C# try-catch-else

Question

One thing that has bugged me with exception handling coming from Python to C# is that in C# there doesn't appear to be any way of specifying an else clause. For example, in Python I could write something like this (Note, this is just an example. I'm not asking what is the best way to read a file):

try
{
    reader = new StreamReader(path);
}
catch (Exception)
{
    // Uh oh something went wrong with opening the file for reading
}
else
{
    string line = reader.ReadLine();
    char character = line[30];
}

From what I have seen in most C# code people would just write the following:

try
{
    reader = new StreamReader(path);
    string line = reader.ReadLine();
    char character = line[30];
}
catch (Exception)
{
    // Uh oh something went wrong, but where?
}

The trouble with this is that I don't want to catch out of range exception coming from the fact that the first line in the file may not contain more than 30 characters. I only want to catch exceptions relating to the reading of the file stream. Is there any similar construct I can use in C# to achieve the same thing?

Answer 1

You can do something similar like this:

bool passed = true;
try
{
    reader = new StreamReader(path);
}
catch (Exception)
{
    passed = false;
}
if (passed)
{
    // code that executes if the try catch block didnt catch any exception
}

Answer 2

If you happen to be in a loop, then you can put a continue statement in the catch blocks. This will cause the remaining code of that block to be skipped.

If you are not in a loop, then there is no need to catch the exception at this level. Let it propagate up the call stack to a catch block that knows what to do with it. You do this by eliminating the entire try/catch framework at the current level.

I like try/except/else in Python too, and maybe they will get added to C# some day (just like multiple return values were). But if you think about exceptions a little differently, else blocks are not strictly necessary.

Answer 3

There might not be any native support for try { ... } catch { ... } else { ... } in C#, but if you are willing to shoulder the overhead of using a workaround, then the example shown below might be appealing:

using System;

public class Test
{
    public static void Main()
    {
        Example("ksEE5A.exe");
    }

    public static char Example(string path) {
        var reader = default(System.IO.StreamReader);
        var line = default(string);
        var character = default(char);
        TryElse(
            delegate {
                Console.WriteLine("Trying to open StreamReader ...");
                reader = new System.IO.StreamReader(path);
            },
            delegate {
                Console.WriteLine("Success!");
                line = reader.ReadLine();
                character = line[30];
            },
            null,
            new Case(typeof(NullReferenceException), error => {
                Console.WriteLine("Something was null and should not have been.");
                Console.WriteLine("The line variable could not cause this error.");
            }),
            new Case(typeof(System.IO.FileNotFoundException), error => {
                Console.WriteLine("File could not be found:");
                Console.WriteLine(path);
            }),
            new Case(typeof(Exception), error => {
                Console.WriteLine("There was an error:");
                Console.WriteLine(error);
            }));
        return character;
    }

    public static void TryElse(Action pyTry, Action pyElse, Action pyFinally, params Case[] pyExcept) {
        if (pyElse != null && pyExcept.Length < 1) {
            throw new ArgumentException(@"there must be exception handlers if else is specified", nameof(pyExcept));
        }
        var doElse = false;
        var savedError = default(Exception);
        try {
            try {
                pyTry();
                doElse = true;
            } catch (Exception error) {
                savedError = error;
                foreach (var handler in pyExcept) {
                    if (handler.IsMatch(error)) {
                        handler.Process(error);
                        savedError = null;
                        break;
                    }
                }
            }
            if (doElse) {
                pyElse();
            }
        } catch (Exception error) {
            savedError = error;
        }
        pyFinally?.Invoke();
        if (savedError != null) {
            throw savedError;
        }
    }
}

public class Case {
    private Type ExceptionType { get; }
    public Action<Exception> Process { get; }
    private Func<Exception, bool> When { get; }

    public Case(Type exceptionType, Action<Exception> handler, Func<Exception, bool> when = null) {
        if (!typeof(Exception).IsAssignableFrom(exceptionType)) {
            throw new ArgumentException(@"exceptionType must be a type of exception", nameof(exceptionType));
        }
        this.ExceptionType = exceptionType;
        this.Process = handler;
        this.When = when;
    }

    public bool IsMatch(Exception error) {
        return this.ExceptionType.IsInstanceOfType(error) && (this.When?.Invoke(error) ?? true);
    }
}

Answer 4

Catch a specific class of exceptions

try
{
    reader = new StreamReader(path);
    string line = reader.ReadLine();
    char character = line[30];
}
catch (IOException ex)
{
    // Uh oh something went wrong with I/O
}
catch (Exception ex)
{
    // Uh oh something else went wrong
    throw; // unless you're very sure what you're doing here.
}

The second catch is optional, of course. And since you don't know what happened, swallowing this most general exception is very dangerous.

Answer 5

Exceptions are used differently in .NET; they are for exceptional conditions only.

In fact, you should not catch an exception unless you know what it means, and can actually do something about it.

