怎么才能把小写字符串提取出来啊

string str = txtString.EditValue.ToString();
char[] c = str.ToCharArray();

        for (int i = 0; i < str.Length; i++)
        {
            if ((char)str[i] > 'a' && (char)str[i] < 'z')
            {
                foreach (char l in c)
                {
                    txtLowString.EditValue = string.Format(l.ToString());
                }
            }
            else
            {

            }

for里面你嵌套个foreach干嘛，

             if ((char)str[i] > 'a' && (char)str[i] < 'z')
            {

                    txtLowString.EditValue += str[i];

            }