Is there any proposals or specification that use spread operator within collection initializer?

[JerryJian]

Just like what was asked in the title, is there any proposals or specification that use spread operator within collection initializer?
Code example:

class Options
{
    public List<string> SelectedIds { get; } = [];
}

class ViewModel
{
    public List<Checkable> CheckList { get; } = new();

    public async Task DoWork()
    {
        var options = new Options
        {
            SelectedIds =
            {
                ..CheckList.Where(t => t.IsChecked).Select(t => t.Id),
            }
        };
    }
}

Thanks!

[huoyaoyuan]

Unlike the old collection initializer, collection initializer doesn't work with the Add method on collections. So support for additions into existing collection is new.

For this proposal, there is an alternative approach with List<string> SelectedIds { get; init; } = [].

[JerryJian]

The code in the description is a hand writing example. The most common one I encounter is ProcessStartInfo.ArgumentList, which doesn't has setter.

Unlike the old collection initializer, collection initializer doesn't work with the Add method on collections. So support for additions into existing collection is new.

I'm aware of this just now, I will study it.

[JerryJian]

Unlike the old collection initializer, collection initializer doesn't work with the Add method on collections. So support for additions into existing collection is new.

But I use ILSpy to view the IL code like this:

public class InstallerXSite
{
	public InstallerXSite()
	{
		new ProcessStartInfo
		{
			ArgumentList = { "1", "2" }
		};
	}
}

.class public auto ansi beforefieldinit InstallerXLib.InstallerXSite
	extends [System.Runtime]System.Object
{
	// Methods
	.method public hidebysig specialname rtspecialname 
		instance void .ctor () cil managed 
	{
		// Method begins at RVA 0x2050
		// Header size: 1
		// Code size: 47 (0x2f)
		.maxstack 8

		IL_0000: ldarg.0
		IL_0001: call instance void [System.Runtime]System.Object::.ctor()
		IL_0006: nop
		IL_0007: nop
		IL_0008: newobj instance void [System.Diagnostics.Process]System.Diagnostics.ProcessStartInfo::.ctor()
		IL_000d: dup
		IL_000e: callvirt instance class [System.Runtime]System.Collections.ObjectModel.Collection`1<string> [System.Diagnostics.Process]System.Diagnostics.ProcessStartInfo::get_ArgumentList()
		IL_0013: ldstr "1"
		IL_0018: callvirt instance void class [System.Runtime]System.Collections.ObjectModel.Collection`1<string>::Add(!0)
		IL_001d: nop
		IL_001e: callvirt instance class [System.Runtime]System.Collections.ObjectModel.Collection`1<string> [System.Diagnostics.Process]System.Diagnostics.ProcessStartInfo::get_ArgumentList()
		IL_0023: ldstr "2"
		IL_0028: callvirt instance void class [System.Runtime]System.Collections.ObjectModel.Collection`1<string>::Add(!0)
		IL_002d: nop
		IL_002e: ret
	} // end of method InstallerXSite::.ctor

} // end of class InstallerXLib.InstallerXSite

We can see it calls Add method to add the list one by one.

[huoyaoyuan]

Sorry for the typo. I was to mean that collection expression doesn't work with Add method.

[JerryJian]

Ok, thanks a lot. Finnaly I found that I could write an Add(IEnumerable<T> collections) extension method to achieve this, but the name Add may cause missunderstanding.