Is there any way in C# to improve exception messages?
I'm tired of getting an unexpected error like Index was out of range or Key not found in dictionary without the most important informations.
I mean, if index was out of bounds, tell me what was the value of index.
If the key does not exist, tell me what key does not exist.
I know I can use try catch with more detailed message.
try
{
dictionary[key] = value;
}
catch (KeyNotFoundException)
{
Console.WriteLine("Key not found. Key value: " + key)
}
However I'm quite tired of this, especially since recreating bugs is not always easy.
EDIT:
I've created simple dictionary inheritance.
using System.Collections.Generic;
public class ExcDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
new public TValue this[TKey key]
{
get
{
try
{
return base[key];
}
catch (KeyNotFoundException)
{
throw new KeyNotFoundException("KeyNotFoundException: The given key (" + key.ToString() + ") was not present in the dictionary.");
}
}
set
{
base[key] = value;
}
}
}
A possible solution is to use a wrapper class around Dictionary.
Something like:
public class CustomDictionary<TKey, TValue>
{
private readonly Dictionary<TKey, TValue> _dictionary;
public CustomDictionary(Dictionary<TKey, TValue> dictionary)
{
_dictionary = dictionary;
}
public TValue this[TKey key]
{
get
{
try
{
return _dictionary[key];
}
catch (KeyNotFoundException)
{
Console.WriteLine("CustomDictionary: key not found in GET. Key value: " + key);
throw; // could also 'return default;' instead, depending on what you want to get when key is not found.
}
}
set
{
try
{
_dictionary[key] = value;
}
catch (KeyNotFoundException)
{
Console.WriteLine("CustomDictionary: key not found in SET. Key value: " + key);
}
}
}
}
Notes:
you can avoid the try catch statements by testing if the key is contained in the dictionary instead. Try catch block can be costly in performance. See doc on ContainsKey: https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2.containskey?view=net-6.0
I would also define an extension method .ToCustomDictionary like the Linq method .ToDictionary as a helper to make this class easy easy to use in your code.
Some doc ref:
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/indexers/using-indexers.
This is a very simple "decorator" around the Dictionary, and if you need to access other Dictionary methods, you could either just create them, or expose the original dictionary as a public instead of a private field.
Alternative solution:
Create your SafeSet extension method:
public static class DictionaryExtensions
{
public static void SafeSet<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key, TValue value)
{
if (dictionary.ContainsKey(key))
{
dictionary[key] = value;
}
else
{
// Here you can just add the missing key without any error!
dictionary.Add(key, value);
// of course you could also display an error message / throw your own exception if you want.
}
}
}
// Usage:
var anyDictionary = new Dictionary<int, string>();
anyDictionary.SafeSet(42, "I'll be added to the dictionary");
anyDictionary.SafeSet(42, "Now I'll replace the existing value as usual.");
Related
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How to get null instead of the KeyNotFoundException accessing Dictionary value by key?
I currently have lots of Dictionary<string, T> uses in my project, and most of them look like so:
if (myDic.ContainsKey("some key"))
localVar = myDic["some key"];
It's not very effecient too, as it does two calls to the dictionary, which can be resource consuming. TryGetValue() is a cool thing, but it just doesn't do it in one line.
I just want to get null if there is no such key from var v = myDic[key]. How do I do that?
You may use an extension method with TryGetValue:
public static U GetValueByKeyOrNull<T, U>(this Dictionary<T, U> dict, T key)
where U : class
{
U value;
dict.TryGetValue(key, out value);
return value;
}
thanks to which you'll be able to write
somedict.GetValueByKeyOrNull("key1")
In the end trying to do this very thing I came up with a variant using a deriving from dictionary class with explicit interface implementation: How to get null instead of the KeyNotFoundException accessing Dictionary value by key?
