C# - Dictionary with generic array as value

Question

In my class, I want to use a dictionary with the following declaration:

Dictionary<string, T[]>

Since the operations of my class are exactly the same for all generic types, I do not wish to define my class as generic (which means I would have to create a separate instance of my class for each generic type I insert into the dictionary ?).

One alternative I'm attempting is to use Dictionary<string, object> instead:

public void Add<T>(string str, T value)
{
  // Assuming key already exists
  var array = (T[]) dictionary[str];
  array[0] = value;
}

However, when iterating over the dictionary, how do I cast the object value back to an array ?

foreach(string strKey in dictionary.Keys)
{
  var array = (T[]) dictionary[strKey];   // How to cast here ?
  //...
  array[0] = default(T);
}

Thanks.

What does an "cast an object back to its generic type" mean? — Ani
– Ani, Commented Jan 9, 2011 at 22:39
You have to add some code that shows how.where you want to introduce <T> — Henk Holterman
– Henk Holterman, Commented Jan 9, 2011 at 22:40
The dictionary is used in a buffer object. When it's time to flush the buffer, I want to iterate over the dictionary and read all values in all arrays regardless of the type (all value types btw) — alhazen
– alhazen, Commented Jan 9, 2011 at 22:43

Ben Voigt · Accepted Answer · 2011-01-09 22:48:27Z

2

You don't cast to T[], you use System.Array, the base class for all arrays.

var dictionary = new Dictionary<string, Array>();
foreach( var item in dictionary )
    item.Value.SetValue(null, 0);

Other operations you can do without having to cast to a specific array type are Array.Clear and Buffer.BlockCopy.

edited Jan 9, 2011 at 22:48

answered Jan 9, 2011 at 22:41

Ben Voigt

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4 Comments

Ani Over a year ago

+1: Nice. I was about to say "this won't work for value-types", but then I saw this: "If SetValue is used to assign null to an element of an array of value types, all fields of the element are initialized to zero." msdn.microsoft.com/en-us/library/kk3bwkb0.aspx

Ben Voigt Over a year ago

@Ani: Exactly. Although that did have me worried for a moment while I was thinking about the answer.

alhazen Over a year ago

Thanks. For the general case, how can I reset to default of T: item.Value.SetValue(default(T), 0) ?

Ani Over a year ago

@alhazen: This behaves as you expect - it should be the same as having used a default(T), (assuming you knew T at compile-time of course).

npinti · Accepted Answer · 2011-01-09 22:41:52Z

2

Since the operations of my class are exactly the same for all generic types

What you can do, is define all the methods which are the same in an interface and make your generic classes implement that interface. Once you do that, all that you need to do is to create a dictionary which takes Strings as keys and arrays of type InterfaceName. This should allow you to have generic values (instead of the object) and you will not need to type cast either.

answered Jan 9, 2011 at 22:41

npinti

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2 Comments

alhazen Over a year ago

In my case, T is always a value type, i.e. the dictionary value is usually int[] or float[]

Ben Voigt Over a year ago

"operations are the same" appears to be talking about operations on the collection, not the operations on elements of the arrays inside the collection.

Kirk Broadhurst · Accepted Answer · 2011-01-09 22:44:07Z

1

You should use a generic class. The advantage of using a generic class if that the object retrieved from the dictionary is type-safe and does not require casting - it will already be of the required type.

You say that 'the operations of my class are exactly the same for all generic types' - if that means what I think it means then it is merely reinforcing the usefulness of a generic class in your case.

answered Jan 9, 2011 at 22:44

Kirk Broadhurst

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2 Comments

Ben Voigt Over a year ago

The way I read the question, he's got a collection of arrays. The arrays are not all the same type, so making the containing collection generic doesn't help.

Kirk Broadhurst Over a year ago

@Ben Voigt - Interesting, I don't read that but I see what you mean. I was focusing on the declaration Dictionary<string, T[]>, (not the same as Dictionary<string, object[]>)

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