Moving right along through the magical world of our .NET Exception Handling series, today we're going to cover the System.InvalidOperationException. The System.InvalidOperationException is a fairly common exception, as it is typically thrown when there's a failed attempt to invoke a method, caused by something other than invalid arguments passed to that method.
In this article, we'll examine where System.InvalidOperationException sits within the .NET exception hierarchy, look at why System.InvalidOperationExceptions typically appear, and see how to deal with them should you encounter one yourself. Let's get this party started!
The Technical Rundown
- All .NET exceptions are derived classes of the
System.Exceptionbase class, or derived from another inherited class therein. System.SystemExceptionis inherited from theSystem.Exceptionclass.System.InvalidOperationExceptionis inherited from theSystem.SystemExceptionclass.
When Should You Use It?
Unlike many exceptions in .NET, you may have noticed that System.InvalidOperationException resides quite close to the top of the exception hierarchy, as a direct descendent of System.SystemException. This typically indicates a .NET exception that is very common, or is applicable in many scenarios, and System.InvalidOperationException(arguably) falls into both camps.
The challenge with describing the System.InvalidOperationException is that it can occur in a wide variety of scenarios. As mentioned in the introduction, a System.InvalidOperationException is thrown when a method invocation fails for a reason other than invalid arguments. The official documentation gives a few potential examples of what might cause this, including:
- When an
IEnumerator.MoveNextcall is made on a collection that has since been modified after the enumerator was generated (more on this one later). - Calling
ResourceSet.GetStringif the resource was already closed prior to this call. - Modifying
XMLvia theXContainer.Addmethod, if doing so would generate invalidXML. - Attempting to modify the UI from a secondary thread that is not the main/UI thread.
This is by no means an exhaustive list, so it's impossible to cover all potential scenarios that a System.InvalidOperationException could occur. Instead, to continue our examination, we'll just take a look at one particular means by which a System.InvalidOperationException is typically raised: Modifying a collection after the enumeration of that collection has been generated, then continuing to utilize the enumeration.
To illustrate this in action, we've got our full code example below:
using System.Collections.Generic;
using Utility;namespace Airbrake.InvalidOperationException
{
class Program
{
static void Main(string[] args)
{
ValidExample();
Logging.Log("-----------------");
InvalidExample();
}private static void InvalidExample()
{
try
{
List<Book> books = new List<Book>();
books.Add(new Book("The Stand", "Stephen King"));
books.Add(new Book("Moby Dick", "Herman Melville"));
books.Add(new Book("Fahrenheit 451", "Ray Bradbury"));
books.Add(new Book("A Game of Thrones", "George R.R. Martin"));
books.Add(new Book("The Name of the Wind", "Patrick Rothfuss"));Logging.Log($"Total Book count: {books.Count}.");
Book newBook = new Book("Robinson Crusoe", "Daniel Defoe");
foreach (var book in books)
{
Logging.Log($"Current book title: {book.Title}, author: {book.Author}.");
if (!books.Contains(newBook))
{
books.Add(newBook);
Logging.Log($"Adding new book: {newBook.Title}, author: {newBook.Author}.");
}
Logging.Log($"New total Book count: {books.Count}.");
}
}
catch (System.InvalidOperationException exception)
{
Logging.Log(exception);
}
}private static void ValidExample()
{
try
{
List<Book> books = new List<Book>();
books.Add(new Book("The Stand", "Stephen King"));
books.Add(new Book("Moby Dick", "Herman Melville"));
books.Add(new Book("Fahrenheit 451", "Ray Bradbury"));
books.Add(new Book("A Game of Thrones", "George R.R. Martin"));
books.Add(new Book("The Name of the Wind", "Patrick Rothfuss"));Logging.Log($"Total Book count: {books.Count}.");
Book newBook = new Book("Robinson Crusoe", "Daniel Defoe");
int maxCount = books.Count - 1;
for (int index = 0; index <= maxCount; index++)
{
Logging.Log($"Current book title: {books[index].Title}, author: {books[index].Author}.");
if (!books.Contains(newBook))
{
books.Add(newBook);
Logging.Log($"Adding new book: {newBook.Title}, author: {newBook.Author}.");
}
}
Logging.Log($"New total Book count: {books.Count}.");
}
catch (System.InvalidOperationException exception)
{
Logging.Log(exception);
}
}
}public interface IBook
{
string Author { get; set; }
string Title { get; set; }
}public class Book : IBook
{
public string Title { get; set; }
public string Author { get; set; }public Book() { }
public Book(string title, string author)
{
Title = title;
Author = author;
}
}
}using System;
using System.Diagnostics;namespace Utility
{
/// <summary>
/// Houses all logging methods for various debug outputs.
/// </summary>
public static class Logging
{
/// <summary>
/// Outputs to <see cref="System.Diagnostics.Debug.WriteLine"/> if DEBUG mode is enabled,
/// otherwise uses standard <see cref="Console.WriteLine"/>.
