需要帮助来理解MVCContrib Grid中的这段代码

Question

I'm a bit new to the concept of "Action" in C# and delegate in general.

I'm trying to study how to build a custom html component in MVC, and I chose the grid component of MVCContrib to start.

To add columns, typically we do

 <%= Html.Grid(Model).Columns(column =>
            {
                column.For(model => model.Date).Format("{0:d}");
                column.For(model => model.DayAmount);
                column.For(model => model.LeaveType);
            })
            %> 

and I see the source of Columns like the following

public IGridWithOptions<T> Columns(Action<ColumnBuilder<T>> columnBuilder)
        {
            var builder = new ColumnBuilder<T>();
            columnBuilder(builder);

            foreach (var column in builder)
            {
                if (column.Position == null) 
                {
                    _gridModel.Columns.Add(column);
                } 
                else
                {
                    _gridModel.Columns.Insert(column.Position.Value, column);   
                }
            }

            return this;
        }

What I'm confused of is the Action parameter In this instance, Type is CustomBuilder, so when did the "CustomBuilder" object got instantiated?

I suppose, i can rewrite the calling statement above as

Html.Grid(Model).Columns(delegate(CustomBuilder<T> column)
{
});

or a bit more explicit as

Html.Grid(Model).Columns(new Action<CustomBuilder<T>>(delegate(CustomBuilder<T> column)
                      {
                      });
);

So are we saying, when the Action was instantiated with the "new" keyword above, the param "CustomBuilder" was instantiated as well?

Lastly, in the

" public IGridWithOptions<T> Columns(Action<ColumnBuilder<T>> columnBuilder) "

function,

the first two lines are

var builder = new ColumnBuilder<T>();
columnBuilder(builder);

What do they do? Looks like it's instantiating ColumBuilder object and pass it as a parameter to Action method columBuilder. Is this where you instantiate the parameter?

Thank you all.

Answer 1

It's nothing to do with action concept.
The delegates is present in .net from the beginning so you should start with the first step. Should build the wall before the roof.
Delegates
Lambda Expressions
Expression trees
But you should know about generic classes and methods, extension methods...

Answer 2

Got it after read this excellent article.

http://www.codeproject.com/Articles/47887/C-Delegates-Anonymous-Methods-and-Lambda-Expressio

while it's talking about Func, the concept applies to Action, which does not return any results.

Looks like the magic happens here

var builder = new ColumnBuilder(); columnBuilder(builder);

I obviously didn't understand the fact that delegate, is just a pointer to a function (anonymous or not). You still need to supply the parameter when calling it. (Duh!).

All cleared up now.

Thank you.