How to set collection inline?

问题:

For example:

DataTable table = new DataTable() 
{ 
  Columns = new DataColumnCollection(
     { 
         new DataColumn("col1"), 
         new DataColumn("col2")
     })
});

回答1:

You are talking about the Collection Initialiser feature added in C# 3. It is done like this:

DataTable table = new DataTable() 
{ 
    Columns = 
    { 
        new DataColumn("col1"), 
        new DataColumn("col2")
    }
};

This does not call a collection constructor, it uses the collection which already exists in the DataTable.

This is short-hand for Columns.Add(), so it doesn't require Columns to have a setter.

You were so close with the code in your question!

回答2:

The Columns property does not have a setter so you can only modify it.

How about this:

DataTable table = new DataTable();
table.Columns.AddRange(new[] { new DataColumn("col1"), new DataColumn("col2") });

If you want to do with one statement in a lambda:

DataTable table = (() => {
    var table = new DataTable();
    table.Columns.AddRange(new[] { new DataColumn("col1"),
                                   new DataColumn("col2") });
    return table;})();

回答3:

You probably need to remove the paretheses around that collection initializer for DataColumnCollection, and remove the unmatched, final )

Those are syntactical issues, though. The underlying problems are that the Columns property has no setter, and the DataColumnCollection has no public constructor.

Basically, you have to instantiate and then call .Columns.Add().

If this is something you have to do a lot in your code, you could create helper classes that would give you friendlier syntax:

DataTable table = DataTableFactory.CreateTableWithColumns("col1", "col2");

回答4:

There are 2 reasons why this won't work:

1) the Columns property is read-only 2) the DataColumnCollection class does not have a constructor that accepts a collection of columns to initialize it.
Best you can do is create the table in one line and add the columns in another:

DataTable table = new DataTable();
table.Columns.AddRange( new []
     { 
         new DataColumn("col1"), 
         new DataColumn("col2")
     });

To answer your other question, IF Columns had a setter and IF DataColumnCollection accepted columns in its constructor the syntax would be:

DataTable table = new DataTable() 
{ 
  Columns = new DataColumnCollection( new DataColumn[]
     { 
         new DataColumn("col1"), 
         new DataColumn("col2")
     })
});

回答5:

The class DataColumnCollection has no constructor so you can't manually create an instance. The compiler's error message should be pretty self-explanatory, saying something along the lines of:

The type 'System.Data.DataColumnCollection' has no constructors defined

You can add columns to the DataTable.Columns collection by using the Add() method:

DataTable table = new DataTable();
table.Columns.Add(new DataColumn("col1"));
table.Columns.Add(new DataColumn("col2"));

回答6:

You can't use that syntax as the Columns property is readonly. I'd use the technique suggested by Gabe.