input: from typing import Union
  
  def convert_to_csharp(code: str) -> Union[str, None]:
      """ Convert a given Python function to a C# method. If the conversion is
      not possible, return None.
      >>> convert_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}'
      >>> convert_to_csharp("def greet(name):\n    print(f\"Hello, {name}!\")")
      None
      """
output:     import re
  
      # Check if the code is a valid Python function
      if not re.match(r"^\s*def\s+\w+\s*\(.*\)\s*:", code):
          return None
  
      # Extract function name and parameters
      match = re.match(r"^\s*def\s+(\w+)\s*\((.*)\)\s*:", code)
      if not match:
          return None
      func_name = match.group(1)
      params = match.group(2).split(",")
      param_list = []
      for param in params:
          param = param.strip()
          if ":" in param:
              param_name, param_type = param.split(":")
              param_list.append(f"{param_name.strip()} {param_type.strip()}")
          else:
              param_list.append(param.strip())
  
      # Convert parameters to C# format
      csharp_params = ", ".join(param_list)
  
      # Convert function body to C# format
      csharp_body = ""
      indent_level = 0
      for line in code.splitlines()[1:]:
          line = line.rstrip()
          if line.endswith(":"):
              line = line[:-1] + " {"
              indent_level += 1
          elif line.startswith("return "):
              line = line.replace("return ", "return ") + ";"
          elif line.startswith("print("):
              return None  # C# does not have a direct equivalent to Python's print
          elif line.strip() == "":
              continue
          else:
              line += ";"
  
          csharp_body += "    " * indent_level + line + "\n"
  
          if line.endswith("{"):
              indent_level += 1
          elif line.endswith("}"):
              indent_level -= 1
  
      # Construct the final C# method
      csharp_code = f"public int {func_name}({csharp_params})\n{{\n{csharp_body}}}"
  
      return csharp_code