C＃和只读引用类型（例如，DataTable）

Question

Let's say I have the following C# class:

class MyClass ()
{
    public readonly DataTable dt = new DataTable();
    ...
}

What's the meaning of readonly for reference types? I just implemented this code and the user is atill able to modify the datatable.

How can I prevent the user from writing or mutating my datatable (or any object in general)? Ie, just read access. Obviously using properties wouldn't help here.

Answer 1

Readonly means that you can not reassign the variable - eg later on you can not assign a new DataTable to dt.

As for making the object itself read-only - that entirely depends on the object itself. There is no global convention for making objects immutable.

I didn't see anything specific to achieve this with .NET's DataTable but some options would be to

1. Ensure that the user doesn't have modify permissions in the database itself (most secure)
2. Look for ReadOnly attributes on whatever grid/controls you are binding to it (also clear to the users then that this is read-only)

Answer 2

you can make a class like this:

class MyClass
{
    private DataTable dt = new DataTable();
    public MyClass()
    {
       //initialize your table
    }
    //this is an indexer property which make you able to index any object of this class
    public object this[int row,int column] 
    {
        get
        {
            return dt.Rows[row][column];
        }
    }

    /*this won't work (you won't need it anyway)
     * public object this[int row][int col]*/
    //in case you need to access by the column name
    public object this[int row,string columnName]
    {
        get 
        {
            return dt.Rows[row][columnName];
        }
    }



}

and use it like this example here:

 //in the Main method
 MyClass e = new MyClass();
 Console.WriteLine(e[0, 0]);//I added just one entry in the table

ofcourse if you wrote this statement

e[0,0]=2;

it will produce an error similar to this: Property or indexer MyNameSpace.MyClass.this[int,int] cannot be assigned to --it is read only.

Answer 3

readonly表示Datatable将是运行时常量，而不是编译时常量