C# – Why is there no key specified with .NET KeyNotFoundException

cdictionarynet

Why is there no key specified with .NET KeyNotFoundException (in message/property)? Is it because of performance reasons? Is it because when Dictionary finds out there isnt any requested object by the hash it already doesnt know the context of the original key? I couldnt find an answer to this question anywhere.

Best Answer

This may not be the official answer, but...

C# and VB.NET - the main .NET languages - are statically typed languages(yea, I know, they support dynamic typing, but still). This means properties in classes are typed. What would the type of KeyNotFoundException.Key be?

Dictionary<string, string> dict1 = new Dictionary<string, string>();
try
{
    Console.WriteLine(dict1["one"]);
}
catch (KeyNotFoundException e)
{
    // is e.Key a string?
}

Dictionary<int, int> dict2 = new Dictionary<int, int>();
try
{
    Console.WriteLine(dict2[1]);
}
catch (KeyNotFoundException e)
{
    // is e.Key an int?
}

If you wanted to properly support this, you'd have to use generics - KeyNotFoundException<string> and KeyNotFoundException<int>. But this would complicate the API just so you can get information you already have.

Related Solutions

Architecture – Why does the .NET framework have no concept of classes as first-class types

.NET (the CLR) was the third generation of Microsoft's Component Object Model (COM), which in the early days referred to as the "COM+ runtime". Microsoft Visual Basic and the COM/ActiveX controls market has had much more influence on the specific CLR architectural compatability choices than Borland Delphi. (Admittedly the fact that that Delphi had adopted ActiveX controls certainly helped grow the ActiveX ecosystem, but COM/ActiveX existed before Delphi)

COM's architecture was worked out in C (not C++) and focused on interfaces, rather than classes. Furthermore supported object composition, meaning that a COM object could in fact be made up of several different objects, with the IUnknown interface linking them together. But IUnknown had no role in COM object creation, which was designed to be as language-independent as possible. Object creation was generally handled by IClassFactory, and reflection by ITypeLibrary and related interfaces. This separation of concerns was independent of implementation language, and the features of each core COM interface were kept minimal and orthogonal.

So as a result of the popularity of COM and ActiveX controls, .NET architecture was built to support on the COM IUnknown, IClassFactory, and ITypeLibrary. In COM, these interfaces were not necessarily on the same object, so lumping these together didn't necessarily make sense.

.NET Lookup Table – Is There a Key/Key Lookup Table Type in .NET?

There is no such datatype, probably because this is a very special requirement which can be easily solved by utilizing a dictionary and adding simply both pairs

 theDictionary.Add("foo", "bar");
 theDictionary.Add("bar", "foo");

Obviously, you can put this into a generic function like

void MyDictAdd(Dictionary<T,T> dict, T key1, T key2)
{
    dict.Add(key1,key2);
    dict.Add(key2,key1);
}

Best Answer

Related Solutions

Architecture – Why does the .NET framework have no concept of classes as first-class types

.NET Lookup Table – Is There a Key/Key Lookup Table Type in .NET?

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