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Best practices for exceptions

  • Članek

Proper exception handling is essential for application reliability. You can intentionally handle expected exceptions to prevent your app from crashing. However, a crashed app is more reliable and diagnosable than an app with undefined behavior.

This article describes best practices for handling and creating exceptions.

Handling exceptions

The following best practices concern how you handle exceptions:

Use try/catch/finally blocks to recover from errors or release resources

For code that can potentially generate an exception, and when your app can recover from that exception, use try/catch blocks around the code. In catch blocks, always order exceptions from the most derived to the least derived. (All exceptions derive from the Exception class. More derived exceptions aren't handled by a catch clause that's preceded by a catch clause for a base exception class.) When your code can't recover from an exception, don't catch that exception. Enable methods further up the call stack to recover if possible.

Clean up resources that are allocated with either using statements or finally blocks. Prefer using statements to automatically clean up resources when exceptions are thrown. Use finally blocks to clean up resources that don't implement IDisposable. Code in a finally clause is almost always executed even when exceptions are thrown.

Handle common conditions to avoid exceptions

For conditions that are likely to occur but might trigger an exception, consider handling them in a way that avoids the exception. For example, if you try to close a connection that's already closed, you'll get an InvalidOperationException. You can avoid that by using an if statement to check the connection state before trying to close it.

if (conn.State != ConnectionState.Closed)
{
    conn.Close();
}

If conn.State <> ConnectionState.Closed Then
    conn.Close()
End IF

If you don't check the connection state before closing, you can catch the InvalidOperationException exception.

try
{
    conn.Close();
}
catch (InvalidOperationException ex)
{
    Console.WriteLine(ex.GetType().FullName);
    Console.WriteLine(ex.Message);
}

Try
    conn.Close()
Catch ex As InvalidOperationException
    Console.WriteLine(ex.GetType().FullName)
    Console.WriteLine(ex.Message)
End Try

The approach to choose depends on how often you expect the event to occur.

  • Use exception handling if the event doesn't occur often, that is, if the event is truly exceptional and indicates an error, such as an unexpected end-of-file. When you use exception handling, less code is executed in normal conditions.

  • Check for error conditions in code if the event happens routinely and could be considered part of normal execution. When you check for common error conditions, less code is executed because you avoid exceptions.

    Note

    Up-front checks eliminate exceptions most of the time. However, there can be race conditions where the guarded condition changes between the check and the operation, and in that case, you could still incur an exception.

Call Try* methods to avoid exceptions

If the performance cost of exceptions is prohibitive, some .NET library methods provide alternative forms of error handling. For example, Int32.Parse throws an OverflowException if the value to be parsed is too large to be represented by Int32. However, Int32.TryParse doesn't throw this exception. Instead, it returns a Boolean and has an out parameter that contains the parsed valid integer upon success. Dictionary<TKey,TValue>.TryGetValue has similar behavior for attempting to get a value from a dictionary.

Catch cancellation and asynchronous exceptions

It's better to catch OperationCanceledException instead of TaskCanceledException, which derives from OperationCanceledException, when you call an asynchronous method. Many asynchronous methods throw an OperationCanceledException exception if cancellation is requested. These exceptions enable execution to be efficiently halted and the callstack to be unwound once a cancellation request is observed.

Asynchronous methods store exceptions that are thrown during execution in the task they return. If an exception is stored into the returned task, that exception will be thrown when the task is awaited. Usage exceptions, such as ArgumentException, are still thrown synchronously. For more information, see Asynchronous exceptions.

Design classes so that exceptions can be avoided

A class can provide methods or properties that enable you to avoid making a call that would trigger an exception. For example, the FileStream class provides methods that help determine whether the end of the file has been reached. You can call these methods to avoid the exception that's thrown if you read past the end of the file. The following example shows how to read to the end of a file without triggering an exception:

class FileRead
{
    public static void ReadAll(FileStream fileToRead)
    {
        ArgumentNullException.ThrowIfNull(fileToRead);

        int b;

        // Set the stream position to the beginning of the file.
        fileToRead.Seek(0, SeekOrigin.Begin);

        // Read each byte to the end of the file.
        for (int i = 0; i < fileToRead.Length; i++)
        {
            b = fileToRead.ReadByte();
            Console.Write(b.ToString());
            // Or do something else with the byte.
        }
    }
}

Class FileRead
    Public Sub ReadAll(fileToRead As FileStream)
        ' This if statement is optional
        ' as it is very unlikely that
        ' the stream would ever be null.
        If fileToRead Is Nothing Then
            Throw New System.ArgumentNullException()
        End If

        Dim b As Integer

        ' Set the stream position to the beginning of the file.
        fileToRead.Seek(0, SeekOrigin.Begin)

        ' Read each byte to the end of the file.
        For i As Integer = 0 To fileToRead.Length - 1
            b = fileToRead.ReadByte()
            Console.Write(b.ToString())
            ' Or do something else with the byte.
        Next i
    End Sub
End Class

Another way to avoid exceptions is to return null (or default) for most common error cases instead of throwing an exception. A common error case can be considered a normal flow of control. By returning null (or default) in these cases, you minimize the performance impact to an app.