/// </summary>
/// <param name="value">Value to be output to log.</param>
public static void Log(object value)
{
#if DEBUG
Debug.WriteLine(value);
#else
Console.WriteLine(value);
#endif
}/// <summary>
/// When <see cref="Exception"/> parameter is passed, modifies the output to indicate
/// if <see cref="Exception"/> was expected, based on passed in `expected` parameter.
/// <para>Outputs the full <see cref="Exception"/> type and message.</para>
/// </summary>
/// <param name="exception">The <see cref="Exception"/> to output.</param>
/// <param name="expected">Boolean indicating if <see cref="Exception"/> was expected.</param>
public static void Log(Exception exception, bool expected = true)
{
string value = $"[{(expected ? "EXPECTED" : "UNEXPECTED")}] {exception.ToString()}: {exception.Message}";
#if DEBUG
Debug.WriteLine(value);
#else
Console.WriteLine(value);
#endif
}
}
}
We won't get into detail about the Utility namespace, but instead we'll look at the Airbrake.InvalidOperationException namespace, where our problematic code resides. In this somewhat realistic example, we've got an interface called IBook, which specifies a few fields (Author and Title) for our Bookclass:
public interface IBook
{
string Author { get; set; }
string Title { get; set; }
}public class Book : IBook
{
public string Title { get; set; }
public string Author { get; set; }public Book() { }
public Book(string title, string author)
{
Title = title;
Author = author;
}
}
Now, using our Book class, our InvalidExample method generates a List<Book> collection called books, and adds a few of my favorites selections to the list. From there, we confirm that our collection contains the five Books we added, then we generate our Enumerator using the foreach loop block. Inside our loop, we output the current bookinstance, then create a newBook of Robinson Crusoe. We then call books.Add to add our newBook to the collection, and continue the loop using our previously established enumeration:
private static void InvalidExample()
{
try
{
List<Book> books = new List<Book>();
books.Add(new Book("The Stand", "Stephen King"));
books.Add(new Book("Moby Dick", "Herman Melville"));
books.Add(new Book("Fahrenheit 451", "Ray Bradbury"));
books.Add(new Book("A Game of Thrones", "George R.R. Martin"));
books.Add(new Book("The Name of the Wind", "Patrick Rothfuss"));Logging.Log($"Total Book count: {books.Count}.");
Book newBook = new Book("Robinson Crusoe", "Daniel Defoe");
foreach (var book in books)
{
Logging.Log($"Current book title: {book.Title}, author: {book.Author}.");
if (!books.Contains(newBook))
{
books.Add(newBook);
Logging.Log($"Adding new book: {newBook.Title}, author: {newBook.Author}.");
}
Logging.Log($"New total Book count: {books.Count}.");
}
}
catch (System.InvalidOperationException exception)
{
Logging.Log(exception);
}
}
The trouble is, our enumeration was established back when we had only five books, but before our second call to foreach, we've modified the collection and added a sixth book. Sure enough, this throws a System.InvalidOperationException:
Total Book count: 5.
Current book title: The Stand, author: Stephen King.
Adding new book: Robinson Crusoe, author: Daniel Defoe.
New total Book count: 6.
Exception thrown: 'System.InvalidOperationException' in mscorlib.dll
[EXPECTED] System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
If adding new items to the collection during the enumeration is a necessity, the best way to do it is to generate a static enumeration beforehand; one that doesn't directly access the modified collection each loop. For example, using for, instead of foreach, allows us to reference pre-determined indexes of the collection items, rather than direct collection items during the loop. Here is our slightly modified code to illustrate a successful fix:
private static void ValidExample()
{
try
{
List<Book> books = new List<Book>();
books.Add(new Book("The Stand", "Stephen King"));
books.Add(new Book("Moby Dick", "Herman Melville"));
books.Add(new Book("Fahrenheit 451", "Ray Bradbury"));
books.Add(new Book("A Game of Thrones", "George R.R. Martin"));
books.Add(new Book("The Name of the Wind", "Patrick Rothfuss"));Logging.Log($"Total Book count: {books.Count}.");
Book newBook = new Book("Robinson Crusoe", "Daniel Defoe");
int maxCount = books.Count - 1;
for (int index = 0; index <= maxCount; index++)
{
Logging.Log($"Current book title: {books[index].Title}, author: {books[index].Author}.");
if (!books.Contains(newBook))
{
books.Add(newBook);
Logging.Log($"Adding new book: {newBook.Title}, author: {newBook.Author}.");
}
}
Logging.Log($"New total Book count: {books.Count}.");
}
catch (System.InvalidOperationException exception)
{
Logging.Log(exception);
}
}
The key is to detach enumeration from the collection that is changing. We've initially stated that our enumerator will only loop through a total of the first five items (indexes), regardless of what is added to the collection afterward. This produces an expected result without a System.InvalidOperationException:
Total Book count: 5.
Current book title: The Stand, author: Stephen King.
Adding new book: Robinson Crusoe, author: Daniel Defoe.
Current book title: Moby Dick, author: Herman Melville.
Current book title: Fahrenheit 451, author: Ray Bradbury.
Current book title: A Game of Thrones, author: George R.R. Martin.
Current book title: The Name of the Wind, author: Patrick Rothfuss.
New total Book count: 6.
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