That is
public interface INullValueDictionary<T, U>
where U : class
{
U this[T key] { get; }
}
public class NullValueDictionary<T, U> : Dictionary<T, U>, INullValueDictionary<T, U>
where U : class
{
U INullValueDictionary<T, U>.this[T key]
{
get
{
U val;
dict.TryGet(key, out val);
return val;
}
}
}
and use it instead of the original dictionary everywhere:
//create some dictionary
NullValueDictionary<int, string> dict = new NullValueDictionary<int, string>
{
{1,"one"}
};
//have a reference to the interface
INullValueDictionary<int, string> idict = dict;
string val = idict[2]; // null
val = idict[1]; // "one"
I don't like to deal with null so my implementation will look like this:
interface Maybe<T> {
bool HasValue {get;}
T Value {get;}
}
class Nothing<T> : Maybe<T> {
public bool HasValue { get { return false; } }
public T Value { get { throw new Exception(); } }
public static const Nothing<T> Instance = new Nothing<T>();
}
class Just<T> : Maybe<T> {
private T _value;
public bool HasValue { get { return true; } }
public T Value { get { return _value; } }
public Just(T val) {
_value = val;
}
}
Maybe is a object that can contain value or not. Note that Nothing class contains static field Instance. We can use this value instead of creating new value each time we need to return Nothing from function.
Now, we need to create our own dictionary class:
class MyDictionary<TKey, TValue>
{
private Dictionary<TKey, TValue> _dict;
...
public Maybe<TValue> this[TKey key] {
TValue val;
if (_dict.TryGetValue(key, out val)) {
return new Just<TValue>(val);
return Nothing<TValue>.Instance;
}
}
Advantage of this approach is not clear, because C# doesn't have pattern matching. But it can be emulated with dynamic:
void ProcessResult(Just<string> val) {
Console.WriteLine(val);
}
void ProcessResult(Nothing<string> n) {
Console.WriteLine("Key not found");
}
var dict = new MyDictionary<string, string>();
...
dynamic x = dict["key"];
ProcessResult(x);
I think that this is very clear way to express the fact that dictionary can't always return meaningful result. Also it is obvious for reader that function overload ProcessResult(Just<T>) will be called only for values that present in dictionary and other overload will be called in case when key is not found.
Pros:
Type serves as a specification.
Dictionary can contain both value and reference types.
Cons:
More keystrokes.
Little more complexity to deal with.
I decided to do it like this:
class MyDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
public new TValue this[TKey key]
{
get
{
TValue value;
return TryGetValue(key, out value) ? value : default(TValue);
}
set { base[key] = value; }
}
}
It lets me use it like any other dictionary, through square brackets. Since I'm not going to use this with value types as TValue, I think it's good enough a solution.
This question already has answers here:
Is there an IDictionary implementation that, on missing key, returns the default value instead of throwing?
(17 answers)
Closed 5 years ago.
In some certain scenario it appeared to be useful for me to have a short-spoken, readable way to get null instead of the KeyNotFoundException while accessing dictionary value by key, when there is no such key in the dictionary.
The first thing that came into my mind was an extension method:
public static U GetValueByKeyOrNull<T, U>(this Dictionary<T, U> dict, T key)
where U : class //it's acceptable for me to have this constraint
{
if (dict.ContainsKey(key))
return dict[key];
else
//it could be default(U) to use without U class constraint
//however, I didn't need this.
return null;
}
But it's not very short-spoken actually, when you write something like this:
string.Format("{0}:{1};{2}:{3}",
dict.GetValueByKeyOrNull("key1"),
dict.GetValueByKeyOrNull("key2"),
dict.GetValueByKeyOrNull("key3"),
dict.GetValueByKeyOrNull("key4"));
I'd say, it would be much better to havesomething close to base syntax: dict["key4"].
Then I came up with an idea to have a class with a private dictionary field, which exposed the functionality I need:
public class MyDictionary<T, U> //here I may add any of interfaces, implemented
//by dictionary itself to get an opportunity to,
//say, use foreach, etc. and implement them
// using the dictionary field.
where U : class
{
private Dictionary<T, U> dict;
public MyDictionary()
{
dict = new Dictionary<T, U>();
}
public U this[T key]
{
get
{
if (dict.ContainsKey(key))
return dict[key];
else
return null;
}
set
{
dict[key] = value;
}
}
}
But it seems a little overhead to get the slight change in the basic behaviour.
One more workaround could be to define a Func in the current context like this:
Func<string, string> GetDictValueByKeyOrNull = (key) =>
{
if (dict.ContainsKey(key))
return dict[key];
else
return null;
};
so it could be utilized like GetDictValueByKeyOrNull("key1").
