using System;

var sourceString = "AABBCCCDAA";

var resultString = "";

char? lastChar = null;

for (int index = 0; index < sourceString.Length; index++)

{

    if (sourceString[index] != lastChar)

    {

        resultString += sourceString[index];

        lastChar = sourceString[index];

    }

}

Console.WriteLine(resultString);