自定义词典只读属性

Question

I created a custom dictionary to implement the IDataErrorInfo interface on it, but when I try to set a value to an instance of this Dictionary:

Custom Dictionary:

public class MyDictionary<TKey, TValue> : Dictionary<TKey, TValue>, IDataErrorInfo
{

    public string Error
    {
        get { throw new NotImplementedException(); }
    }



    public string this[string columnName]
    {
        get { throw new NotImplementedException(); }
    }
}

Then:

MyDictionary<string, string> myDict = new MyDictionary<string, string>();
    Dictionary["key"] = "value";

I get the following error:

Property or indexer 'MyDictionary.this[string]' cannot be assigned to -- it is read only

I guess I'm missing some kind of Getter/Setter. But shouldn't this be inherited from base class Dictionary? How can I create it?

Answer 1

Your problem is, that IDataErrorInfo implements the this[string] indexer too.

public string this[string columnName]
{
    get { ...; }
}

If you want to access the indexer of Dictionary you need to cast your variable to Dictionary (one of the problems of inheritance)

((Dictionary<string,string>)variableName)["key"] = "something";

or even better (since you'll probably never use IDataErrorInfo manually), implement the interface explicitly :

string IDataErrorInfo.this[string columnName]
{
    get { return ...; }
}

That way you can use the Dictionary like usual and the IDataErrorInfo indexer by casting

dictionaryVariable["key"] = "something";
string error = ((IDataErrorInfo)dictionaryVariable)["key"];

Answer 2

Dictionary has a method called Add which you should use to add to the dictionary. You can also use .Contains to ensure the key isn't already used in the dictionary.