Could you, please, give me any more suggestions or help to choose a better one?
Here is my solution from my personal library, implemented as an extension method. I am only posting it because it is implemented from the dictionary interface and allows an optional default value to be passed in.
Implementation
public static TV GetValue<TK, TV>(this IDictionary<TK, TV> dict, TK key, TV defaultValue = default(TV))
{
TV value;
return dict.TryGetValue(key, out value) ? value : defaultValue;
}
Usage
MyDictionary.GetValue("key1");
MyDictionary.GetValue("key2", -1);
MyDictionary.GetValue("key3")?.SomeMethod();
You can't get the syntax you want with an extension method, and as others have said overriding a method/operator to change its behavior is generally not a great idea. I think the best you can do is shorten the name you use.
That's if you need to keep to the IDictionary interface. If you aren't interfacing with any code that expects an IDictionary, then you are free to define your own interface and having the [] operator work differently isn't a problem.
Whatever you end up calling the function, you'll want to implement it like this:
public static U Get<T, U>(this Dictionary<T, U> dict, T key)
where U : class
{
U val;
dict.TryGetValue(key, out val);
return val;
}
It just does one lookup, compared with 2 for your implementations.
In the end I came up with a variant using a deriving from dictionary class with explicit interface implementation:
public interface INullValueDictionary<T, U>
where U : class
{
U this[T key] { get; }
}
public class NullValueDictionary<T, U> : Dictionary<T, U>, INullValueDictionary<T, U>
where U : class
{
U INullValueDictionary<T, U>.this[T key]
{
get
{
U val;
this.TryGetValue(key, out val);
return val;
}
}
}
So it exposes the functionality I need the following way:
//create some dictionary
NullValueDictionary<int, string> dict = new NullValueDictionary<int, string>
{
{1,"one"}
};
//have a reference to the interface
INullValueDictionary<int, string> idict = dict;
try
{
//this throws an exception, as the base class implementation is utilized
Console.WriteLine(dict[2] ?? "null");
}
catch { }
//this prints null, as the explicit interface implementation
//in the derived class is used
Console.WriteLine(idict[2] ?? "null");
Add a DictionaryExtension class
public static class DictionaryExtension
{
public static TValue GetValueOrDefault<TKey, TValue>
( this IDictionary<TKey, TValue> dictionary,TKey key)
{
TValue value;
return dictionary.TryGetValue(key, out value) ? value : default(TValue);
}
}
And it can return Default value if not found key in dictionary.
The Default is null if this is reference type .
_dic.GetValueOrDefault();
Worth pointing out that the HybridDictionary does this by default.
You lose the generic typeing, but get the null-if-not-found functionality.
And (at least theoretically) you get performance benefits at low numbers of values, I think.
I premise by saying that I would not use this. The new keyword, while useful in this case, can create bugs which are really hard to find. Other than that, you can try this class.
class MyDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
public new TValue this[TKey key]
{
get
{
TValue value;
return TryGetValue(key, out value) ? value : default(TValue);
}
set { base[key] = value; }
}
}
I've done this before and it worked pretty well to just inherit the regular Dictionary class and just hide the indexer. It's really clean to do it this way so you automatically acquire all the reliability and familiarity of the Regular Dictionary class.
public class NullSafeDict<TKey, TValue> : Dictionary<TKey, TValue> where TValue : class
{
public new TValue this[TKey key]
{
get
{
if (!ContainsKey(key))
return null;
else
return base[key];
}
set
{
if (!ContainsKey(key))
Add(key, value);
else
base[key] = value;
}
}
}
I've seen posts here on how to make a dictionary that has multiple values per key, like one of the solutions presented in this link:
Multi Value Dictionary
it seems that I have to use a List<> as the value for the keys, so that a key can store multiple values.
the solution in the link is fine if you want to add values. But my problem now is how to remove specific values from a single key.
I have this code for adding values to a dictionary:
private Dictionary<TKey, List<TValue>> mEventDict;
// this is for initializing the dictionary
public void Subscribe(eVtEvtId inEvent, VtEvtDelegate inCallbackMethod)
{
if (mEventDict.ContainsKey(inEvent))
{
mEventDict[inEvent].Add(inCallbackMethod);
}
else
{
mEventDict.Add(inEvent, new List<TValue>() { v });
}
}
// this is for adding values to the dictionary.
