如果我没理解错的话，你有一个带小数部分的二进制数，就像

10.1 (binary)

你想把它转换成一个对应的十进制数，在上面的例子中，它是

2.5 (decimal)

如果这是你的任务，你可以用

private static decimal BinaryToDecimal(string value, char decimalSeparator = '.') {
  value = value.Trim();

  int sign = value.StartsWith('-') ? -1 : 1;

  value = value.Trim('-');

  int index = value.IndexOf(decimalSeparator);

  string integerPart = index >= 0 ? value.Substring(0, index) : value;

  decimal result = string.IsNullOrEmpty(integerPart)
    ? 0
    : Convert.ToInt64(integerPart, 2);

  string fractionalPart = index >= 0 ? value.Substring(index + 1) : "";

  for (int i = 0; i < fractionalPart.Length; ++i)
    result += (decimal)(fractionalPart[i] - '0') / (1L << (i + 1));

  return sign * result;
}

用法：

decimaltxtbox.Text = $"{BinaryToDecimal(binarytxtbox.Text)}";

演示：

using System.Linq;

...

string[] tests = {
  "0",
  "-0",
  "100",
  "-100",
  "10.",
  "-10.",
  "1.1",
  "101.101",
  "-101.101",
  ".1011",
  "-.11011",
  "100.00",
  ".", // <- It seems, you treat it as a valid input
};

string report = string.Join(Environment.NewLine, tests
  .Select(test => $"{test,10} => {BinaryToDecimal(test)}"));

Console.Write(report);

输出：

0 => 0
        -0 => 0
       100 => 4
      -100 => -4
       10. => 2
      -10. => -2
       1.1 => 1.5
   101.101 => 5.625
  -101.101 => -5.625
     .1011 => 0.6875
   -.11011 => -0.84375
    100.00 => 4
         . => 0

如果您正在寻找 reverse（即从十进制到二进制）转换，请查看
如何将十进制分数转换为十六进制分数？