Question

I was wondering if there was a reasonable way to customize messages on exceptions that are thrown by the .NET framework? Below is a chunk of code that I write often, in many different scenarios to achieve the effect of providing reasonable exception messages to my users.

public string GetMetadata(string metaDataKey)
{
  // As you can see, I am doing what the dictionary itself will normally do, but my exception message has some context, and is therefore more descriptive of the actual problem.
  if (!_Metadata.ContainsKey(metaDataKey))
  {
    throw new KeyNotFoundException(string.Format("There is no metadata that contains the key '{0}'!", metaDataKey));
  }

  // This will throw a 'KeyNotFoundException' in normal cases, which I want, but the message "The key is not present in the dictionary" is not very informative.  This is the exception who's message I wish to alter.
  string val = _Metadata[metaDataKey].TrimEnd();
  return val;
}

As you can see, I am essentially producing duplicate code just to use a different (better) message.

Edit:
What I am looking for, essentially is something like this:

KeyNotFoundException.SetMessage("this is my custom message!")
{
// OK, now this will send off the message I want when the exception appears!
// Now I can avoid all of that silly boilerplate!
string val = _Metadata[metaDataKey].TrimEnd();
}

At any rate, i don’t think that such a feature exists, but if it did I would be very pleased indeed. Has anyone tackled this type of problem before? It’s looking like I am going to wind up needed some type of extension method in the end…

Answer 1

Here is a solution that I came up with, but I would like to note that it is more of a patch than anything. It does work, but probably isn’t suitable for all applications. I couldn’t even think of a good name for it either.

  public class ContextDictionary<TKey, TValue> : Dictionary<TKey, TValue>
  {
    public TValue this[TKey key, string context]
    {
      get
      {
        if (!this.ContainsKey(key))
        {
          throw new KeyNotFoundException(string.Format("There is no {0} that contains the key '{1}'!", context, key));
        }
        return this[key];
      }
      set { this[key] = value; }
    }
  }

So now I can say something like this, and get the more descriptive exception message that I really want.

var _MetaData = new ContextDictionary<string,string>();
string val = _Metadata[metaDataKey, "metadata"].TrimEnd();