UnicodeCategory.NonSpacingMark)
    {
        //...
    }
}

I don't know but maybe it would be possible to try for each possible Char to normalize it (the corresponding string) to the "D" form and then build information from that.

However, I haven't found a way to split character like œ to oe so in my case, I handle a few characters manually.