Question

I’m trying to create an array of the class DataTableColumn, but unfortunately I’m going crazy with that.
By now that’s what I tried to do without any success

public interface IDataTableColumn<out TValue> {
    string Expression { get; }
    DataTableFilterType FilterType { get; }

    TValue Cast(string value);
}

public class DataTableColumn<TValue> : IDataTableColumn<TValue> {
    public DataTableColumn(string expression, DataTableFilterType filterType = DataTableFilterType.Equal) {
        Expression = expression;
        FilterType = filterType;
    }

    public string Expression { get; private set; }
    public DataTableFilterType FilterType { get; private set; }

    public TValue Cast(string value) {
        return value.As<TValue>();
    }
}

My array SHOULD be like

private readonly IDataTableColumn<object>[] _columns = {
    new DataTableColumn<int>("Id"), // ERROR
    new DataTableColumn<string>("Description", DataTableFilterType.StartsWith), // SUCCESS
    new DataTableColumn<DateTime?>("Date"), // ERROR
};

Actually working like that

private readonly dynamic[] _columns = {
    new DataTableColumn<int>("Id"),
    new DataTableColumn<string>("Description", DataTableFilterType.StartsWith),
    new DataTableColumn<DateTime?>("Date"),
};

I think that using dynamic isn’t the best way to do that.. someone shed light, please!

EDIT

damn I forgot the error

Cannot implicitly convert type
DataTableColumn< int >’ to
‘IDataTableColumn< object >’. An
explicit conversion exists (are you
missing a cast?)

Cannot implicitly convert type
DataTableColumn< System.DateTime? >’ to
‘IDataTableColumn< object >’. An
explicit conversion exists (are you
missing a cast?)

Answer 1

I believe it is because int and DateTime are value types which are not covariant with object (they require boxing). I receive the following error when I compile your code:

covar.cs(23,13): error CS0266: Cannot implicitly convert type
    'DataTableColumn<System.DateTime>' to 'IDataTableColumn<object>'.
    An explicit conversion exists (are you missing a cast?)

Per MSDN, “Variance in Generic Interfaces (C# and Visual Basic)”:

Variance in generic interfaces is supported for reference types only. Value types do not support variance. For example, IEnumerable<int> (IEnumerable(Of Integer) in Visual Basic) cannot be implicitly converted to IEnumerable<object> (IEnumerable(Of Object) in Visual Basic), because integers are represented by a value type.

DataTableColumn<string> works because it derives directly from object.