// if the "key" (inEvent) is not yet present in the dictionary,
// the key will be added first before the value
my problem now is removing a specific value from a key. I have this code:
public void Unsubscribe(eVtEvtId inEvent, VtEvtDelegate inCallbackMethod)
{
try
{
mEventDict[inEvent].Remove(inCallbackMethod);
}
catch (ArgumentNullException)
{
MessageBox.Show("The event is not yet present in the dictionary");
}
}
basically, what I did is just replace the Add() with Remove() . Will this work?
Also, if you have any problems or questions with the code (initialization, etc.), feel free to ask.
Thanks for the advice.
TylerOhlsen's answer is a step in the right direction, but it has 6 key lookups (calls to Remove, ContainsKey, and the indexer). This can be reduced to three by using TryGetValue:
private Dictionary<TKey, List<TValue>> mEventDict;
public void Subscribe(TKey inEvent, TValue inCallbackMethod)
{
List<TValue> list;
if (mEventDict.TryGetValue(inEvent, out list))
list.Add(inCallbackMethod);
else
mEventDict.Add(inEvent, new List<TValue> { inCallbackMethod });
}
public bool Unsubscribe(TKey inEvent, TValue inCallbackMethod)
{
List<TValue> list;
if (!mEventDict.TryGetValue(inEvent, out list))
return false;
bool removed = list.Remove(inCallbackMethod);
if (list.Count == 0)
mEventDict.Remove(inEvent);
return removed;
}
If you don't care about removing empty lists:
public bool Unsubscribe(TKey inEvent, TValue inCallbackMethod)
{
List<TValue> list;
if (!mEventDict.TryGetValue(inEvent, out list))
return false;
return list.Remove(inCallbackMethod);
}
If you don't need to report whether the item was in the list (and therefore removed from it), change the return type to void, and (in the first version) get rid of the removed variable.
Will it work? Not exactly the way you intended...
Your method parameters will need to be of the generic types.
List(T).Remove does not throw an ArgumentNullException.
You might want to clean up your dictionary if your list becomes empty.
The caller might not care if the callback was ever subscribed when they unsubscribe, but you have that information so you might as well return it. This information could be helpful for troubleshooting/logging purposes.
This is what I would recommend...
private Dictionary<TKey, List<TValue>> mEventDict;
public void Subscribe(TKey inEvent, TValue inCallbackMethod)
{
if (!mEventDict.ContainsKey(inEvent))
mEventDict.Add(inEvent, new List<TValue>());
mEventDict[inEvent].Add(inCallbackMethod);
}
public bool Unsubscribe(TKey inEvent, TValue inCallbackMethod)
{
if (!mEventDict.ContainsKey(inEvent))
return false;
bool removed = mEventDict[inEvent].Remove(inCallbackMethod);
if (mEventDict[inEvent].Count == 0)
mEventDict.Remove(inEvent);
return removed;
}
NOTE: I have not tested this code, so just try it out. Also, this code is not thread safe.
#phoog - so I want to keep the Unsubscribe method as void . After modifying your code, this is what I came up with...
public void Unsubscribe(TKey inEvent, TValue inCallbackMethod)
{
List<TValue> list;
bool mRemoved = false.
if (mEventDict.TryGetValue(inEvent, out list))
{
list.Remove(inCallbackMethod);
mRemoved = true;
}
}
is the listRemoved variable necessary? But then again, I think nothing will happen if the inCallbackMethod cannot be found in the list.
All I want is a dictionary which tells me which key it couldn't find, rather than just saying The given key was not present in the dictionary.
I briefly considered doing a subclass with override new this[TKey key], but felt it was a bit hacky, so I've gone with implementing the IDictionary interface, and passing everything through directly to an inner Dictionary, with the only additional logic being in the indexer:
public TValue this[TKey key]
{
get
{
ThrowIfKeyNotFound(key);
return _dic[key];
}
set
{
ThrowIfKeyNotFound(key);
_dic[key] = value;
}
}
private void ThrowIfKeyNotFound(TKey key)
{
if(!_dic.ContainsKey(key))
throw new ArgumentOutOfRangeException("Can't find key [" + key + "] in dictionary");
}
Is this the right/only way to go? Would newing over the this[] really be that bad?
