input: Write a function to convert a given Python program to a C# program. The function should accept the Python code as a string and return the equivalent C# code as a string. If the input string is empty, return "Invalid input".
  assert convert_python_to_csharp("def power(x, y):\n    if y == 0:\n        return 1\n    else:\n        return x * power(x, y - 1)") == "public int Power(int x, int y)\n{\n    if (y == 0)\n    {\n        return 1;\n    }\n    else\n    {\n        return x * Power(x, y - 1);\n    }\n}"
  assert convert_python_to_csharp("") == "Invalid input"
output: def convert_python_to_csharp(python_code):
      if not python_code:
          return "Invalid input"
      
      lines = python_code.split('\n')
      csharp_code = []
      
      for line in lines:
          if line.startswith("def "):
              line = line.replace("def ", "public ")
              line = line.replace(":", "")
              line = line.replace("):\n", ")\n{\n")
              line = line.replace("(", "(int ")
              line = line.replace(", ", ", int ")
              line = line.replace("):\n", ")\n{\n")
              csharp_code.append(line)
          elif line.startswith("    if "):
              line = line.replace("if ", "if (")
              line = line.replace(":", ")")
              line = line.replace("\n", "\n    {\n")
              csharp_code.append(line)
          elif line.startswith("    else:"):
              line = line.replace("else:", "else")
              line = line.replace("\n", "\n    {\n")
              csharp_code.append(line)
          elif line.startswith("        return "):
              line = line.replace("return ", "return ")
              line = line.replace("\n", ";\n    }\n")
              csharp_code.append(line)
          elif line.startswith("    return "):
              line = line.replace("return ", "return ")
              line = line.replace("\n", ";\n    }\n")
              csharp_code.append(line)
      
      csharp_code.append("}")
      return "\n".join(csharp_code)