C# – Declare a dictionary inside a static class

cdictionarynet

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".

Best Answer

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.

Related Solutions

Python – How to sort a list of dictionaries by a value of the dictionary

The sorted() function takes a key= parameter

newlist = sorted(list_to_be_sorted, key=lambda d: d['name'])

Alternatively, you can use operator.itemgetter instead of defining the function yourself

from operator import itemgetter
newlist = sorted(list_to_be_sorted, key=itemgetter('name'))

For completeness, add reverse=True to sort in descending order

newlist = sorted(list_to_be_sorted, key=itemgetter('name'), reverse=True)

C# – Should ‘using’ directives be inside or outside the namespace

There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:

// File1.cs
using System;
namespace Outer.Inner
{
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

Now imagine that someone adds another file (File2.cs) to the project that looks like this:

// File2.cs
namespace Outer
{
    class Math
    {
    }
}

The compiler searches Outer before looking at those using directives outside the namespace, so it finds Outer.Math instead of System.Math. Unfortunately (or perhaps fortunately?), Outer.Math has no PI member, so File1 is now broken.

This changes if you put the using inside your namespace declaration, as follows:

// File1b.cs
namespace Outer.Inner
{
    using System;
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

Now the compiler searches System before searching Outer, finds System.Math, and all is well.

Some would argue that Math might be a bad name for a user-defined class, since there's already one in System; the point here is just that there is a difference, and it affects the maintainability of your code.

It's also interesting to note what happens if Foo is in namespace Outer, rather than Outer.Inner. In that case, adding Outer.Math in File2 breaks File1 regardless of where the using goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any using directive.

Best Answer

Related Solutions

Python – How to sort a list of dictionaries by a value of the dictionary

C# – Should ‘using’ directives be inside or outside the namespace

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