Refactor nested IF statement for clarity

I want to refactor this mumbo jumbo of a method to make it more readible, it has way to many nested IF's for my liking.

How would you refactor this?

public static void HandleUploadedFile(string filename)
{
  try
  {
    if(IsValidFileFormat(filename)
    {
      int folderID = GetFolderIDFromFilename(filename);
      if(folderID > 0)
      {
        if(HasNoViruses(filename)
        {
          if(VerifyFileSize(filename)
          {
            // file is OK
            MoveToSafeFolder(filename);
          }
          else
          {
            DeleteFile(filename);
            LogError("file size invalid");
          }
        }
        else
        {
          DeleteFile(filename);
          LogError("failed virus test");
        }
      }
      else
      {
        DeleteFile(filename);
        LogError("invalid folder ID");
      }
    }
    else
    {
      DeleteFile(filename);
      LogError("invalid file format");
    }
  }
  catch (Exception ex)
  {
    LogError("unknown error", ex.Message);
  }
  finally
  {
    // do some things
  }
}

Answers


I would reverse the conditions in the test to if bad then deleteAndLog as the example below. This prevent nesting and puts the action near the test.

try{
    if(IsValidFileFormat(filename) == false){
        DeleteFile(filename);
        LogError("invalid file format");
        return;
    }

    int folderID = GetFolderIDFromFilename(filename);
    if(folderID <= 0){
        DeleteFile(filename);
        LogError("invalid folder ID");
        return;
    }
    ...

}...

Guard clauses.

For each condition, negate it, change the else block into the then block, and return.

Thus

if(IsValidFileFormat(filename)
{
   // then
}
else
{
   // else
}

Becomes:

if(!IsValidFileFormat(filename)
{
    // else 
    return;     
}
// then

If you are not against using exceptions, you could handle the checks without nesting.

Warning, air code ahead:

public static void HandleUploadedFile(string filename)
{
  try
  {
    int folderID = GetFolderIDFromFilename(filename);

    if (folderID == 0)
      throw new InvalidFolderException("invalid folder ID");

    if (!IsValidFileFormat(filename))
      throw new InvalidFileException("invalid file format!");

    if (!HasNoViruses(filename))
      throw new VirusFoundException("failed virus test!");

    if (!VerifyFileSize(filename))
      throw new InvalidFileSizeException("file size invalid");

    // file is OK
    MoveToSafeFolder(filename);
  }
  catch (Exception ex)
  {
    DeleteFile(filename);
    LogError(ex.message);
  }
  finally
  {
    // do some things
  }
}

One possible approach is to have single if statements that check for when the condition isn't true. Have a return for each one of these checks. This turns your method into a sequence of 'if' blocks instead of a nest.


There's not a lot to refactor here, as you keep the 3 tests separately due to the fact that the error messages relate to the test performed. You could opt for having the test methods report back the error to log so you don't have them in the if/else tree, which could make things simpler abit as you then could simply test for an error and log it + delete the file.


In David Waters reply, I don't like the repeated DeleteFile LogError pattern. I would either write a helper method called DeleteFileAndLog(string file, string error) or I would write the code like this:

public static void HandleUploadedFile(string filename)
{
    try
    {
        string errorMessage = TestForInvalidFile(filename);
        if (errorMessage != null)
        {
             LogError(errorMessage);
             DeleteFile(filename);
        }
        else
        {
            MoveToSafeFolder(filename);
        }
    }
    catch (Exception err)
    {
        LogError(err.Message);
        DeleteFile(filename);
    }
    finally { /* */ }
}

private static string TestForInvalidFile(filename)
{
    if (!IsValidFormat(filename))
        return "invalid file format.";
    if (!IsValidFolder(filename))
        return "invalid folder.";
    if (!IsVirusFree(filename))
        return "has viruses";
    if (!IsValidSize(filename))
        return "invalid size.";
    // ... etc ...
    return null;
 }

It's the elses above that throw my eye. Here's an alternative, inside the try {}

You can make this even shorter by returning after MoveToSafeFolder (Even though you're returning the finally block will be executed.) Then you don't need to assign an empty string to errorMessage, and you don't need to check is errorString empty before deleting the file and logging the message). I didn't do it here because many find early returns offensive, and I'd agree in this instance, since having the finally block execute after the return is unintuitive for many people.

