C# nested dictionaries

Question

What is wrong with my syntax? I want to be able to get the value "Genesis" with this info["Gen"]["name"]

    public var info = new Dictionary<string, Dictionary<string, string>> {
    {"Gen", new Dictionary<string, string> {
    {"name", "Genesis"},
    {"chapters", "50"},
    {"before", ""},
    {"after", "Exod"}
    }},
    {"Exod", new Dictionary<string, string> {
    {"name", "Exodus"},
    {"chapters", "40"},
    {"before", "Gen"},
    {"after", "Lev"}
    }}};

Aside from the actual cause of the error, why are you using a Dictionary<string, string> as the value anyway, when it appears you really want a simple Chapter class? — Jon Skeet
– Jon Skeet, Commented Mar 19, 2013 at 13:46
Yeah I think the issue is "var" because it is directly in a class and not used in a method. What should I use instead? — user1898657
– user1898657, Commented Mar 19, 2013 at 13:48
I agree with Jon Skeet, I think that creating a class would be the correct route and if he wanted a Collection then he could create a List<T> of the Class itself. — MethodMan
– MethodMan, Commented Mar 19, 2013 at 13:51

Moha Dehghan · Accepted Answer · 2023-04-03 12:15:29Z

You cannot define a class field using var.

Change var to Dictionary<string, Dictionary<string, string>>:

public Dictionary<string, Dictionary<string, string>> info =
    new Dictionary<string, Dictionary<string, string>>
    {
        {
            "Gen",
            new Dictionary<string, string>
            {
                {"name", "Genesis"},
                {"chapters", "50"},
                {"before", ""},
                {"after", "Exod"}
            }
        },
        {
            "Exod",
            new Dictionary<string, string>
            {
                {"name", "Exodus"},
                {"chapters", "40"},
                {"before", "Gen"},
                {"after", "Lev"}
            }
        }
    };

See here for more information about var keyword and its usage.

In C# 9, with the introduction of target-typed new expressions, you can rewrite the field definition as:

public Dictionary<string, Dictionary<string, string>> info = new()
{
    {
        "Gen",
        new Dictionary<string, string>
        {
            {"name", "Genesis"},
            {"chapters", "50"},
            {"before", ""},
            {"after", "Exod"}
        }
    },
    {
        "Exod",
        new Dictionary<string, string>
        {
            {"name", "Exodus"},
            {"chapters", "40"},
            {"before", "Gen"},
            {"after", "Lev"}
        }
    }
};

Soner Gönül · Accepted Answer · 2013-03-19 13:56:48Z

From MSDN;

var can only be used when a local variable is declared and initialized in the same statement; the variable cannot be initialized to null, or to a method group or an anonymous function.

var cannot be used on fields at class scope.

Variables declared by using var cannot be used in the initialization expression.

Just change your var to Dictionary<string, Dictionary<string, string>>. Like;

public Dictionary<string, Dictionary<string, string>> info =
    new Dictionary<string, Dictionary<string, string>>{}

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