How to convert Turkish chars to English chars in a string?

var text = "ÜST";
var unaccentedText  = String.Join("", text.Normalize(NormalizationForm.FormD)
        .Where(c => char.GetUnicodeCategory(c) != UnicodeCategory.NonSpacingMark));

You can use the following method for solving your problem. The other methods do not convert "Turkish Lowercase I (\u0131)" correctly.

public static string RemoveDiacritics(string text)
{
    Encoding srcEncoding = Encoding.UTF8;
    Encoding destEncoding = Encoding.GetEncoding(1252); // Latin alphabet

    text = destEncoding.GetString(Encoding.Convert(srcEncoding, destEncoding, srcEncoding.GetBytes(text)));

    string normalizedString = text.Normalize(NormalizationForm.FormD);
    StringBuilder result = new StringBuilder();

    for (int i = 0; i < normalizedString.Length; i++)
    {
        if (!CharUnicodeInfo.GetUnicodeCategory(normalizedString[i]).Equals(UnicodeCategory.NonSpacingMark))
        {
            result.Append(normalizedString[i]);
        }
    }

    return result.ToString();
}

I'm not an expert on this sort of thing, but I think you can use string.Normalize to do it, by decomposing the value and then effectively removing an non-ASCII characters:

using System;
using System.Linq;
using System.Text;

class Test
{
    static void Main()
    {
        string text = "\u00DCST";
        string normalized = text.Normalize(NormalizationForm.FormD);
        string asciiOnly = new string(normalized.Where(c => c < 128).ToArray());
        Console.WriteLine(asciiOnly);
    }    
}

It's entirely possible that this does horrible things in some cases though.