How to use indexers with Extension Methods having out parameter and function calls

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

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

score 4 · Answer 1 · answered Jun 25 '10 at 10:47

4

Well, there are two issues here:

C# doesn't (by and large) support named indexers¹
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.

answered Jun 25 '10 at 10:47

Jon Skeet

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@Jon: What kind of indexer i need to create here in that case. http://msdn.microsoft.com/en-us/library/6x16t2tx.aspx. I read this link to create indexer but how will it work within extension class. Is it possible to work out using this within static class as i need to create indexer also static – Shantanu Gupta Jun 25 '10 at 10:51
I too would prefer the method specified above in my ques until unless i have named indexer for which i have to start working on C# 4 – Shantanu Gupta Jun 25 '10 at 10:53
Why only supporting named indexers on COM? – simendsjo Jun 25 '10 at 10:59
@Jon: How it is possible to use named indexers in 2K8 with datatables and datasets and many more. i.e. `(new DataTable).Rows[0]["mycol"]`. Doesn't .net opens this indexers to every one in previous versions itself – Shantanu Gupta Jun 25 '10 at 11:04
@Shantanu: That's using the `Rows` property, which returns a `DataRowCollection` which itself has an indexer. See my second bullet point - you can't create extension properties. – Jon Skeet Jun 25 '10 at 11:09
@simendsjo: I believe there was a desire to support it more widely, but the cost was seen to be greater than the value provided. The C# team only has limited resources. – Jon Skeet Jun 25 '10 at 11:10
@Jon: I read related article on msdn for C# 4. But still i would like to know named indexers are available to Collections in all previous version ( – Shantanu Gupta Jun 25 '10 at 11:14
@Shantanu: Named indexers just aren't supported in C# - the idea is that it's generally better to write a property which returns an indexable collection. I'm not sure to what extent I agree with that, but that's the explanation usually given. – Jon Skeet Jun 25 '10 at 12:43

score 1 · Answer 2 · edited Aug 03 '13 at 18:32

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.

How to use indexers with Extension Methods having out parameter and function calls

2 Answers2

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