Declare a Dictionary inside a static class

Question

How to declare a static dictionary object inside a static class? I tried

public static class ErrorCode
{
    public const IDictionary<string , string > ErrorCodeDic
        =new Dictionary<string, string>()
             {
            {"1","User name or password problem"}     
};
}

But the compiler complains that "A const field of a reference type other than string can only be initialized with null".

Answer 1

public static class ErrorCode
{
    public const IDictionary<string , string > m_ErrorCodeDic;

    public static ErrorCode()
    {
      m_ErrorCodeDic = new Dictionary<string, string>()
             { {"1","User name or password problem"} };             
    }
}

Probably initialise in the constructor.

Answer 2

You can use the static/class constructor to initialize your dictionary:

public static class ErrorCode
{
    public const IDictionary<string, string> ErrorCodeDic;
    public static ErrorCode()
    {
        ErrorCodeDic = new Dictionary<string, string>()
            { {"1", "User name or password problem"} };
    }
}

Answer 3

Make the Dictionary a static, and never add to it outside of your static object's ctor. That seems to be a simpler solution than fiddling with the static/const rules in C#.

Answer 4

The correct syntax ( as tested in VS 2008 SP1), is this:

public static class ErrorCode
{
    public static IDictionary<string, string> ErrorCodeDic;
     static ErrorCode()
    {
        ErrorCodeDic = new Dictionary<string, string>()
            { {"1", "User name or password problem"} };
    }
}

Answer 5

If you want to declare the dictionary once and never change it then declare it as readonly:

private static readonly Dictionary<string, string> ErrorCodes
    = new Dictionary<string, string>
{
    { "1", "Error One" },
    { "2", "Error Two" }
};

If you want to dictionary items to be readonly (not just the reference but also the items in the collection) then you will have to create a readonly dictionary class that implements IDictionary.

Check out ReadOnlyCollection for reference.

BTW const can only be used when declaring scalar values inline.

Answer 6

The problem with your initial example was primarily due to the use of const rather than static; you can't create a non-null const reference in C#.

I believe this would also have worked:

public static class ErrorCode
{
    public static IDictionary<string, string> ErrorCodeDic
        = new Dictionary<string, string>()
            { {"1", "User name or password problem"} };
}

Also, as Y Low points out, adding readonly is a good idea as well, and none of the modifiers discussed here will prevent the dictionary itself from being modified.

Answer 7

OK - so I'm working in ASP 2.x (not my choice...but hey who's bitching?).

None of the initialize Dictionary examples would work. Then I came across this: http://kozmic.pl/archive/2008/03/13/framework-tips-viii-initializing-dictionaries-and-collections.aspx

...which hipped me to the fact that one can't use collections initialization in ASP 2.x.