For value types, consider whether to use Nullable<T> or default as the error indicator for your app. By using Nullable<Guid>, default becomes null instead of Guid.Empty. Sometimes, adding Nullable<T> can make it clearer when a value is present or absent. Other times, adding Nullable<T> can create extra cases to check that aren't necessary and only serve to create potential sources of errors.

Restore state when methods don't complete due to exceptions

Callers should be able to assume that there are no side effects when an exception is thrown from a method. For example, if you have code that transfers money by withdrawing from one account and depositing in another account, and an exception is thrown while executing the deposit, you don't want the withdrawal to remain in effect.

public void TransferFunds(Account from, Account to, decimal amount)
{
    from.Withdrawal(amount);
    // If the deposit fails, the withdrawal shouldn't remain in effect.
    to.Deposit(amount);
}

Public Sub TransferFunds(from As Account, [to] As Account, amount As Decimal)
    from.Withdrawal(amount)
    ' If the deposit fails, the withdrawal shouldn't remain in effect.
    [to].Deposit(amount)
End Sub

The preceding method doesn't directly throw any exceptions. However, you must write the method so that the withdrawal is reversed if the deposit operation fails.

One way to handle this situation is to catch any exceptions thrown by the deposit transaction and roll back the withdrawal.

private static void TransferFunds(Account from, Account to, decimal amount)
{
    string withdrawalTrxID = from.Withdrawal(amount);
    try
    {
        to.Deposit(amount);
    }
    catch
    {
        from.RollbackTransaction(withdrawalTrxID);
        throw;
    }
}

Private Shared Sub TransferFunds(from As Account, [to] As Account, amount As Decimal)
    Dim withdrawalTrxID As String = from.Withdrawal(amount)
    Try
        [to].Deposit(amount)
    Catch
        from.RollbackTransaction(withdrawalTrxID)
        Throw
    End Try
End Sub

This example illustrates the use of throw to rethrow the original exception, making it easier for callers to see the real cause of the problem without having to examine the InnerException property. An alternative is to throw a new exception and include the original exception as the inner exception.

catch (Exception ex)
{
    from.RollbackTransaction(withdrawalTrxID);
    throw new TransferFundsException("Withdrawal failed.", innerException: ex)
    {
        From = from,
        To = to,
        Amount = amount
    };
}

Catch ex As Exception
    from.RollbackTransaction(withdrawalTrxID)
    Throw New TransferFundsException("Withdrawal failed.", innerException:=ex) With
    {
        .From = from,
        .[To] = [to],
        .Amount = amount
    }
End Try

Capture and rethrow exceptions properly

Once an exception is thrown, part of the information it carries is the stack trace. The stack trace is a list of the method call hierarchy that starts with the method that throws the exception and ends with the method that catches the exception. If you rethrow an exception by specifying the exception in the throw statement, for example, throw e, the stack trace is restarted at the current method and the list of method calls between the original method that threw the exception and the current method is lost. To keep the original stack trace information with the exception, there are two options that depend on where you're rethrowing the exception from:

  • If you rethrow the exception from within the handler (catch block) that's caught the exception instance, use the throw statement without specifying the exception. Code analysis rule CA2200 helps you find places in your code where you might inadvertently lose stack trace information.
  • If you're rethrowing the exception from somewhere other than the handler (catch block), use ExceptionDispatchInfo.Capture(Exception) to capture the exception in the handler and ExceptionDispatchInfo.Throw() when you want to rethrow it. You can use the ExceptionDispatchInfo.SourceException property to inspect the captured exception.

The following example shows how the ExceptionDispatchInfo class can be used, and what the output might look like.

ExceptionDispatchInfo? edi = null;
try
{
    var txt = File.ReadAllText(@"C:\temp\file.txt");
}
catch (FileNotFoundException e)
{
    edi = ExceptionDispatchInfo.Capture(e);
}

// ...