Answer 6

Sounds like you want to do the second thing only if the first thing succeeded. And maybe catching different classes of exception is not appropriate, for example if both statements could throw the same class of exception.

try
{
    reader1 = new StreamReader(path1);
    // if we got this far, path 1 succeded, so try path2
    try
    {
        reader2 = new StreamReader(path2);

    }
    catch (OIException ex)
    {
        // Uh oh something went wrong with opening the file2 for reading
        // Nevertheless, have a look at file1. Its fine!
    }
}
catch (OIException ex)
{
    // Uh oh something went wrong with opening the file1 for reading.
    // So I didn't even try to open file2
}

Answer 7

Catch more specific exceptions.

try {
   reader = new StreamReader(path);
   string line = reader.ReadLine();
   char character = line[30];
}
catch(FileNotFoundException e) {
   // thrown by StreamReader constructor
}
catch(DirectoryNotFoundException e) {
   // thrown by StreamReader constructor
}
catch(IOException e) {
   // some other fatal IO error occured
}

Further, in general, handle the most specific exception possible and avoid handling the base System.Exception.

Answer 8

I have taken the liberty to transform your code a bit to demonstrate a few important points.

The using construct is used to open the file. If an exception is thrown you will have to remember to close the file even if you don't catch the exception. This can be done using a try { } catch () { } finally { } construct, but the using directive is much better for this. It guarantees that when the scope of the using block ends the variable created inside will be disposed. For a file it means it will be closed.

By studying the documentation for the StreamReader constructor and ReadLine method you can see which exceptions you may expect to be thrown. You can then catch those you finde appropriate. Note that the documented list of exceptions not always is complete.

// May throw FileNotFoundException, DirectoryNotFoundException,
// IOException and more.
try {
  using (StreamReader streamReader = new StreamReader(path)) {
    try {
      String line;
      // May throw IOException.
      while ((line = streamReader.ReadLine()) != null) {
        // May throw IndexOutOfRangeException.
        Char c = line[30];
        Console.WriteLine(c);
      }
    }
    catch (IOException ex) {
      Console.WriteLine("Error reading file: " + ex.Message);
    }
  }
}
catch (FileNotFoundException ex) {
  Console.WriteLine("File does not exists: " + ex.Message);
}
catch (DirectoryNotFoundException ex) {
  Console.WriteLine("Invalid path: " + ex.Message);
}
catch (IOException ex) {
  Console.WriteLine("Error reading file: " + ex.Message);
}

Answer 9

After seeing the other suggested solutions, here is my approach:

try {
    reader = new StreamReader(path);
}
catch(Exception ex) {
    // Uh oh something went wrong with opening the file stream
    MyOpeningFileStreamException newEx = new MyOpeningFileStreamException();
    newEx.InnerException = ex;
    throw(newEx);
}
    string line = reader.ReadLine();
    char character = line[30];

Of course, doing this makes sense only if you are interested in any exceptions thrown by opening the file stream (as an example here) apart from all other exceptions in the application. At some higher level of the application, you then get to handle your MyOpeningFileStreamException as you see fit.

Because of unchecked exceptions, you can never be 100% certain that catching only IOException out of the entire code block will be enough -- the StreamReader can decide to throw some other type of exception too, now or in the future.

Answer 10

You could write it like:

bool success = false;
try {
    reader = new StreamReader(path);
    success = true;
}
catch(Exception) {
    // Uh oh something went wrong with opening the file for reading
}
finally {
    if(success) {
        string line = reader.ReadLine();    
        char character = line[30];
    }
}   

Answer 11

Is there any similar construct I can use in C# to acheive the same thing?

No.

Wrap your index accessor with an "if" statement which is the best solution in your case in case of performance and readability.

if (line.length > 30) {
   char character = line [30];
} 

Answer 12

You can nest your try statements, too

Answer 13

More idiomatically, you would employ the using statement to separate the file-open operation from the work done on the data it contains (and include automatic clean-up on exit)

try {
  using (reader = new StreamReader(path))
  {
    DoSomethingWith(reader);
  }
} 
catch(IOException ex)
{
  // Log ex here
}

It is also best to avoid catching every possible exception -- like the ones telling you that the runtime is about to expire.

Answer 14

You can have multiple catch clauses, each specific to the type of exception you wish to catch. So, if you only want to catch IOExceptions, then you could change your catch clause to this:

try
{
    reader = new StreamReader(path);
    string line = reader.ReadLine();
    char character = line[30];
}
catch (IOException)
{    
}

Anything other than an IOException would then propagate up the call stack. If you want to also handle other exceptions, then you can add multiple exception clauses, but you must ensure they are added in most specific to most generic order. For example:

try
{
    reader = new StreamReader(path);
    string line = reader.ReadLine();
    char character = line[30];
}
catch (IOException)
{    
}
catch (Exception)
{
}

Answer 15

You can do this:

try
{
    reader = new StreamReader(path);
}
catch (Exception)
{
    // Uh oh something went wrong with opening the file for reading
}

string line = reader.ReadLine();
char character = line[30];

But of course, you will have to set reader into a correct state or return out of the method.