Sounds like a good fit for an extension method:
public static class SomeUtilClass {
public static TValue VerboseGetValue<TKey, TValue>(
this IDictionary<TKey, TValue> data, TKey key)
{
TValue result;
if (!data.TryGetValue(key, out result)) {
throw new KeyNotFoundException(
"Key not found: " + Convert.ToString(key));
}
return result;
}
}
This will then work on all your existing dictionaries whenever you call VerboseGetValue, for example:
var data = new Dictionary<int, string> { { 123, "abc" } };
Console.WriteLine(data.VerboseGetValue(123));
Console.WriteLine(data.VerboseGetValue(456));
Instead of doing ContainsKey and checking for the presence of the key before touching the underlying dictionary, why not do
get {
try {
return _dic[key];
}
catch (ArgumentOutOfRangeException) {
throw new ArgumentOutOfRangeException(......);
}
}
That way, you only pay for the extra checking in the failure case - the success case, which is hopefully more common, doesn't have to do an extra dictionary lookup. This is good for get, but set is more difficult since the default behaviour of set is to always work. If you don't want that then you would need to check for the existence of the key first.
If you want to do this, you are going to have to roll your own in one way or another. But I'm going to question WHY you would want to do this?
I need to add key/object pairs to a dictionary, but I of course need to first check if the key already exists otherwise I get a "key already exists in dictionary" error. The code below solves this but is clunky.
What is a more elegant way of doing this without making a string helper method like this?
using System;
using System.Collections.Generic;
namespace TestDictStringObject
{
class Program
{
static void Main(string[] args)
{
Dictionary<string, object> currentViews = new Dictionary<string, object>();
StringHelpers.SafeDictionaryAdd(currentViews, "Customers", "view1");
StringHelpers.SafeDictionaryAdd(currentViews, "Customers", "view2");
StringHelpers.SafeDictionaryAdd(currentViews, "Employees", "view1");
StringHelpers.SafeDictionaryAdd(currentViews, "Reports", "view1");
foreach (KeyValuePair<string, object> pair in currentViews)
{
Console.WriteLine("{0} {1}", pair.Key, pair.Value);
}
Console.ReadLine();
}
}
public static class StringHelpers
{
public static void SafeDictionaryAdd(Dictionary<string, object> dict, string key, object view)
{
if (!dict.ContainsKey(key))
{
dict.Add(key, view);
}
else
{
dict[key] = view;
}
}
}
}
Just use the indexer - it will overwrite if it's already there, but it doesn't have to be there first:
Dictionary<string, object> currentViews = new Dictionary<string, object>();
currentViews["Customers"] = "view1";
currentViews["Customers"] = "view2";
currentViews["Employees"] = "view1";
currentViews["Reports"] = "view1";
Basically use Add if the existence of the key indicates a bug (so you want it to throw) and the indexer otherwise. (It's a bit like the difference between casting and using as for reference conversions.)
If you're using C# 3 and you have a distinct set of keys, you can make this even neater:
var currentViews = new Dictionary<string, object>()
{
{ "Customers", "view2" },
{ "Employees", "view1" },
{ "Reports", "view1" },
};
That won't work in your case though, as collection initializers always use Add which will throw on the second Customers entry.
What's wrong with...
dict[key] = view;
It'll automatically add the key if it's non-existent.
simply
dict[key] = view;
From the MSDN documentation of Dictionary.Item
The value associated with the
specified key. If the specified key is
not found, a get operation throws a
KeyNotFoundException, and a set
operation creates a new element with
the specified key.
My emphasis
As usual John Skeet gets in there with lighting speed with the right answer, but interestingly you could also have written your SafeAdd as an Extension Method on IDictionary.
public static void SafeAdd(this IDictionary<K, T>. dict, K key, T value)...
Although using the indexer is clearly the right answer for your specific problem, another more general answer to the problem of adding additional functionality to an existing type would be to define an extension method.
Obviously this isn't a particularly useful example, but something to bear in mind for the next time you find a real need:
public static class DictionaryExtensions
{
public static void SafeAdd<TKey, TValue>(this Dictionary<TKey, TValue> dict,
TKey key, TValue value)
{
dict[key] = value;
}
}