Console.WriteLine("I was here.");

if (edi is not null)
    edi.Throw();

If the file in the example code doesn't exist, the following output is produced:

I was here.
Unhandled exception. System.IO.FileNotFoundException: Could not find file 'C:\temp\file.txt'.
File name: 'C:\temp\file.txt'
   at Microsoft.Win32.SafeHandles.SafeFileHandle.CreateFile(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.Strategies.FileStreamHelpers.ChooseStrategyCore(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.StreamReader.ValidateArgsAndOpenPath(String path, Encoding encoding, Int32 bufferSize)
   at System.IO.File.ReadAllText(String path, Encoding encoding)
   at Example.ProcessFile.Main() in C:\repos\ConsoleApp1\Program.cs:line 12
--- End of stack trace from previous location ---
   at Example.ProcessFile.Main() in C:\repos\ConsoleApp1\Program.cs:line 24

Throwing exceptions

The following best practices concern how you throw exceptions:

Use predefined exception types

Introduce a new exception class only when a predefined one doesn't apply. For example:

Note

While it's best to use predefined exception types when possible, you shouldn't raise some reserved exception types, such as AccessViolationException, IndexOutOfRangeException, NullReferenceException and StackOverflowException. For more information, see CA2201: Do not raise reserved exception types.

Use exception builder methods

It's common for a class to throw the same exception from different places in its implementation. To avoid excessive code, create a helper method that creates the exception and returns it. For example:

class FileReader
{
    private readonly string _fileName;

    public FileReader(string path)
    {
        _fileName = path;
    }

    public byte[] Read(int bytes)
    {
        byte[] results = FileUtils.ReadFromFile(_fileName, bytes) ?? throw NewFileIOException();
        return results;
    }

    static FileReaderException NewFileIOException()
    {
        string description = "My NewFileIOException Description";

        return new FileReaderException(description);
    }
}

Class FileReader
    Private fileName As String


    Public Sub New(path As String)
        fileName = path
    End Sub

    Public Function Read(bytes As Integer) As Byte()
        Dim results() As Byte = FileUtils.ReadFromFile(fileName, bytes)
        If results Is Nothing
            Throw NewFileIOException()
        End If
        Return results
    End Function

    Function NewFileIOException() As FileReaderException
        Dim description As String = "My NewFileIOException Description"

        Return New FileReaderException(description)
    End Function
End Class

Some key .NET exception types have such static throw helper methods that allocate and throw the exception. You should call these methods instead of constructing and throwing the corresponding exception type:

Tip

The following code analysis rules can help you find places in your code where you can take advantage of these static throw helpers: CA1510, CA1511, CA1512, and CA1513.

If you're implementing an asynchronous method, call CancellationToken.ThrowIfCancellationRequested() instead of checking if cancellation was requested and then constructing and throwing OperationCanceledException. For more information, see CA2250.

Include a localized string message

The error message the user sees is derived from the Exception.Message property of the exception that was thrown, and not from the name of the exception class. Typically, you assign a value to the Exception.Message property by passing the message string to the message argument of an Exception constructor.

For localized applications, you should provide a localized message string for every exception that your application can throw. You use resource files to provide localized error messages. For information on localizing applications and retrieving localized strings, see the following articles:

Use proper grammar

Write clear sentences and include ending punctuation. Each sentence in the string assigned to the Exception.Message property should end in a period. For example, "The log table has overflowed." uses correct grammar and punctuation.

Place throw statements well

Place throw statements where the stack trace will be helpful. The stack trace begins at the statement where the exception is thrown and ends at the catch statement that catches the exception.

Don't raise exceptions in finally clauses

Don't raise exceptions in finally clauses. For more information, see code analysis rule CA2219.

Don't raise exceptions from unexpected places

Some methods, such as Equals, GetHashCode, and ToString methods, static constructors, and equality operators, shouldn't throw exceptions. For more information, see code analysis rule CA1065.

Throw argument validation exceptions synchronously

In task-returning methods, you should validate arguments and throw any corresponding exceptions, such as ArgumentException and ArgumentNullException, before entering the asynchronous part of the method. Exceptions that are thrown in the asynchronous part of the method are stored in the returned task and don't emerge until, for example, the task is awaited. For more information, see Exceptions in task-returning methods.

Custom exception types

The following best practices concern custom exception types:

End exception class names with Exception

When a custom exception is necessary, name it appropriately and derive it from the Exception class. For example:

public class MyFileNotFoundException : Exception
{
}

Public Class MyFileNotFoundException
    Inherits Exception
End Class

Include three constructors

Use at least the three common constructors when creating your own exception classes: the parameterless constructor, a constructor that takes a string message, and a constructor that takes a string message and an inner exception.

For an example, see How to: Create user-defined exceptions.

Provide additional properties as needed

Provide additional properties for an exception (in addition to the custom message string) only when there's a programmatic scenario where the additional information is useful. For example, the FileNotFoundException provides the FileName property.

See also