Hope this helps

            string errorMessage = "invalid file format";
            if (IsValidFileFormat(filename))
            {
                errorMessage = "invalid folder ID";
                int folderID = GetFolderIDFromFilename(filename);
                if (folderID > 0)
                {
                    errorMessage = "failed virus test";
                    if (HasNoViruses(filename))
                    {
                        errorMessage = "file size invalid";
                        if (VerifyFileSize(filename))
                        {
                            // file is OK                                        
                            MoveToSafeFolder(filename);
                            errorMessage = "";
                        }
                    }
                }
            }
            if (!string.IsNullOrEmpty(errorMessage))
            {
                DeleteFile(filename);
                LogError(errorMessage);
            }

I would to something like this:

public enum FileStates {

MoveToSafeFolder = 1,

InvalidFileSize = 2,

FailedVirusTest = 3,

InvalidFolderID = 4,

InvalidFileFormat = 5,
}


public static void HandleUploadedFile(string filename) {
    try {
        switch (Handledoc(filename)) {
            case FileStates.FailedVirusTest:
                deletefile(filename);
                logerror("Virus");
                break;
            case FileStates.InvalidFileFormat:
                deletefile(filename);
                logerror("Invalid File format");
                break;
            case FileStates.InvalidFileSize:
                deletefile(filename);
                logerror("Invalid File Size");
                break;
            case FileStates.InvalidFolderID:
                deletefile(filename);
                logerror("Invalid Folder ID");
                break;
            case FileStates.MoveToSafeFolder:
                MoveToSafeFolder(filename);
                break;
        }
    }
    catch (Exception ex) {
        logerror("unknown error", ex.Message);
    }
}

private static FileStates Handledoc(string filename) {
    if (isvalidfileformat(filename)) {
        return FileStates.InvalidFileFormat;
    }
    if ((getfolderidfromfilename(filename) <= 0)) {
        return FileStates.InvalidFolderID;
    }
    if ((HasNoViruses(filename) == false)) {
        return FileStates.FailedVirusTest;
    }
    if ((VerifyFileSize(filename) == false)) {
        return FileStates.InvalidFileSize;
    }
    return FileStates.MoveToSafeFolder;
}

How about this?

public static void HandleUploadedFile(string filename)
{  
   try  
   {    
       if(!IsValidFileFormat(filename))        
       { DeleteAndLog(filename, "invalid file format"); return; }
       if(GetFolderIDFromFilename(filename)==0) 
       { DeleteAndLog(filename, "invalid folder ID");   return; }
       if(!HasNoViruses(filename))             
       { DeleteAndLog(filename, "failed virus test");   return; }
       if(!!VerifyFileSize(filename))          
       { DeleteAndLog(filename, "file size invalid");   return; }
       // --------------------------------------------------------
       MoveToSafeFolder(filename); 
   }
   catch (Exception ex)  { LogError("unknown error", ex.Message); throw; }
   finally {    // do some things  }
}     
private void DeleteAndLog(string fileName, string logMessage)
{
    DeleteFile(fileName);
    LogError(logMessage));
}

or, even better, ... this:

public static void HandleUploadedFile(string filename)
{  
   try  
   {    
       if(ValidateUploadedFile(filename))       
           MoveToSafeFolder(filename); 
   }
   catch (Exception ex)  { LogError("unknown error", ex.Message); throw; }
   finally {    // do some things  }
} 
private bool ValidateUploadedFile(string fileName)
{
   if(!IsValidFileFormat(filename))        
    { DeleteAndLog(filename, "invalid file format"); return false; }
   if(GetFolderIDFromFilename(filename)==0) 
    { DeleteAndLog(filename, "invalid folder ID");   return false; }
   if(!HasNoViruses(filename))             
    { DeleteAndLog(filename, "failed virus test");   return false; }
   if(!!VerifyFileSize(filename))          
    { DeleteAndLog(filename, "file size invalid");   return false; }
   // ---------------------------------------------------------------
   return true;

}    
private void DeleteAndLog(string fileName, string logMessage)
{
    DeleteFile(fileName);
    LogError(logMessage));
}

NOTE: You shouldn't be catching and swallowing generic Exception without rethrowing it...


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