如何使用具有扩展方法的索引器具有参数和函数调用

Question

Is it possible to use indexers with extension methods.

eg. Consider it as an example only.

    public static object SelectedValue(this DataGridView dgv, string ColumnName)
    {            
        return dgv.SelectedRows[0].Cells[ColumnName].Value;
    }

EDIT

1. usage mygrid.SelectedValue("mycol")

2. How to use it as an indexer mygrid.SelectedValue["mycol"] rather than above one.

3. Is it possible to use it like this as well ? mygrid.SelectedValue["mycol"](out somevalue);

What are the syntax of getting this kind of values. Any simple example or link will work.

Answer 1

Well, there are two issues here:

• C# doesn't (by and large) support named indexers ¹
• C# doesn't support extension properties, so you can't make SelectedValue a property returning something indexable instead

So no, the syntax you've specified there won't work. You could get this to work:

mygrid.SelectedValue()["mycol"]

but that's a bit ugly. I'd stick with the method form if I were you.

---

¹ C# 4 supports calling named indexers on COM objects.

Answer 2

Let me try to clarify the usage and intentions of Extension Method .

Consider a Extension Method

public static bool IsNullOrEmpty(this string source)
{
    return source == null || source == string.Empty;
}

Now you extend your string class with this Extension Method

var myString = "Hello World";
Assert.AreEqual(myString.IsNullOrEmpty(), false);

This is what .NET does on compilation:

public static bool IsNullOrEmpty(string source)
{
    return source == null || source == string.Empty;
}

Using our old school

var myString = "Hello World";
Assert.AreEqual(IsNullOrEmpty(myString), false);

Extension method is nothing but a visualization to what we were used to do.

Well, extending indexers could be possible but Microsoft did not think about it.